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Influence of Habitat on Colonization of Collard Plants by Pieris rapae

 

WILLIAM JAMES CROMARTIE, JR.

Department of Entomology, Cornell University, Ithaca, NY 14853

ABSTRACT

The imported cabbageworm, Pieris rapae (L.) fails to colonize host plants set out in woods and woodland edge habitats, while successfully occupying plots set out in adjacent meadows.

The imported cabbageworm, Pieris rapae (L.), feeds on many wild and cultivated plants of the family Cruciferae (Harcourt 1963, Shapiro 1974). There is little detailed information, however, on the patterns of behavior that lead to habitat and host selection in the field. In the course of studies on the colonization of collards (Brassica oleracea) by herbivorous insects (Cromartie 1975), I had the opportunity to follow the colonization of P. rapae on experimental plots set out in different habitats. In particular, I wanted to determine whether P. rapae would colonize woodland and .woodland edge habitats if suitable hosts were made available, or if it has a preference for open areas as suggested by Klots (1951) and Voss and Wagner (1956). Hicks and Tahvanainen (1974) demonstrated distinct habitat preferences on the part of several crucifer-feeding species of flea beetles' of the genera Phyllotreta and Psylliodes.

Materials and Methods

I conducted an experiment in the summer of 1972 using 8 plots of 10 collard plants (Brassica oleracea, c.v. Vates) each: 4 in open meadows, 2 in a forest, and 2 on the border between meadow and forest. The study site was located cat 13 km southeast of Ithaca, N.Y., near the village of Brooktondale. The first meadow was an east-facing slope bordered on all sides by tall hedgerows and forest. The forest site lay on the south edge of this meadow along a small stream, in a strip about 50 m wide, giving way to a field on the south. The 2nd meadow was also an east-facing slope about 100 m uphill from the first.

Collards were transplanted from greenhouse flats to 20-cm-diem by 10-cm-high plastic pots, thus assuring uniform soil conditions in all habitats. Plants were assigned randomly to the 8 plots. Within each plot, plants were spaced cat 0.9 m apart. Vegetation in the plots and in a surrounding strip 0.5 m wide was cut down to 0.1 m high. The plots on the wood land edge were placed so that the plants were within the trees and Liz in the meadow beyond.

Successive trials, using fresh plants each time, were run from 16 June to 11 July, 28 July to 19 Aug., and 23 Aug. to 15 Sept. At the conclusion of each run, eggs and larvae of P. rapae were collected from all plants in each plot and lumped into one sample.

Results and Discussion

Table I shows the numbers of P. rapae eggs and larvae for each of the 3 trials. Only 3 larvae, all 5th instars, were found in the woods. In the edge plots only 11 larvae occurred, 9 of them in one plot on 15 Sept. The 5th instar larvae found in the woods and edge plots may have come from eggs laid while the plants were hardening off outside the greenhouse. On 15 Sept., however, the age distribution of the larvae suggests that at least I or 2 females oviposited in edge plot 2. Eggs and larvae were numerous in the field plots. Their age distribution (especially on 15 Sept. ) suggests most oviposition occurred over a period of a few days.

These results provide quantitative evidence of P. rapae's preference for oviposition in open, sunny sites and suggest that the female is much less likely to utilize plants in the cooler, shaded woodland and woodland edge habitats. In the Ithaca area, P. rapae's principal wild hosts, Barbarea vulgaris and Lepidium virginicum, are plants of open, sunny successional habitats and roadsides (Shapiro 1974, 1975, Slansky 19746). During the period of my experiments, failure of P. rapae females to enter the woods was probably the reason no oviposition took place there. The number of eggs and larvae found on edge plot 2 on 15 Sept. indicate that once a female discovers these plants they will be attacked, but that the likelihood of discovery is less for plants growing in partial shade along the woodland boundary than in the open. Light, temperature, and availability of nectar plants may all affect female behavior, but their actual roles have not been studied. Shapiro (1974) reports that P. rapae adults are found in woods early in the spring. Sampling at that time in the Ithaca area, however, yielded no eggs nor larvae on the woodland crucifer Dentaria (B. Rathcke, pers. comm.), although Dentaria has been shown to be an extremely suitable larval foodplant in the laboratory (Slansky 1974). (Pieris virginiensis Edw., a Dentaria specialist, is now rare or absent from the Ithaca area [Shapiro 1974, F. Slansky, pers. comm.].) P. rapae apparently fails to exploit this potential host even when, in early spring, woods and fields differ relatively lass in microclimate than later, when the canopy has leafed out. The reasons why Dentaria enjoys this apparent protection from P. rapae remain to be investigated.

REFERENCES CITED

Cromartie, W. J. 1975. The effect of stand size and vegetational background on the colonization of cruciferous plants by herbivorous insects. J. Appl. Ecol. 12: 517-33.

Harcourt, D. G. 1963. Biology of cabbage caterpillars in eastern Canada. Proc. Entomol. Soc. Ontario 93: 6 1-75.

Hicks, K. L., and J. O. Tahvanainen. 1974. Niche differentiation by crucifer-feeding flea beetles (Coleoptera: Chrysomelidae). Am. Midl. Nat. 9l: 40623.

Klots, A. B. 1951. A field guide to the butterflies. Houghton Mifflin Co., New York. 349 pp.

Shapiro, A. 1974. Butterflies and skippers of New York State. Search Agric. 4: 1-59. 1975. Ecological and behavioral aspects of coexistence in six crucifer-feeding pierid butterflies in central Sierra Nevada. Am. Midl. Nat. 93: 42~33.

Voss, E. G., and W. H. Wagner. 1956. Notes on Pieris virginiensis and Erora laeta--two butterflies hitherto unreported from Michigan. Lepid. News 10: 18-24.

1-Lepidoptera: Pieridae.

2-Received for publication Mar. 3, 1975.

3- Present address: ENVL, Stockton State College, Pomona NJ 08240.

4- Coleoptera: Chrysomelidae.

5- Slansky, F. 1974. Energetic and nutritional interactions between larvae of the imported cabbage butterfly. Pieris rapae L. and cruciferous food-plants. Ph.D. thesis. Dept. of Entomology, Cornell Univ. 303 pp. 7'

Copyright 1975


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