It's a moth-eat-moth world: Groundcherry moth performs better as a frugivore than a budworm when not eaten by another moth

Background/Question/Methods

Life history feeding polymorphism is when a species utilizes multiple feeding strategies at one life stage. This may signify either 1) chronic intraspecific competition over a preferred resource or 2) variation in the success of one strategy over another under different contexts. Larvae of Symmetrischema lavernella specialize on the reproductive tissue of their host plant, Physalis spp. (groundcherry), but feed on two distinct resources differing in apparent exposure to potential parasites and predators. They either enter a large floral bud and feed within a single fruit that develops around it (frugivore), or enter a single small floral bud and consume the reproductive tissue of the developing flower (budworm). To test if these two strategies traded off risk for food quality, we compared body size, development time, predation, and parasitism between feeding strategies at several field sites at a single location.

Results/Conclusions

Larvae developed in the same amount of time under both strategies, but frugivores weighed significantly more than budworms and survived better under some circumstances. While both strategies were at risk of being parasitized by ichneumonid and braconid parasitoid wasps, budworms disappeared much more frequently due to unknown causes (unknown predators or bud drop by the host plant). Interestingly, the main cause of mortality among frugivores was predation by a larger, externally feeding specialist frugivore, the moth Heliothis subflexa.  In each of the 3+ annual generations, frugivores occurred before budworms on the same plants, indicating that it is the preferred strategy. The budworm strategy, which utilizes smaller floral buds, appears to occur when larger buds are not available, apparently as a result of intraspecific competition. Although the frugivore strategy was preferred and equally or more successful under the field conditions tested, the larger, externally feeding frugivore H. subflexa often reaches very dense populations and may favor the use of the budworm strategy at other sites or other years, thus maintaining the polymorphism even in the absence of intraspecific competition.