Abstract
This study examined defensive behaviors of Mythimna separata (Lepidoptera: Noctuidae) larvae varying in body size in response to two parasitoids varying in oviposition behavior; Microplitis mediator females sting the host with the ovipositor after climbing onto it while Meteorus pulchricornis females make the sting by standing at a close distance from the host. Mythimna separata larvae exhibited evasive (escaping and dropping) and aggressive (thrashing) behaviors to defend themselves against parasitoids M. mediator and M. pulchricornis. Escaping and dropping did not change in probability with host body size or parasitoid species. Thrashing did not vary in frequency with host body size, yet performed more frequently in response to M. mediator than to M. pulchricornis. Parasitoid handling time and stinging likelihood varied depending not only on host body size but also on parasitoid species. Parasitoid handling time increased with host thrashing frequency, similar in slope for both parasitoids yet on a higher intercept for M. mediator than for M. pulchricornis. Handling time decreased with host size for M. pulchricornis but not for M. mediator. The likelihood of realizing an ovipositor sting decreased with thrashing frequency of both small and large hosts for M. pulchricornis, while this was true only for large hosts for M. mediator. Our results suggest that the thrashing behavior of M. separata larvae has a defensive effect on parasitism, depending on host body size and parasitoid species with different oviposition behaviors.
Highlights
Since successful parasitism by parasitoids results in the death of the host, natural selection should favor the evolution of host defenses against parasitoids
Our data support the hypothesis that M. separata larvae adjust their defensive behaviors in response to different species of parasitoids that differ in oviposition behaviors
We showed that the larvae, whether small or large in size, thrashed the head more frequently in response to M. mediator than to M. pulchricornis, though they dropped and escaped in the same likelihood in response to them
Summary
Since successful parasitism by parasitoids results in the death of the host, natural selection should favor the evolution of host defenses against parasitoids. Many studies have shown that host larval resistance to attacking parasitoids increases with age (and body size) (Gross, 1993; Firlej et al, 2010; Kageyama & Sugiura, 2016) Such resistance can be achieved by a combination of morphology (e.g., increasingly tough exoskeleton) and behaviors associated with host age (or size) (Brodeur, Geervliet & Vet, 1998; Yazdani, Glatz & Keller, 2015; Ameri, Rasekh & Michaud, 2014; Kageyama & Sugiura, 2016). These defensive mechanisms often increase host handling time, shape parasitoid host-preference (Lucas, Coderre & Brodeur, 1997; Potting, Vermeulen & Conlong, 1999) and likely decrease parasitism success by parasitoids (Gross, 1993)
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