Polyphagous caterpillars of Spodoptera litura switch from a trap crop to the main crop, improve fitness, and shorten generation time

Trap crops are used for pulling the pest load from the main crops toward themselves. Here, we report a switching by a polyphagous pest, Spodoptera litura, from trap crop castor to main crop eggplant during its mid-larval development. In the eggplant-castor mixed field, adults oviposit exclusively on castor. However, 3rd and 4th instar castor-grown larvae migrate to eggplants. Host choice assays for adults and larvae conducted in the controlled environment and the nutritional indices of castor and eggplant-feeding larvae revealed that the natural enemies did not influence the host switch. The host switch was rather associated with larvae's enhanced digestion and metabolism. It shortened the development time and increased the eclosion success on eggplant. Mother-selected castor is suitable for the early instars and eggplant is for the later instars, indicating that the host switching was not because of the mother's erroneous host choice but has evolved as an advantageous behavior. S. litura's ability to switch hosts in the same ontogenic phase and reduce the generation time can contribute to an additional generation within a cropping season and endanger the critical growth stage of fruiting. Our findings support the 'mother knows the best' concept and annex that the larvae can also possess the ability to know the best. Such ability in different ontogenic stages can be fundamental to the exceptional adaptability of species like S. litura.

Automated summary

Trap crops are used to divert pest pressure away from main crops. In this study, we found that polyphagous pests, like Spodoptera litura, can switch hosts during their larval development, from castor to eggplant. This host switching is not influenced by natural enemies, but rather by the larvae's enhanced digestion and metabolism. The ability of S. litura to switch hosts within the same developmental phase may contribute to an additional generation within a cropping season.