Availability and location of cocooning sites for diapausing codling moth larvae (Cydia pomonella (L.)) (Lepidoptera: Tortricidae) on mature and young apple trees T.L. Blomefield 1 2
1,2
2 & J.H. Giliomee *
ARC Infruitec-Nietvoorbij, Private Bag X5026, Stellenbosch, 7599 South Africa Department of Botany and Zoology, University of Stellenbosch, Private Bag X1, Matieland, 7602 South Africa
Mature larvae of the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), a key pest of apples and pears, leave infested fruit and spin cocoons in which they transform to pupae. These larvae may either immediately transform to pupae from which moths emerge two to three weeks later, or they overwinter in a state of diapause (Headlee 1929; Steiner 1929; Cutright 1937; Gould & Geissler 1941; Yothers & Carlson 1941; MacLellan 1960; Geier 1963; Wearing & Skilling 1975a,b; Howell 1994; Lacey et al. 2006). Cocooning sites are usually in crevices, under loose bark or in the splintered ends of broken branches on the host tree, and sometimes in loose debris on the ground or in the soil around the tree trunk (Steiner 1929; Yothers & Carlson 1941; MacLellan 1960; Geier 1963; Wearing & Skilling 1975a. Geier (1964) studied the population dynamics of codling moth in Australia and concluded that under optimal weather conditions and negligible parasitism, cocoon shelter, together with larval food, are the main factors limiting the abundance of the insect. Since the availability of larval food only becomes restrictive at very high infestation levels, and the mortality of larvae in cocoons spun on the ground is extremely high (Wearing & Skilling 1975a), the number of cocooning sites on the tree becomes the most important factor determining how many individuals will survive the winter and develop into the spring flight of the new season. On young trees with fewer cocooning sites, competition for these sites may be a major limitation to codling moth increase (Wearing & Skilling 1975b). This study was undertaken to establish the relative abundance and location of available cocooning sites on young and mature Golden Delicious and Granny Smith apple trees in the Western Cape of South Africa. The study was undertaken in August 1991, prior to spring moth emergence, in two apple orchards on the Overberg Research Farm, 1 km north of Grabouw (34°09’S 19°02’E). This farm consists of two sections, Elgin and Bellevue. The orchards on *To whom correspondence should be addressed. E-mail:
[email protected]
these two sections are separated from one another by a railway line and thickets of eucalypts and pine trees. No insecticides were applied to either orchard for seven years, and codling moth infestation was therefore high at both sites. The orchard on the Elgin section consisted of mature, 27-year-old trees, 0.7 ha in size and consisted of 200 trees with a 6.1 × 6.1 m spacing. There were 180 Golden Delicious (GD) and 20 Granny Smith (GS) trees in a 9:1 planting. The trees were 3 to 4 m high. The orchard on the Bellevue section was seven years old, 0.9 ha in size, and consisted of equal rows of GD, GS and Top Red cultivars. The planting distance was 3 × 5 m and the trees were 2 to 3 m high. The trees in both orchards had been pruned to a vase-shape with no central leader. Several primary scaffold branches angled outward from the top of the trunk. The drip area under the trees was treated with a herbicide and was relatively free of plant material, while the ground cover in the work row was a mixture of grasses and herbaceous weeds. Long-term rainfall for the Elgin area was obtained from Agomet, ARC InfruitecNietvoorbij. In the mature orchard (Elgin), 10 GD and 9 GS trees were randomly selected and inspected for cocoons, while in the young orchard (Bellevue) 20 randomly selected trees of each cultivar were similarly inspected. From the soil surface to the top of the tree all loose bark, crevices, torn branches, pruning wounds and woolly apple aphid damage sites were thoroughly searched for live larvae. The position of each overwintering site (on the trunk, primary scaffold branches, or side branches), and distance (cm) from the soil surface was recorded. The mean number of larvae found on GD and GS trees were compared using pairwise t-tests at the 5 % level of significance. To monitor the possible emergence of adults of the spring generation from the soil, 10 ground traps were placed at random, one trap per tree and 30 cm from the trunk, under the trees in the Bellevue orchard. Each trap was made from galvanized sheeting in the form of a box, 55 cm square and 10 cm high. A 38 cm square African Entomology 20(1): 182–186 (2012)
Short communications
183
Table 1. Distribution of diapausing Cydia pomonella larvae on different parts of mature and young Golden Delicious and Granny Smith trees. Cultivar
Mean number of larvae/tree (± S.E.)
Percentage of live larvae found on: Trunk
Primary branches
Side branches
Mature trees Golden Delicious Granny Smith
13.9 ± 1.91 5.7 ± 0.94
15.1 13.7
24.5 41.2
60.4 45.1
Young trees Golden Delicious Granny Smith
0.5 ± 0.20 2.0 ± 0.61
30.0 23.0
50.0 74.4
20.0 2.6
hole was cut out of the roof of the trap. The opening was covered with fly-screen gauze to allow rain through to the soil beneath the trap. Without disturbing the soil, the vegetation under the trap was cut to ground level and a thin layer of white building sand spread over the soil surface. A sticky bottom of a pheromone trap, on which a codling moth pheromone lure had been placed, was placed in each trap to facilitate the inspection of the boxes for codling moth adults. On the mature, 27-year-old GD and GS trees a mean of 13.9 and 5.7 live larvae per tree were recorded respectively (Table 1). The maximum number of larvae found on a GD and GS tree was, respectively, 24 and 11. On GD trees most of the larvae (60.4 %) overwintered on the side branches, while on the GS trees the percentage of larvae on the primary (41.2 %) and side (45.1 %) branches was similar. On both cultivars the majority of the larvae were located in old decaying pruning cuts, or in pruning cuts that did not have a smooth surface or were not properly sealed (Table 2). The second most important overwintering site on these mature trees was under loose bark. Most of
the cocoon sites contained single larvae, although some of the sites contained up to seven larvae (Fig. 1). These latter sites were large pruning cuts or branches that had cracked or splintered. Although most of the larvae on GD (74.1 %) and GS (68.6 %) were found in cocooning sites within 1.4 m of the soil surface, some larvae were located above 2 m, particularly on GD trees (Fig. 2). On the smaller seven-year-old trees the mean number of larvae per GD and GS tree was 0.5 and 2.0, respectively (Table 1). The maximum number of larvae found on a GD and GS tree was respectively three and 10. The primary branches accounted for 50 % of the overwintering larvae on the GD trees but 74.4 % of the larvae on GS trees. On GD trees most larvae (90 %) were located under loose bark and in pruning wounds, while on GS trees most larvae (82.1 %) overwintered in broken branches and under loose bark. On two occasions eight and nine larvae were found on GS trees in sites where branches had broken under the weight of the fruit. If these last two groups of larvae were excluded, the mean number of larvae per GS tree decreased from 2.0 to 1.1. As in the case
Table 2. Percentage of diapausing Cydia pomonella larvae in different overwintering sites on unsprayed mature and young Golden Delicious and Granny Smith trees in spring. Cultivar
Percentage of live larvae found in: Cracks
Under loose bark
Pruning cuts
Woolly apple aphid damage
Torn/broken branches
Mature trees Golden Delicious Granny Smith
5.8 9.8
13.7 11.8
69.8 78.4
2.1 0
8.6 0
Young trees Golden Delicious Granny Smith
0 0
50 38.5
40 17.9
10 0
0 43.6
184
African Entomology Vol. 20, No. 1, 2012
Fig. 1. Percentage of larval overwintering sites with one or more larvae on mature (A) and young (B) Golden Delicious (GD) and Granny Smith (GS) trees.
of the mature trees, the majority of larvae were found in cocooning sites within 1.4 m from the soil surface (Fig. 2). In one GS tree a group of nine larvae were located in a broken branch 2.6 to 2.8 m above the soil surface. All the overwintering sites on GD trees contained single larvae, while on GS trees 41 % of the cocooning sites contained single larvae. No moths were collected from the emergence boxes placed under the trees in the Bellevue orchard. In view of the heavy codling moth infestation in these unsprayed orchards with a fruit infestation of 90 to 100 %, the relatively low number of larvae found diapausing on the trees was probably a function of the number of sites available for cocooning, rather than the number of larvae emerging from infested fruit. One can assume that the high competition for cocooning sites resulting from the
very high populations would have meant that most or all available sites would have been occupied. Competition would have been particularly severe on young trees with fewer cocooning sites. In a trial where codling moth larvae were released on trees, Wearing & Skilling (1975b) concluded that the upper limit for the number of cocoons per tree on 8 to 10-year-old-trees, 2.5 to 3.6 m in height and with a trunk circumference of 17 to 29 cm, was a mean of five cocoons. This was reached when a density of 12 larvae were released per tree. This is substantially higher than that obtained for the seven-year-old trees in this study. However, in mature commercial orchards, where codling moth is under an effective integrated control programme, it is unlikely that competition for cocooning sites will limit the overwintering population. Geier (1964) found that the mean number of over-
Short communications
185
Fig. 2. Vertical distribution of overwintering Cydia pomonella larvae on mature (A) and young (B) Golden Delicious (GD) and Granny Smith (GS) trees.
wintering larvae per tree varied between 0.80 and 2.50 where trees received various insecticide treatments. He concluded that repeated applications of an effective insecticide would reduce the densities of winter populations to below the level where cocooning sites becomes a limiting factor. In this study significantly more diapausing larvae were found on mature GD than GS trees (t18 = 3.8520, P = 0.0012). The reason for this is unknown, but we speculate that it is at least partly due to the wood of the GD tree being softer than that of the GS tree, which we observed when digging into old pruning wounds and where branches had broken. Many of the older pruning wounds on the GD trees appeared to be in a more
advanced stage of decay than those on GS trees, thus possibly providing more cocooning sites. Furthermore, it appeared to us that the GS trees had less loose bark and greater areas of smooth bark, thus resulting in fewer overwintering sites. In the young orchard significantly more larvae were recorded on GS than GD trees (t38 = 2.2587, P = 0.0297). However, if the two torn GS branches previously mentioned, which yielded 8 and 9 larvae each, are excluded there was no significant difference (t36 = 1.7319, P = 0.0919). The absence of moths in the emergence boxes placed under the trees was not unexpected, as the drip area under the trees was relatively clean of vegetation, rocks and prunings. The soil surface
186
African Entomology Vol. 20, No. 1, 2012
was also hard and had a high content of small stones. Studies undertaken on the movement of mature fifth instar larvae after being released under apple trees, and the number of larvae found overwintering on the tree, in the soil, leaf litter and other debris on the soil surface, have indicated that the tree is the preferred overwintering site (Headlee 1929, Steiner 1929; Cutright 1937; Gould & Geissler 1941; MacLellan 1960; Wearing & Skilling 1975b). Furthermore, predation of cocoons on the ground could be as high as 100 % during the winter months (Jaynes & Marucci 1947; MacLellan 1960; Wearing & Skilling 1975a). Wearing and Skilling (1975a) also observed that cocoons in the soil were thin-walled compared to those spun on the tree. This could increase mortality during the winter months of the Western Cape when 57.1 % of the long-term average rainfall of 1041.66 mm falls between April and August. During this period the soils have a high moisture content and are often waterlogged. The high proportion of diapausing larvae found in pruning wounds indicates that pruning wounds are important overwintering sites. This emphasizes the importance of the proper training and pruning of apple trees from the time of planting. Large-scale corrective pruning at a later stage increases the number of large pruning wounds
and thus overwintering sites. Pruning in such a way that the pruning cut is very smooth, and immediately sealing the wound, will limit the availability of cocooning sites and therefore infestation. The wood of a well-cut pruning wound, particularly on a GD tree, will be less prone to decomposition and the sealant will prevent or delay the onset of decay. Once a cocooning site has been established in a pruning wound, the site can be exploited by successive generations of moths. It is possible to reduce the overwintering population in the soil by ensuring the drip area under the trees is clear of any ground cover and debris and the ground cover in the work row is properly maintained. Cultural practices that reduce rough bark areas and pruning, such as smaller trees in higher density plantings, will contribute to more effective codling moth management by reducing the availability of cocooning sites. Furthermore, smaller trees will facilitate more effective orchard sanitation, mating disruption and the use of biopesticides and entomopathogenic nematodes for the control of codling moth. ACKNOWLEDGEMENTS The authors would like to thank P. Kleinhans, M. Knipe and N. Du Plessis for technical assistance.
REFERENCES CUTRIGHT, C.R. 1937. Codling moth biology and control investigations. Ohio Agricultural Experiment Station Bulletin 583: 1–45. HEADLEE, T.J. 1929. An operation in practical control of codling moth in a heavily infested district. Journal of Economic Entomology 22: 89–97. HOWELL, J.F. 1994. Understanding sources of codling moth infestation. Good Fruit Grower 5: 30–32. GEIER, P.W. 1963. The life history of codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), in the Australian Capital Territory. Australian Journal of Zoology 11: 323–367. GEIER, P.W. 1964. Population dynamics of codling moth, Cydia pomonella (L.) (Tortricidae) in the Australian Capital Territory. Australian Journal of Zoology 12: 381–416. GOULD, W. & GEISSLER, G. 1941. Hibernating codling moth larvae. Journal of Economic Entomology 34: 445–450. JAYNES, H.A. & MARUCCI, P.E. 1947. Effect of artificial control practices on the parasites and predators of the codling moth. Journal of Economic Entomology 40: 9–25. LACEY, L.A., ARTHURS, S.P., UNRAH, T.R., HEADRICK, H. & FRITTS, R. 2006. Entomopathogenic nematodes
for the control of codling moth (Lepidoptera: Tortricidae) in apple and pear orchards: effect of nematode species and seasonal temperatures, adjuvants, application equipment, and post-application irrigation. Biological Control 37: 214–223. MACLELLAN, C.R. 1960. Cocooning behaviour of overwintering codling moth larvae. Canadian Entomologist 92: 469–479. STEINER, L.F. 1929. Miscellaneous codling moth studies. Journal of Economic Entomology 22: 648–654. WEARING, C.H. & SKILLING, L. 1975a. Integrated control of apple pests in New Zealand 4. Survival of fifth-instar larvae of codling moth on the ground. New Zealand Journal of Zoology 2: 245–255. WEARING, C.H. & SKILLING, L. 1975b. Integrated control of apple pests in New Zealand 5. Effect of larval density on the cocooning behaviour of fifthinstar codling moth larvae on young trees. New Zealand Journal of Zoology 2: 257–263. YOTHERS, M.A. & CARLSON, F.W. 1941. Orchard observations of the emergence of codling moths from two-year-old larvae. Journal of Economic Entomology 34: 109–110. Accepted 27 December 2011
