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. 2000 Feb 15;97(4):1634-9.
doi: 10.1073/pnas.030532797.

Chemical Defense Against Predation in an Insect Egg

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Free PMC article

Chemical Defense Against Predation in an Insect Egg

T Eisner et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

The larva of the green lacewing (Ceraeochrysa cubana) (Neuroptera, Chrysopidae) is a natural predator of eggs of Utetheisa ornatrix (Lepidoptera, Arctiidae), a moth that sequesters pyrrolizidine alkaloids from its larval foodplant (Fabaceae, Crotalaria spp.). Utetheisa eggs are ordinarily endowed with the alkaloid. Alkaloid-free Utetheisa eggs, produced experimentally, are pierced by the larva with its sharp tubular jaws and sucked out. Alkaloid-laden eggs, in contrast, are rejected. When attacking an Utetheisa egg cluster (numbering on average 20 eggs), the larva subjects it to an inspection process. It prods and/or pierces a small number of eggs (on average two to three) and, if these contain alkaloid, it passes "negative judgement" on the remainder of the cluster and turns away. Such generalization on the part of the larva makes sense, because the eggs within clusters differ little in alkaloid content. There is, however, considerable between-cluster variation in egg alkaloid content, so clusters in nature can be expected to range widely in palatability. To check each cluster for acceptability must therefore be adaptive for the larva, just as it must be adaptive for Utetheisa to lay its eggs in large clusters and to apportion alkaloid evenly among eggs of a cluster.

Figures

Figure 1
Figure 1
(A) C. cubana (adult). (B) Same (larva), feeding on a natural Utetheisa egg cluster (five eggs have already been partly or wholly sucked out). (C) Cluster of (+) eggs exposed days beforehand to a C. cubana larva in a choice test (nine eggs have gone on to develop normally; the 10th died as a consequence of being pierced when inspected by the larva). Bars = (A) 2 mm: (B) 1 mm; (C) 0.5 mm.
Figure 2
Figure 2
Stereo pair of photos, depicting the front end of a C. cubana larva (the arrow denotes one of the jaws) (×54).
Figure 3
Figure 3
Fate of eggs in (−) and (+) Utetheisa egg clusters (10 eggs per cluster) offered as a choice in laboratory tests with C. cubana larvae (n = 10 tests). Column on right gives normal hatching incidence of (+) eggs (n = 10 clusters of 20 eggs each). Error bars = SEM.
Figure 4
Figure 4
Fate of (+) and (−) Utetheisa egg clusters (n = 26 clusters per category) staked out on C. mucronata plants in the field.
Figure 5
Figure 5
Fate of eggs of natural Utetheisa egg clusters offered to C. cubana larvae in the laboratory, plotted as a function of cluster size (n = 24 clusters). Numbers in parentheses give numbers of clusters per size grouping. Error bars = SEM.
Figure 6
Figure 6
Size frequency distribution of Utetheisa egg clusters in nature (n = 127 clusters).
Figure 7
Figure 7
(Left) Alkaloid (usaramine) content of eggs from 15 natural Utetheisa egg clusters. Two numbers are given for each cluster: mean + SE, derived from analyses of five individual eggs (solid column), and averages, calculated from analysis of a five-egg sample (striped column). (Right) Alkaloid (monocrotaline) concentration in Utetheisa egg, egg interior, and egg shell (n = five for each sample).
Figure 8
Figure 8
Fate of (−) Utetheisa eggs (in clusters of 10 eggs) treated by topical addition of either monocrotaline free base (in methanol solution) or monocrotaline N-oxide (in methanol/dichloromethane solution) presented together with their respective solvent-treated control clusters to C. cubana larvae (n = six for each test).

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