Pear rust mite

Experimental & Applied Acarology. 17 (1993) 215-224

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Pear rust mite, Epitrimeruspyri (Acari: Eriophyidae) oviposition and nymphal development on Pyrus and non-Pyrus hosts J.C. Bergha* arid C,R. Weiss b aAgriculnlre Canada Research Station, Summerland, B.C. Canada, VOH I ZO b P.O. Box 328, Kaleden, B.C. VOH IKO (Accepted 8 January' 1993) ABSTRACT J.C. Bergh and C,R. Weiss. Pear rust mite, Epitrimerus pyri (Acari: Eriophyidae) oviposition and nymphal development on Pyrus and non-Pyrus hosts. Exp. Appl. Acarol., 17: 215-224. The ovi~positionalresponse of deutogynes of the pear rust mite, Epitriments pyri Nalepa, and the rate of development of E. pyri nymphs differed according to the host plant to which mites were expo~d. Among the fourteen Pyrus hosts examined, leaves from the Clapp's Favorite cultivar of P. communis elicited the strongest egg-laying response and the fastest rate of nymphal development, whereas egg-laying was least and nymphal development slowest on leaves from P. caller3'ana. Among the three non-Pyrus hosts, E. pyri oviposited and completed nymphal development on apple and quince leaves, but would not deposit eggs on apricot. Antixenosis appears to be the primary mechansim of resistance, although antibiosis may be operating to a lesser extent. These results are discussed in relation to the resistance of Pyn~s to other arthropod pests. INTRODUCTION

The host range of most species of eriophyid mites is narrow, often limited to members of a single botanical genus or family (Jeppson et al., 1975). Within the range of plants upon which a given eriophyid species can develop, hosts can vary in their suitablility for development and growth of mite populations. Host suitability/susceptibility to eriophyid mites has been inferred from field observations of whether mites were present or absent (Easterbrook, 1978, 1979), and from estimates of the relative severity of infestation (Yothers and Mason, 1930; Albrigo et al., 1987). Other studies used more quantitative assessments of suitability by comparing the numbers of mites counted on foliage samples from different hosts (Das and Sengupta, 1958, 1962: Herbert, 197,-l), or by inoculating host plants with mites under controlled conditions and then rating suitability based on plant damage or on the relative numbers of mites found some days or weeks later (Connin, 1956: Rice and Strong, 1962: Gibson, 1974; Harvey and Seifers, 1991; Harvey and Martin, 1992). *To whom correspondence should be addressed. Present address: Department of Biology, Nova Scotia Agricultural College, P.O. Box 550. Truro. N.S. Canada V0H 120. © Science and Technology Letters. All rights reserved.

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Some studies (e.g. Connin, 1956; Rice and Strong, 1962; Harvey and Martin, 1992) revealed pronounced differences in the suitability/susceptibility of the host plants tested, but did not address the question of why differences occurred. Such differences can be important in the breeding of crop plants resistant to eriophyid mites, as has been shown for the resistance of some Ribes species to the black currant gall mite, Cecidophyopsis ribis (Westwood) (Pavlova, 1963, 1964, 1969; Anderson, 1971; Knight et al., 1974). Recently, Herr ( 1991) investigated the mechanism of resistance in Ribes to C. ribis and concluded that both antixenosis and antibiosis were operating. The pear rust mite, Epitrimerus pyri Nalepa, is a pest of pear, Pyrus communis Linnaeus, (Keifer, 1952) and has been reported from many of the world's peargrowing regions. In England, Easterbrook (1978) examined several cultivars of P. communis and several Pyrus species, and found pear rust mites on all cultivars, and on some, but not all pear species. Easterbrook (1978) also found E. pyri on quince but not on apple, and reported that adult mites transferred to apple or hawthorn seedlings did not survive. Resistance of Pyrus to several insect and mite pests has been reported (review: Layne and Quamme, 1975), with particular emphasis on pear psylla, Cacopsylla pyricola Foerster (Westigard et al., 1970; Harris, 1973, 1975; Chang and Philogene, 1976; Quamme, 1984; Butt et al., 1988, 1989). This paper reports experiments designed to identify possible sources of resistance to the pear rust mite by examining E. pyri oviposition and nymphal development on a range of hosts. MATERIALS AND METHODS

Mites In October and November, 1991, 1-yr-old branches of Bartlett pear harbouring overwintering pear rust mite deutogynes were collected from an orchard in the Okanagan Valley, British Columbia, and stored in the dark at I °C and 85-95% r.h. Branches with mites were held under these conditions for seven to nine months before being used in experiments. Leaves and confinement of mites Young leaves from current year branches were used exclusively, although the youngest leaves were not used because they were previously found to deteriorate sooner than older leaves. To accomodate the cells in which mites were confined, only cultivars and species that produced leaves with a flat blade could be used. All experiments were initiated within 24 h of collecting leaves from trees. Tile cells used to confine mites within a small area of leaf surface consisted of 4 mm lengths of glass tubing (9 mm inside diameter, 11 mm outside diameter). To prevent mites from escaping, the inner surface of each cell was coated with anhydrous lanolin. A cell was attached to the upper leaf surface using Elmer's School

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glue (Borden Company, Willowdale, Ontario), and centered over the midvein near the proximal end of the leaf. To prevent water from seeping along the midvein and dissolving the glue, a line of anhydrous lanolin was applied across the midvein at both ends of the leaf. Leaves with cells were placed on square pieces of cotton in Petri dishes containing distilled water. To facilitate the location of E. pyri eggs and nymphs, trichomes on the section of midvein contained within the cell were removed with tweezers.

Deutogyne oviposition in relation to host plant A single cell was glued to each of 10 leaves of cultivars of Pyrus communis (Aurora, Bartlett, Bosc, Clapp's Favorite, Flemish Beauty, Magness, Old Home, Seckel), Pyrus species (betulaefolia Bunge, calleo'ana Decaisne, ovoidea Rehder, pyrifolia (Burm.) Nakai cv. 20th Century, usseriensis Maximowicz), a Pyn~s hybrid (Ure, communis x usseriensis), quince A (Cydonia oblonga Mill.), apple (Malus pumila Mill.), and apricot (Prunus armeniaca L. cv. Tilton). Apple leaves were from seedlings propagated in a greenhouse; all others were from orchards on the Agriculture Canada Station at Summerland. To collect mites for this experiment, sections of branches with 2-3 buds were removed from cold storage and the top of each branch section was wrapped tightly in Parafilm (American Can Company, Greenwich, CT). The branches were oriented vertically by impaling them on nails protruding through a wood block, and held at 20"C and 13:11 L:D. After 24 h, the branch sections were examined under a dissecting microscope, and mites found walking on the Parafilm at the top of each were collected using a single-hair tool and placed ir~dividually in cells. Leaves with cells were held in an environmental chamber at 15 + I°C, with a 13 h photophase. Light intensity during the photophase was 8,500 lux. The eggs deposited by each female were counted at 3-d intervals for 12 d using a Wild M8 dissecting microscope (Wild Heerbrugg, Ltd., Switzerland) at about 50x magnification. Eggs were destroyed by pricking them with a pin as they were counted. Occasionally, a nymph would be found in the cell. Based on data showing the relationship between the rate of pear rust mite nymphal development and temperature (Bergh, unpublished), these nymphs were assumed to have eclosed from eggs deposited tw O intervals previously, and the data were corrected accordingly. The mean number of eggs deposited per 3-day interval was calculated for each host. Mites that were missing or dead after the first 3 days were replaced, but none were replaced thereafter. The mean total number of eo,~s laid by mites that were alive on day 12 were compared among hosts using PROC GLM and the Student-Newman-Keuls multiple range test (SAS Institute, 1985). All statistical comparisons were at the 5% significance level. •Vymphal development in relation to host plant Selection of the host plants used in this experiment was based on the results of the

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oviposition experiment. Several hosts on which egg-laying was lowest, intermediate, and highest were chosen. To collect mites for this experiment, branches were removed from cold storage and buds were examined under a dissecting microscope. Buds with many overwintering deutogynes were removed and placed on young Flemish Beauty leaves on cotton in Petri dishes with distilled water. After 6 days, 3-5 mites were placed in a cell on each of 10 leaves per host plant. Leaves were examined at 24-h intervals until 2 or more eggs had been laid. The deutogynes and all but two of the deposited eggs were destroyed and the location of the two eggs remaining on each leaf was mapped. Leaves were held at 15 + 1° C and 13:11 L:D and the stage of development was recorded at 24-h intervals: When one of the two eggs hatched during a 24-h interval, the nymph was located and the remaining egg was destroyed. When both nymphs eclosed during the same interval, one nymph was destroyed. On some leaves from some hosts, females would not oviposit or the nymph would die on the leaf surface or would leave the leaf surface and become trapped in the lanolin. Because we intended to follow the complete development of I 0 mites per host, additional leaves were prepared as required. The mean number of days from eclosion to adult was compared among host plants using the Kruskal-Wallis test with tied ranks and Dunn's multiple comparisons test with tied ranks (Zar, 1984). RESULTS

Deutogyne oviposition in relation to host plant The mean number of eggs deposited on leaves of each host plant during each 3day interval is shown in Fig. 1. Host plants are arranged in ascending order of mean total number of eggs laid after 12 days. Mites showed a pre-oviposition period, and no eggs were laid on most leaves during the first 3-day interval. On day 6. eggs had been deposited on some or all leaves of all hosts except apricot, and the number of eggs laid during the second interval varied widely among hosts. The number of eggs deposited during the third interval increased on all but two hosts (quince and apricot), representing in many cases, a two- to four-fold increase in egg numbers relative to the second interval. During the final 3-day interval, the number of eggs laid on hosts that had previously received the most eggs tended to decline or remain nearly the same, whereas the number of eggs deposited on hosts that had received relatively few e~,,,s~,,,previously tended to continue to increase. The number of mites that remained alive and/or present on leaves over the 12day experiment tended to be greater on hosts on which the average number of eggs deposited during each 3-day interval was greater than on those those that received the fewest eggs (Fig. 1). The mean total number of eggs laid by mites that were alive and present on day 12 differed significantly among hosts (F0.05,14.94 = 9.05; P < 0.001 ) (Fig.2). Signif-

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Fig. 1. M e a n n u m b e r o f e g g s deposited per three-day interval by E. py,ri d e u t o g y n e s on leaves o f Pyrus a n d non-Pyrus hosts. N u m b e r in bars indicates n u m b e r o f mites alive at each interval. R a n g e o f S E M for each interval: D a y 3 = 0.11 - 0.37, D a y 6 = 0.15 - 1.32, D a y 9 = 0.45 - 1.44, D a y 12 = 0 . 6 4 - 1.67. 35• 30.

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Fig. 2. M e a n total n u m b e r o f e g g s deposited after twelve d a y s by E. pyri d e u t o g y n e s o n leaves o f Pyrus a n d non-Pyrus hosts. Vertical lines indicate standard error o f m e a n . H o s t s with the s a m e letter a b o v e b a r are not different at the 5% significance level. O n l y o n e mite r e m a i n e d alive and present on P. calleryana leaves alter 12 days, and no e g g s were laid on apricot leaves; those hosts were not included in the analysis.

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B E R G b l A N D C. R. "WEISS

icantly more eggs were deposited on Clapp's Favorite leaves than on leaves of any other host. Compared to Clapp's Favorite and to hosts that received the fewest eggs, an intermediate number of eggs were laid on many hosts, and no significant differences resulted from pair-wise comparisons between them. Significantly fewer eggs were deposited on apple leaves than on host leaves on which an intermediate number of eggs were laid, and there was not a significant difference between apple and P. ovoidea. Only one mite remained alive and/or present on P. calleryana over 12 d and therefore this species could not be included in this analysis. No eggs were laid on apricot during the experiment.

Nymphal development in relation to host plant The mean number of days from ectosion to adult differed significantly among host plants (X20.0s.9= 16.92, P