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1. Choice of host size may play a critical role in parasitoid success, a task that takes on added complications when dealing with concealed hosts, but most such studies of insect behaviour have only taken place in the laboratory. 2. This study investigates the success of a wasp (Alabagrus texanus: Braconidae) in finding host caterpillars Herpetogramma theseusalis (Crambidae) of the most effectively handled size hidden in shelters, in both the field and the laboratory. 3. First, the study tested wasp preference and success in parasitizing large, middle‐sized and small caterpillars (> 5, 3–5, 5 mm) caterpillars. Naive wasps attacked large caterpillars more often than did experienced wasps. Wasps responded to increasing caterpillar size by increasing the number of legs used to pin their prey rather than by increasing handling time. 4. The frequencies of visits to shelters in the field containing a majority of either large or middle‐sized caterpillars were then compared, followed by a test providing the wasps with similar choices under controlled laboratory conditions. Wasps most frequently visited shelters containing a majority of middle‐sized caterpillars both in the field and under controlled laboratory conditions. 5. The combined results confirmed that the wasps can size‐select their hosts both in the field and in laboratory tests.

Young unable to move freely depend on their parents to place them in sites with superior resources or protection from unfavorable factors. The preference‐performance hypothesis holds that females should oviposit on sites that maximize the performance of their offspring. Shelters may provide their inhabitants with superior food resources, protection from enemies, and ameliorated physical factors. Accordingly, one might predict that females would prefer shelters to alternatives, since they might provide enemy‐free space. However, Herpetogramma theseusalis (Walker) (Lepidoptera: Crambidae) fern moths oviposited both in shelters constructed on sensitive ferns (Onoclea sensibilis L., Dryopteridaceae) and on exposed fern pinnae, perhaps as a bet‐hedging tactic. The moths laid eggs on the outer parts of the shelters, but after hatching the young caterpillars moved toward the center of the shelters, often beyond the reach of their common larval parasitoid, Alabagrus texanus (Cresson) (Hymenoptera: Braconidae). Although caterpillars in both areas grew at the same rate, those in shelters were parasitized less frequently than those on exposed pinnae. However, shelters senesced from the center to the periphery, depriving the caterpillars of progressively more of their food source and eventually forcing them into the open, potentially increasing their vulnerability. But numbers of wasps decreased simultaneously; hence, most caterpillars were not exposed until danger from the parasitoids had greatly diminished. Thus, offspring of females ovipositing in shelters experienced superior success, consistent with the preference‐performance hypothesis, but through lowered mortality via use of enemy‐free space, rather than superior feeding conditions.

Most attention to size-time trade-offs of insects has focused on herbivore risk, with considerably less attention paid to parasitoids. Here, we focus on parasitoid risk, comparing the fates of unparasitised herbivore hosts and parasitised hosts that protect the parasitoids. Success of a koinobiont parasitoid (host grows after parasitisation) depends on maintaining a delicate balance with its host, thereby ensuring its own survival while the host grows. To evaluate growth rate–mortality rate relationships of host and parasitoid, we compared several aspects of the growth, phenology, and behaviour of unparasitised fern moth [Herpetogramma theseusalis (Walker) (Lepidoptera: Crambidae)] larvae and larvae parasitised by Alabagrus texanus (Cresson) (Hymenoptera: Braconidae), a solitary koinobiont (one parasitoid per host) wasp. Host larvae feed and construct shelters on sensitive fern, Onoclea sensibilis L. (Dryopteridaceae). Alabagrus texanus parasitise early-instar moths in late summer, which overwinter in their host, emerging in mid-summer to pupate and eclose. During the autumn following hatching and the immediately following spring, parasitised and unparasitised moth larvae did not differ in size, took similar time to choose between satisfactory and unsatisfactory foods, and built similar shelters. Prior to any other changes noted, more parasitised than unparasitised larvae also died when severely starved. Parasitised larvae subsequently grew less and pupated later than unparasitised ones (small size, slow growth), but consumed similar amounts of food. Although the numerically dominant parasitoid of fern moths, we concluded that A. texanus do not efficiently exploit their hosts.

The choice of food plants often assumes critical importance for a herbivore. Although many studies have investigated host-plant choice behavior, few have examined preferences (vs. growth and survival) at multiple stages of the life cycle, notwithstanding the importance of identifying the critical stage(s) in an animal’s life history. Fern moths Herpetogramma theseusalis (Lepidoptera: Crambidae) provide an excellent opportunity to test host-plant choice at several stages. Fern moth larvae feed on distantly related ferns, sensitive Onoclea sensibilis and marsh fern Thelypteris palustris, and adults oviposit on both species. We examined newly hatched larvae, overwintered larvae and ovipositing females to test hypotheses predicting when host-plant choice takes place (overwintering and mobility hypotheses: overwintering stage determines choice of substrate vs. most mobile stage chooses) and the basis for choice (optimal oviposition and enemy-free space hypotheses: resource producing highest fecundity vs. lowest losses to enemies). We also evaluated the hypothesis that host-associated fitness trade-offs explain host specialization. Only ovipositing females, the most mobile stage, exhibited a clear preference (for marsh fern), consistent with the mobility hypothesis. However, their preference for marsh fern fits neither the optimal oviposition hypothesis nor the enemy-free space hypothesis; although some larvae initially grew faster on marsh fern, adults reared from the two ferns did not differ significantly in mass and experienced marginally lower parasitism on sensitive fern. Thus, we found no host-associated fitness trade-offs. Overwintering losses in marsh fern plots exceeded those in sensitive fern, and mixed plots supported the most overwintered larvae. Preference for marsh fern suggests that early success drives host-plant choice, an advantage that later disappears. Temporal variability may prevent closer fits to the hypotheses, because both ferns provide the moths with acceptable resources throughout their life cycles.

1. Variables affecting species at the ends of trophic chains may modify the success of members with which they do not directly interact. The majority of such examples involve three trophic levels, but hyperparasitoids provide an excellent opportunity to examine four-level relationships. 2. The gregarious hyperparasitoid Aprostocetus sp. (Hymenoptera: Eulophidae) commonly attacks the primary parasitoid Alabagrus texanus (Hymenoptera: Braconidae), by far the commonest parasitoid of the moth Herpetogramma theseusalis (Lepidoptera: Crambidae). 3. Larvae of this moth feed on ferns of two families, sensitive fern Onoclea sensibilis (Dryopteridaceae) and marsh fern Thelypteris palustris (Thelypteridaceae), in the study area, an old field in Maine, U.S.A. 4. I test the hypotheses that the ferns indirectly affect the reproductive success of the hyperparasitoids and that the ferns produce similar effects at intermediate links. 5. The moths experienced similar success on the two ferns, and the primary parasitoid performed similarly on moths reared from both ferns. The hyperparasitoid parasitized similar proportions of the primary parasitoid from moths that fed on sensitive fern and marsh fern. 6. However, hyperparasitoid broods on primary parasitoids from moths feeding on marsh fern contained approximately one-third more offspring, whose individuals were significantly larger than those from sensitive fern, even though their hosts’ sizes did not differ significantly. 7. An indirect effect, related to the primary producers, thus strongly affected Trophic Level 4 in the absence of a significant effect at intermediate levels. To the best of my knowledge, this relationship has not been previously reported in a multi-year or field-based study of a natural system.