Host preferences of an invasive specialist herbivore and native predators on chemically defended plants

Abstract

Herbivorous insects generally choose host plants that optimize their growth. However, many specialists choose chemically defended host plants that slow their growth, prioritizing protection from natural enemies. Invasive specialist herbivores may have an added advantage in their non‐native range where they lack co‐evolved specialist natural enemies, in addition to being unpalatable to generalist predators. Here, we explore the diet preferences of the invasive yellowmargined leaf beetle, Microtheca ochroloma and those of native natural enemies. We measured M. ochroloma preferences among six Brassica cultivars varying in glucosinolate concentrations. In both field and laboratory experiments, M. ochroloma preferred the relatively less defended host plants, mizuna and Chinese cabbage. M. ochroloma grew quickest on mizuna. Generalist predators also preferred to eat M. ochroloma larvae that had been fed mizuna in choice trials, yet in the field, predatory insects were least commonly found on mizuna, and most commonly found on plants dominated by native herbivores. M. ochroloma consumes plants that increase their susceptibility to predators, but predation pressure in the field appears to be generally low. Our results suggest that invasive specialist herbivores may prioritize growth over protection against natural enemies. Weak top‐down regulation of specialist herbivore invaders might enable them to exploit less‐defended host plants in their novel range that facilitate faster growth and exacerbate crop damage. Invasive pests may be more effectively managed by leveraging plant preferences in the novel host range in trap crop systems.