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The twolined chestnut borer, Agrilus bilineatus (Weber) (Coleoptera: Bupres- tidae) ...... We thank Dr. Dave J. Hall and Paul E. Pingrey, Wisconsin Department of.
The

Canadian

Vol. 114

Entomologist

Ottawa, Canada, May 1982

No. 5

THE BIOLOGY AND ECOLOGY OF THE TWOLINED CHESTNUT BORER, AGRILUS BILINEATUS (COLEOPTERA: BUPRESTIDAE), ON OAKS, QUERCUS

SPP., IN WISCONSIN1

ROBERTA . HAACK'and DANIELM. BENJAMIN Department of Entomology, University of Wisconsin, Madison 53706

Abstract

Can. Ent. 114: 385-396 (1982)

Agrilus bilineatus (Weber) is univoltine in Wisconsin. Adults were present from early June through mid-September; however, peak flight and oviposition occurred during the second half of June. Adults consumed significantly more oak foliage than foliage of hardwoods other than oak. Stressed oaks attracted significantly more adults than did uninjured controls, as measured by density of captured adults on sticky traps. Four larval instars were determined; instar IV was the overwintering stage. Fourth instar larvae generally overwintered in pupal cells in the outer bark or in sapwood if the bark was thin. Larvae, especially third and fourth instats, damaged oaks by girdling the conducting xylem and phloem. Attacked oaks had low root starch content relative to those unattacked. Tree death normally resulted after 2 or 3 years of borer infestation, yet tree death may occur in a single season. Initial attack usually began in the live crown and proceeded downward in succeeding years with no apparent reinfestation of previously killed areas. Mean adult longevity in days for adults fed red oak foliage was 28.1 at 20°, 37.8 at 24", and 8.3 at 30°C. Mean duration of the pupal stage in days was 11.7 at 24' and 8.5 at 30°C.

Au Wisconsin, Agrilus bilineatus (Weber) est univoltine. Les adultes ont CtC observts du dtbut juin a la mi-septembre; cependant le plus fort du vol et de la ponte est survenu durant la seconde moitiC de juin. Les adultes ont consommi significativement plus de feuillage de chgne que de feuillage d'autres bois francs. Des chCnes soumis au stress ont attire significativement plus d'adultes que des ch2nes timoins intacts, d'aprks la densite des adultes capturts au pihge engluk. Quatre stades larvaires ont CtC dCnombrLs, le stade IV Ctant le stade hivernant. Les larves de stade IV ont gCnCralement ete observees hivernant a l'intirieur de cellules pupales logies dans I'tcorce externe ou l'aubier si 1'Ccorce Ctait mince. Les larves, en particulier celles des stades 3 et 4, ont caust du dommage au chCne en "ceinturant" les tissus conducteurs du phlokme et du xylkme. Les ch2nes attaquCs ont montrC une basse teneur en amidon dans la racine compares aux ch2nes non-attaquts. La mortalit6 des arbres s'est gtnkralement produite suite a 2 ou 3 ans d'attaque des perceurs, mais elle peut survenir en une seule saison. L'attaque initiale a gCnCralement commence dans la couronne saine, et a progress6 vers le bas d'une annee a l'autre, apparemment sans rb-infestation des parties dCja mortes. La longCvitC adulte moyenne en jours pour des adultes noums de feuillage de ch2ne rouge a Ctt de 28.1 20°, 37.8 24", et 8.3 B 30°C. La durCe moyenne du stade pupal en jours a tte de 11.7 a 24" et 8.5 30°C.

Introduction

The twolined chestnut borer, Agrilus bilineatus (Weber) (Coleoptera: Buprestidae), is a major factor contributing to the mortality of weakened oaks, Quercus v

'Research supported by the School of Natural Resources. College of Agricultural and Life Sciences, University of Wisconsin-Madison, and the Wisconsin Department o f Natural Resources. 'Resent address: Department of Entomology and Nematology, University of Florida. Gainesville 32611.

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spp., throughout eastern North America. Early investigations of the borer's life history were conducted in Virginia (Chittenden 1909) and Minnesota (Chapman 1915). Later studies were completed in Connecticut (Dunbar and Stephens 1975, 1976) and in New York and Pennsylvania (Cote and Allen 1980). Borer outbreaks in southern Wisconsin from 1976 through 1980 renewed interest in this pest. Outbreaks occurred in areas where trees were weakened by drought, ice-storm damage, defoliation by fall cankerworm, Alsophila pometaria (Harris) (Lepidoptera: Geometridae), and man-made disturbances. We report the results of investigations of the biology and ecology of A. bilineatus in a natural oak-hardwood stand in the Kettle Moraine State Forest, Jefferson County, Wisconsin, and in the laboratories at the University of Wisconsin-Madison in 1979 and 1980.

Materials and Methods Adult activities. The adult flight periods for 1979 and 1980 were determined by trapping adults on 20-cm-wide polyethylene bands coated with Tree Tanglefoot@. Traps were stapled at breast height (1.3 m) around the trunks of healthy oaks (11-51 cm d.b.h.) that showed no symptoms of last-season borer attack. Borer-attacked oaks can be recognized in winter and spring by the presence of last season's foliage on killed branches (Hopkins 1904). We used 4 red oaks (Q. rubra L.), 4 black oaks (Q. velutina Lam.), 6 white oaks (Q. alba L.), and 6 mossycup oaks (Q. macrocarpa M i ~ h x . )in~ 1979, and 6 red, 4 black, and 7 white oaks in 1980. Traps were installed on 24 May 1979 and 30 May 1980. Trees were stressed by basal girdling to ca. 20 mm depth with a chain saw on 31 May 1979 and 4 June 1980. Adults were removed and counted weekly from 31 May through 10 October 1979 and from 4 June through 17 September 1980. Longevity was determined for fed and unfed adults using 10 males and 10 females per treatments at 20°, 25", and 30°C (photoperiod 16L:8D). Adults were reared from naturally infested red oak bark collected on 7 May 1980. Adults were removed from emergence cages daily, sexed, put into individual petri dishes with moist filter paper, and maintained in environmental chambers. The filter paper was kept moist in all treatments, and the fed adults received fresh red oak foliage every 2 days. We removed the dead borers daily and recorded longevity in days. The food preference of adults was investigated by using foliage from eight tree species that comprised the forest type of the study area: red oak, black oak, white oak, mossycup oak, shagbark hickory ( ~ C a v uotraatn (Mill.) K. Koch). trembling aspen (Populus tremuloides Michx.), black cherry (Primus serofina Ehrh.). and basswood (Tilia americana L.). Each of' the 36 bioassays consisted o f I6 foliage discs (2 discslspecies, 1 cm diam.) that were soaked in water for ca. 24 h, and randomly positioned around the perimeter of a petri dish ( l o x 2 cm) on moist filter paper. Three unsexed recently-emerged adults were introduced into each dish and maintained at 24OC (photoperiod 16L:8D). After 48 h the leaf discs were dried and weighed. The weight of consumed foliage per leaf disc was estimated by subtracting the oven-dry weight of each disc after feeding from a standard weight for that species. The standard weights were calculated by averaging the oven-dry weights of 50 similar leaf discs per species that were cut from the same leaves used in the bioassays. The sex ratio was determined for adults reared from naturally infested red and black oak bark in 1980, and for adults captured on the sticky traps used in the 1979 flight period study. The characteristic deep groove on the second abdominal sternite in males and its absence in females was used (Horn 1891). 3Q.

macrocarpa Michx. = mossycup oak, bur oak.

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Adult host preference for stressed trees was studied by comparing the mean density of all adults captured in 1979 on 20 girdled oaks (those used in the 1979 flight period study) and on 20 ungirdled controls. Within a 15-m radius of each girdled oak, we selected a control tree of the same species with similar height and diameter and with no symptoms of borer attack. Sticky traps, as described in the flight period study, were installed on 24 May 1979; captured adults were collected and counted weekly. On 1 January 1980 the crown branches of the controls were observed for retained foliage, and the lower 1 m of trunk of all 40 oaks was debarked and inspected for living larvae and larval galleries to determine if oviposition occurred. The oviposition period was estimated by making weekly observations at ca. 1h intervals from 0800 to 1800 from 31 May through 14 September 1979. The lower trunk (1-2 m) of living oaks with symptoms of borer attack were examined for eggs where females had been observed to enter a bark crack, turn 180°, and move in and out as if ovipositing.

Larval development and survival. Seasonal larval development was studied by collecting larvae from infested red and black oaks felled at regular intervals from 30 June through 10 December 1979; one oak was felled per collection date. Time of infestation for all oaks in this study was considered the same because we observed females ovipositing on them on each of 4 days from 14 June through 23 June 1979. Bolts, 30 cm long, were cut at 2-m intervals from the tree base out along one major crown branch to a final diameter of ca. 5 cm. Bolts were debarked with hammer and chisel to expose larvae in the cambial region. Prepupal larvae were recovered from pupal cells in the outer bark or sapwood (where the bark was thin) by carefully splitting the bark or shaving the sapwood. All larvae were counted, preserved in alcohol, and referenced by collection date. The number of larval instars was determined by evaluating a histogram of urogomphus (anal forceps) lengths for all larvae collected from 30 June through 21 September 1979. Seasonal larval development was approximated by assigning each larva an instar ranking based on the length of its urogomphus and then determining the distribution of larval instars by collection date. Host root-starch content and within-tree borer distribution. Host root-starch content, adult exit hole densities, overwintering larval densities, and the extent of larval girdling were determined for red and black oaks in five host condition categories based on external tree symptoms of borer attack. Forty-five dominant or codominant oaks were selected in early September 1979 when symptoms of currentyear borer attack were most evident. The amount of dead wood in the crown, the presence of last season's foliage on some dead branches, and the percentage of current-year foliage that had prematurely wilted and turned brown (flagging) were used as criteria to categorize the trees. Category I oaks (n = 10) had no symptoms of attack in 1979: some dead branches but with no foliage from last season, no flagging. Category I1 oaks (n = 10) had symptoms of borer attack in which some crown branches died in 1979 after one season of attack: some dead branches but with no foliage from last season, 25-50% flagging. Category I11 oaks (n = 5 ) had symptoms of borer attack in which trees died in 1979 after one season of attack: some dead branches but with no foliage from last season, 100% flagging. Category IV oaks (n = 10) had symptoms of borer attack in which trees died in 1979 after two seasons of attack; 25-50% of the crown branches dead, some dead branches with last season's foliage attached, 100% flagging. Category V oaks (n = 10) had symptoms of borer attack in which trees died in 1978: all crown branches dead,

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some lower crown branches with last season's foliage attached, no current-year foliage produced. A sample of root wood ( 2 x 2 1~ cm) was removed from a major root of each oak about 30 cm below the soil line on 3 November 1979. Samples were bagged, put onto ice, then stored at - 10°C. On 14 January 1980, the samples were analyzed visually for starch following the Wargo (1978) procedure. Sections, 120 p m thick, were cut from the samples with a sliding microtome, stained with 1,KI solution, rinsed with water, and viewed with a dissecting microscope. The starch content of the ray parenchyma cells was rated as high, medium, low, or depleted when compared with relative standards for red oak as shown by Wargo (1975). The ranges of starch concentration determined chemically by Wargo (1975) for red oak were 15-30% for a rating of high, 7-12% for medium, 3-6% for low, and 0-1% for depleted. The numerical value we assigned to each oak's starch rating was the range midpoint: 22.5% high, 9.5% medium, 4.5% low, 0.5% depleted. Twenty-five oaks, five from each category, were felled in early December 1979. Bolts, 30 cm long, were cut at 2-m intervals from tree base out along one major branch to a diameter of ca. 5 cm; bolt length and diameter-inside-bark were recorded. The number of adult exit holes was recorded per m2 of bolt area to indicate areas where complete larval and pupal development and subsequent adult emergence had occurred. The number of living larvae in the cambial region and in pupal cells was recorded per m2 of bolt area to indicate areas of current-year attack. Percent circumferential girdling of each bolt was estimated from larval feeding galleries as a measure of damage to the tree's conductive tissues. It is important to realize that the layer of phloem and xylem that is actively involved in conduction is exceedingly narrow in ring-porous trees,such as oak. The conducting phloem in oak is ca. 0.2 mm thick (Holdheide 1951). Conduction in xylem occurs in the outermost annual ring in red oak, and in the outermost 1 or 2 annual rings in white and mossycup oak (Kozlowski and Winget 1963; Kramer and Kozlowski 1979). Also, water movement in xylem flares circumferentially as it ascends in trees of the red oak group (Kozlowski and Winget 1963). Therefore, a point on the bolt's circumference was considered girdled when at least two galleries crossed an imaginary line drawn lengthwise along the bolt grain at that point. The five unfelled oaks from each category with known root starch levels, and an additional five for category 111, were checked in July 1980 and June 1981 for evidence of further borer attack. In early June 1980, the lower trunks were inspected for ovipositing females and eggs. In mid-July 1980, bark samples were removed from the lower trunk of each oak to note the presence or absence of A . bilineatus larvae. In June 1981, all oaks originally from categories I and I1 were checked for flagging foliage from the 1980 growing season and for adult exit holes.

Pupal development and adult emergence. The period of pupation was determined by collecting all life stages of the borer from naturally infested bark collected at regular intervals from 1 January through 4 June 1980. Bark samples (0.5-1 m2) were removed from the lower trunk of infested red and black oaks, and split with hammer and chisel. We recorded the number of borers recovered by life stage and the number of current-year adult exit holes per m2 of bark area. The rate of pupal development was studied at 24°C and 30°C (photoperiod OL:24D). Prepupal larvae were obtained from red oak bark collected and split on 16 April 1980. Larvae were put into individual plastic cups on moist filter paper, placed into environmental chambers, and checked daily for initiation and termination of the pupal stage. Pieces of bark, with larvae in pupal cells that were exposed

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T H E CANADIAN ENTOMOLOGIST

during extraction, were put into petri dishes on moist filter paper and used to determine pupal stage duration and the length of time for adults to bore through the bark and emerge.

Results and Discussion Adult activities. The flight pattern of borer adults was similar in 1980 and 1979, with first collections on 4 and 7 June, peak collections on 18 and 21 June, and l a g collections on 10 and 14 September, respectively (Fig. 1). The mean density (x r S.E.) of captured borers per dm2 of trap increased from 7.3 & 1.4 (n = 2,736) in 1979 to 29.9k3.9 (n = 12,987) in 1980. These data reflected the increase in the number of dead and dying oaks observed in the study area in September 1978 and 1979, respectively. Borer collections on sticky traps on seven other oaks in the same study area that were girdled on 17 April 1980 showed a similar flight period to that in Fig. 1, but mean density was lower, 6.8/dm2 (n = 1,195) (J. 0.H a a n ~ t a d , ~ pers. comm.). These data suggest that as a result of girdling, host volatiles attractive to A. bilineatus were released in relatively large quantities at first and diminished over time. Similar flight periods were recorded in Connecticut (Dunbar and Stephens 1976) and New York (Cote and Allen 1980). Male and female longevity data were combined for each treatment because they did not differ significantly (P < 0.05 Student's t-test). Some adults died because of a fungal disease early during the experiment; these data were omitted from the analyses. Longevity in days (x k S.E., range) for fed adults was 28.1 k l . 8 , 19-44 (n = 14) at 20"; 37.8k1.9, 26-46 (n = 17) at 24"; and 8 . 3 e 0 . 5 , 7-13 (n = 12) at 30°C; means differed significantly (P < 0.01). Longevity in days for unfed adults was 11.2k 1.2, 4-24 (n = 20) at 20"; 7.7k0.5, 4-12 (n = 17) at 24"; and 5.1 20.3, 3-7 (n = 18) at 30°C; means differed significantly (P < 0.01). Borer adults consumed significantly more oak foliage (except for red oak) than non-oak foliage, more foliage of the white oak group (white and mossycup) than foliage of the red oak group (black and red), and more mossycup oak foliage than all others tested (Table I) (P < 0.05 Student-Newman-Keuls' multiple range test). In Connecticut, scarlet oak (Q. coccinea Muenchh.) was the preferred species, followed by chestnut oak (Q. prinus L.), white oak, black oak, and red oak (Dunbar and Stephens 1976). The sex ratio (8:0 ) was 1:0.99 (n = 462) for reared adults, and 1: 1.16 (n = 1071) for adults captured on sticky traps. The sex ratio was ca. 1:l for adults collected on traps at weekly intervals from 11 June through 23 August 1979.

28 MAY

11

25 JUNE

9

23 JULY

6

20 AUGUST

17

3

SEPT

FIG. 1 . Agrilus bilineatus adult flight period for 1979 (n = 2,736) and 1980 ( n = 12,987) based on weekly sticky-trap collections on gridled oaks: Kettle Moraine State Forest, Jefferson County, Wisconsin. 'Department of Entomology, University of Wisconsin. Madison 53706.

May 1982

THE CANADIAN ENTOMOLOGlST

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Table I . Mean dry-weight consumption 5 S.E., mg) per leaf disc by 3 Agrilus bilineatus adults that were offered foliage of 8 hardwood tree species during a 48-h feeding period: 24"C, photoperiod 16L:8D.* Tree species

Mean consumption

(n)

Quercus macrocarpa Quercus alba Quercus velutina Quercus rubra Carya ovata Populus tremuloides Prunus serotina Tilia americana -

p p

- -

-

*Each bioassay ( n = 36) consisted of 16 leaf discs (Yspecies, 1 cm diam.) and 3 unfed adults. Dry weight consumption per leaf disc was estimated by subtracting the oven-dry weight of each disc after feeding from a standard ovendry weight for that species. Experiments were conducted from 25 to 29 June 1979. tMeans followed by the same letter are not significantly different at the P < 0.05 level of probability (Student-Newman-Keuls' multiple range test).

Significantly more adults were captured on girdled oaks than on uninjured c_ontrols ( P < 0.001 paired t-test). Mean density of captured adults per dm2 of trap (x 2 S.E., range) was 7 . 3 2 1 . 4 , 2.5-28.0 (n = 2736) on girdled oaks, and 0.8 20.3, 0.0-6.1 (n = 293) on controls. Mean densities of captured borers did not differ significantly between the girdled trees of the white oak and red oak groups (P < 0.05 Student's t-test). Later inspection revealed that only the girdled oaks had been attacked by A . bilineatus. Females apparently were able to discern suitable and nonsuitable host material because no larvae or larval galleries were found on those control oaks that had relatively high mean densities (3.0-6.1) of captured adults. Controls within 4 m of a girdled oak attracted significantly more adults than controls at greater distances ( P < 0.01 Student's t-test), probably because they were within the girdled tree's effective radius of possible volatile attractants. The possibility that stressed oaks release volatiles, such as ethanol (Cote and Allen 1980), that are attractive to A . bilineatus adults should be investigated. Females were observed ovipositing on oaks from 14 June through 16 August 1979 (Fig. 2). No adults were seen on oaks after 16 August, although 1-4 adults were captured weekly on traps through mid-September. We observed females ovipositing from 1000 to 1700 h; peak oviposition occurred from 1100 to 1400 h. Groups of 1-7 eggs were laid, with 2-3 most common. Larval development and survival. Four instars were determined from a comparison of u r o g o ~ p h u slengths measured ventrally (n = 549). Mean length of the urogomphus in mm (x k S.E.) was 0.121 20.002 (n = 37) for instar I, 0.24320.002 (n = 94) for instar 11, 0.44820.002 (232) for instar 111, and 0.639?0.003 (n = 186) for . instar IV. In Pennsylvania, mean urogomphus lengths in mm were 0.101 (I), 0.210 (II), 0.407 (111), and 0.612 (IV) (Cote and Allen 1980). First instar larvae were collected from 30 June through 23 August, second instars from 12 July through 21 September, third instars from 18 July through 10 November, and fourth instars from 26 July through 10 December 1979 (Fig. 2). One fourth-instar larva was found initiating pupal cell construction on 12 July 1979; probably it developed during the 1978 season and overwintered in the cambial region as a third or fourth instar. We first observed fourth instar larvae entering the bark to construct pupal cells on 9 August, and larvae were first seen in fully constructed pupal cells on 23 August. Eight percent of the recovered larvae ( n = 78) on 7

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September were in fully constructed pupal cells, 40% ( n = 125) on 21 September, 80% ( n = 107) on 10 October, 92% ( n = 103) on 10 November, and 96% ( n = 68) on 10 December. Generally, fourth instar larvae in the crown were the first to leave the cambial region; those at lower levels left progressively later (Table 11). Similarly, drying of the cambial region appeared to progress from crown to base. When infested oaks were felled and sampled in December 1979, the largest prepupal larvae were recovered from lower trunk samples and the smallest were from crown samples. When the lower trunk of each of the 20 girdled oaks used in the 1979 flight period study was debarked in January 1980, we found living larvae below the line of girdling in all trees, but only dead larvae above the girdle. Tissues above Table 11. Percent of Agrilus bilineatus larvae in pupal cells at various heights above ground for red oaks sampled from 17 August through 10 December 1979: Kettle Moraine State Forest, Jefferson County, Wisconsin. * Percent of larvae in pupal cells at indicated tree height above ground (m) Sampling date

0.5

2.5

4.5

6.5

8.5

10.5

12.5

14.5

August 17 August 23 September 7 September 21 October 10 November 10 December 10

0 0 0 46 76 82 90

0 0 0 41 80 93 96

0 0 0 62 79 96 92

0 0 0 88 83 100 100

0 0 0 84 100 100 100

0 0 10 79 100 100 100

0 0 29 100 100 100 100

0 25 66 100 10 100 100

(86) (123) (78) (125) (107) (103) (68)

*One infested red oak was felled on each sampling date. Sample bolts, 30 cm long, were cut at 2-m intervals from tree base to a final crown-branch diameter of ca. 5 cm. tTotal number of larvae recovered from all bolts of the sampled oak on indicated date.

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the girdle probably became unsuitable for larval development because of desiccation. In another study (Haack and Benjamin 1980), borer larvae failed to complete development in infested oaks felled prior to mid-July at which time only first and second instar larvae were present. Felling during early larval development apparently caused the cambial region to become unsuitable to A. bilineatus larvae faster than if the trees had remained standing. Variation in adult length (4.5-12.5 mm in our study) may reflect the host conditions under which the larvae developed.

Host root-starch content and within-tree borer distribution. Category I oaks had no external symptoms of borer attack; however, two of the five oaks felled and sampled in December 1979 were attacked and had current-year larval galleries (Table 111). Most attacks occurred in the crown, with some branch samples being 80-90% girdled. No branches were 100% girdled; this may explain why no flagging occurred. All larvae recovered from the two attacked oaks had died as first or second instars, usually while boring into the sapwood where they molt. Category I1 oaks had flagging in 25-50% of the crown after one season of attack. No adult exit holes were found, indicating no past successful attack (complete development and adult emergence). Larval densities were highest in the crown samples; no larvae were found in lower trunk samples. Girdling was 100% in all branch samples with flagging; branches with no flagging were either unattacked or girdling was less than 100%. Living larvae were found in 80% of the 4.5-m samples, although symptoms suggested only crown attack. Larval galleries were present in 60% of the 2.5-m samples, but all recovered larvae had died as second instars. Category 111 oaks had 100% flagging of the crown. No adult exit holes were present, living larvae were found in all samples, and percent girdling was nearly 100% in all samples. These data suggest that tree death occurred in a single season. Category 111 oaks probably died because girdling of the trunk below 7-9 m (height above ground of first major branch) was complete, but category I1 oaks were not killed because girdling was incomplete below the first major branch. Category IV oaks had several branches with last season's foliage attached and 100% flagging of all current-year foliage. All upper crown samples had adult exit holes, 100% girdling, but no living larvae. All lower trunk samples had living larvae, 100% girdling, but no adult exit holes. These data suggest that tree death occurred over at least a 2-year period. No sample had both living larvae and exit holes; apparently, females did not oviposit on areas where past successful attack had occurred, or if oviposition did occur the larvae died. Category V oaks had been dead for 1 year from borer attack. Exit holes were found in all samples, no living larvae were present, and girdling was nearly 100% throughout the trees. Mean starch content of root wood sampled in November 1979 decreased from a relatively high level in category I oaks to a low level in category I1 oaks, and to depleted levels in category 111, IV, and V oaks (Fig. 3). Mean starch content was 19.9% for category I; the two oaks with borer attack had ratings of medium while all others had ratings of high. Mean starch content for category I1 was 6.7%; one oak had a rating of high, 4 medium, 1 low, and 4 depleted. Mean starch content was 0.5% for categories 111, IV, and V; all oaks had ratings of depleted. In July 1980, when the five unfelled oaks from each category were inspected for signs of 1980 borer attack, we found eggs and living larvae on category I1 oaks only. Although no signs of attack were found on the lower trunk of category I oaks, cfown attack may have occurred. No living A. bilineatus larvae were found under bark of category 111, IV, and V oaks; this was expected because females normally do not oviposit on dead oaks. In June 1981, the five unfelled category I

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Table 111. Mean Agrilus bilineatus adult exit hole and larval densities 2 S.E.) per rn2 of bark, and percent circumferential girdling k S.E.) by larval feeding galleries at various heights above ground for red and black oaks in 5 host condition categories (5 oakslcategory) in December 1979: Kettle Moraine State Forest, Jefferson County. Wisconsin Sampling height above ground (m) 14.5 12.5 10.5 8.5 6.5 4.5 2.5 0.5

Mean bolt diam. (cm)

Category I* Percent girdling

Living larvae

Percent girdling

Living larvae

Percent girdling

Exit holes

Living larvae

7 11 14 19 23 26 29 33

24217 18?16 94 8 6+ 6 4t 4 3~ 3 2t 2 2% 2

781'- 7 55212 541'-18 33212 31t 9 30k12 0 0

1002 0 1 0 0 0 741'-17 72k20 78k20 60k21 2 1 0

522 11 4329 84k19 123221 901'-13 1141'-27 81214 57t_ 8

9624 1001'-0 10020 10050 10020 10020 1OOLO 96k 4

312 3 138226 73k18 26212 17213 0 0 0

0 0 0 0 16% 7 40% 6 69313 43t 9

Category I1

Category 111

Category IV

Category V Percent girdling 1001'-0 1001'-0 10020 74212 9822 10020 10020 100?0

Exit holes

Percent girdling

20? 2 40?16 44414 36t14 42t 5 35t 7 64215 50t17

10020 10020 look0 9020 10020 10020 10020 10010

*Criteria used to assign oaks to calcgories were based on the external tree symptoms of borer attack in September 1979: Category 1. oaks with no symptom'%of borer attack in 1979; 11, oaks with 25-50% crown death in I979 after one season of attack; 111, oaks died in 1979 after one season of attack: IV, oaks died in 1979 after two seasons nf attack; and V, oaks died in 1978 fmm borer attack.

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HOST CONDITION CATEGORY

FIG. 3. Percent starch content (T 2 S.E.) of root wood collected in November 1979 from red and black oaks in five host condition categories that were based on external tree symptoms of Agrilus bilineatus attack in September 1979: Kettle Moraine State Forest, Jefferson County, Wisconsin. Categories were: 1 ( n = lo), oaks apparently unattacked in 1979; I1 ( n = lo), oaks with some branch death in 1979 after one season of attack; 111 ( n = 5), oaks died in 1979 after one season of attack; IV ( n = lo), oaks died in 1979 after two seasons of attack; and V ( n = lo), oaks died in 1978 from borer attack.

oaks were fully leafed out and had no symptoms or signs of previous-year (1980) borer attack. All unfelled categoly I1 oaks died in 1980; in June 1981, they had old foliage attached to the lower crown branches and adult exit holes on the lower trunk of each. Root starch content of deciduous hardwoods. measured after leaf drop, is a good indicator of a tree's current vigor because it reflects the photosynthetic activity of the tree during the past growing season, and because it responds to environmental stresses during the growing season when imposed (Wargo 1978). Reductions in root starch content have been reported following defoliation (Parker and Houston 1971: Wargo et al. 1972) and drought (Parker 1970). In the present study, reductions in root starch appeared to follow borer attack; larval feeding may have caused such reductions by girdling the conducting xylem and phloem. Xylem girdling affects photosynthate production by causing water deficits above the girdled region, and phloem girdling blocks transport of photosynthates to the roots (Kozlowski 1969). Recall that conduction in xylem occurs almost exclusively in the outermost annual ring in trees of the red oak group. The current annual rings of xylem for red and black oaks in our study measured 0.8-1.8 mm in width. We first observed wilting of the foliage of borer-infested oaks in mid-August at which time most larvae were third and fourth instars (Fig. 2). Mean lateral width in mm of the larval prothorax (x ? S.E.) was 0.19k0.01 (n = 7 ) for instar I, 0.5320.03 (n = 23) for instar 11, 0.89-tO.01 (n = 67) for instar 111, and 1.34k0.02 ( n = 36) for instar IV. These data suggest that the galleries made by first and second instar larvae were not sufficiently deep to disrupt the conducting xylem, but galleries of third and fourth instar larvae were sufficiently deep to girdle the conducting xylem and cause the wilting observed in August. The twolined chestnut borer is reported to prefere~tiallyattack low vigor oaks (Dunbar and Stephens 1976). Consequentrly, attacked oaks may have relatively low root starch content prior to borer attack is suggested by Cote (1977). Therefore, it is uncertain if the relatively low starch ratings of the attacked oaks of categories I, 11, and I11 resulted solely from borer attack or if they were low prior to attack. The oaks in the study area were lightly to moderately defoliated in May of I978 and 1979 by fall cankerworm, and this probably caused some reduction in root starch content. Category I11 oaks may have had depleted ratings prior to borer attack, and if starch content is assumed a good indicator of tree vigor, then this may explain

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why the entire trees were attacked and killed in one season. Likewise, oaks in categories I and I1 may have been only partially attacked, if at all, because of greater host vigor as indicated by higher starch ratings. The high starch ratings of the unfelled category I oaks in 1979 may explain why they were not attacked in 1980. Similarly, the relatively low starch ratings of the unfelled category I1 oaks in 1979 may explain why all were attacked and killed in 1980. Further research is needed to determine if borer-attacked oaks have relatively low starch content prior to attack, or if it is low because of larval girdling, or a combination of the two.

Pupal development and adult emergence. Mean duration in days (i+ S.E., range) for the pupal stage was 11.7-fO.2, 11-12 (n = 9) at 24" and 8.5k0.2, 8-9 (n = 10) at 30°C; means were significantly different (P < 0.01 Student's t-test). All adults began boring (if within a pupal cell) or walking (if on filter paper) on day 2 following eclosion at both temperatures. Once boring was initiated, all adults emerged from the bark on day 3 (n = 5) at 24" and on day 2 (n = 6) at 30°C. In another study (Haack and Benjamin 1981), mean time in days for prepupal larvae to pupate and emerge as adults from infested bark collected in January 1980 was 83.6 at 20", 33.9 at 24", 20.6 at 28", and 19.2 at 30°C. The theoretical developmental threshold temperature for overwintering prepupal larvae was 17.8"C; calculations were based on the 24" and 28°C rearing data. The approximate degree-day requirement for prepupal larvae to develop and emerge as adults from bark was ca. 210°Dn. Pupae were first collected in 1980 on 25 April and peak collections were on 17 May; last collections from the 1978-1979 generation were on 26 July 1979 (Fig. 2). Adults were first recovered from pupal cells on 17 May 1980, but none had begun boring an exit hole. Over half ( 5 8 % , n = 38) were adults on 30 May, but none had exited from the bark; most adults (73%, n = 22) had initiated boring and 23% had completed exit hole construction. Current-year exit holes leading to empty pupal cells were first observed on 4 June. In another study in the same forest, adults were first collected from sticky traps on 31 May 1980 (J. 0. Haanstad, pers. comm.), thereby renewing the cycle. Acknowledgments We thank Dr. Dave J. Hall and Paul E. Pingrey, Wisconsin Department of Natural Resources, for assistance in locating forest stands; John 0 . Haanstad for Agrilus bilineatus collection data; and Kevin D. Haack, Barbara J. Nakagaki, and Beth A. Schuh for assistance in the field and laboratory. References Chapman, R. N. 1915. Observations on the life history of Agrilus bilineatus. J . agric. Res. 3: 283-297. Chittenden, F. H. 1909. The two-lined chestnut borer. U.S. D e p . Agric. Bur. Ent. Circ. 24 (rev. ed.). Cote, W. A,, 111. 1977. The biology of the two-lined chestnut borer and its impact on defoliated oaks. Syracuse, NY: State Univ. of New York. Available from: University Microfilms, Ann Arbor, MI; Publication no. 77-14,553. 102 pp. Dissertation. Cote, W. A , , 111 and D. C. Allen. 1980.Boloy o he w-lndchesnutborer RILUS BILINEATUS, in Pennsylvania and New York. Ann. enl. Soc. Am. 73: 409-413. Dunbar, D. M. and G. R. Stephens. 1975. Association of twolined chestnut borer and shoestring fungus with mortality of defoliated oak in Connecticut. For. Sci. 21: 169-174. 1976.The bionomics of the twolined chestnut borer. pp. 73-83 in J. F. Anderson and H. K. Kaya (Eds.), Perspectives in Forest Entomology. Academic Press, New York. Haack, R. A. and D. M. Benjamin. 1980. Influence of time of summer felling of infested oak trees on larval development and adult emergence of the twolined chestnut borer, Agrilus bilineatus. Univ. Wis. For. Res. Note 236. 1981. Observations on the biology of Phasgonophora sulcata (Hymenoptera: Chalcididae), a larval parasitoid of the twolined chestnut borer, Agrilus bilineatus (Coleoptera: Buprestidae), in Wisconsin. Great Lakes Ent. 14: 113-1 16.

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Holdheide, W. 1951. Anatomie mitteleuropaischer Geholzrinden. pp. 193-367 in H. Freund (Ed.), Handbiich der Mikroskopie in der Technik, Vol. 5. Umsachen-Verlag, Frankfurt. Hopkins, A. D. 1904.Insect injuries to hardwood forest trees. pp. 313-328 in U.S. Dep. Agric. Yearb. Agric. 1903. Gov. Printing Office, Washington, D.C. Horn, G. H. 1891.The species of Agrilus of boreal America. Trans. Am. ent. Soc. 18: 277-336. Kozlowski, T. T. 1969. Tree physiology and forest pests. J. For. 67: 118-123. Kozlowski, T. T. and C. H. Winget. 1963. Patterns of water movement in forest trees. Bot. Gaz. (Chicago) 124: 301-31 1. Kramer, P. J. and T. T. Kozlowski. 1979. Physiology of Woody Plants. Academic Press, New York. Parker, J. 1970. Effects of defoliation and drought on root food reserves in sugar maple seedlings. U . S . Dep. Agric. For. Serv. Res. Pap. NE-169. Parker, J. and D. R. Houston. 1971. Effects of repeated defoliation on root and root collar extractives of sugar maple trees. For Sci. 17: 91-95. Wargo, P. M. 1975. Estimating starch content in roots of deciduous trees-a visual technique. U . S . Dep. Agric. For. Serv. Res. Pap.. NE-313. -1978. Judging vigor of deciduous hardwoods. U . S . Dep. Agric. Agric. Info. Bull. 418. Wargo, P. M., J. Parker, and D. R. Houston. 1972. Starch content in defoliated sugar maples. For. Sci. 18: 203-204. (Received 7 April 1981; accepted 27 November 1981)