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Discovering 'pirate parasitism': Wasps can successfully exploit unsuitable hosts with help from another species

Parasitoid wasps sometimes lay eggs in unsuitable host insects, a behavior that has traditionally been considered accidental. However, researchers at the University of Tsukuba have discovered that parasitism can still be successful in these cases—if another species of wasp lays eggs in the same host simultaneously. They have named this newly discovered parasitic strategy "pirate parasitism."

Adult female parasitoid wasps lay their eggs in the bodies of host arthropods. The offspring develop inside the host, eventually killing it as they emerge as adults. In complex environments, adult female wasps search for suitable hosts but often lay eggs in unsuitable insects (those that cannot serve as hosts), which are sympatric with the actual hosts.

Since the offspring deposited in non-host insects are typically killed by the host's immune system, such egg-laying was previously considered accidental and non-adaptive.

In , which has been published in Scientific Reports, researchers discovered that when the parasitoid wasp, Cotesia kariyai (Hymenoptera: Braconidae) lays eggs in unsuitable host caterpillars, Mythimna loreyi (Lepidoptera: Noctuidae), another parasitoid wasp, Meteorus pulchricornis (Hymenoptera: Braconidae), often lays eggs in the same caterpillars around the same time.

Remarkably, C. kariyai is able to successfully parasitize M. loreyi caterpillars at a certain rate. The researchers have termed this behavior "pirate parasitism."

This phenomenon is believed to occur because C. kariyai, which cannot suppress the immunity of M. loreyi caterpillars on its own, takes advantage of caterpillars whose immune systems have already been suppressed by M. pulchricornis.

The existence of pirate parasitism suggests that the reproductive potential of parasitoid wasps is maintained even in environments where their usual host is temporarily unavailable. This discovery highlights the adaptive significance of egg-laying in non-hosts, a behavior previously considered non-adaptive.

By investigating the prevalence and physiological mechanisms of pirate parasitism, researchers aim to deepen our understanding of both natural and agricultural ecosystems and to inform the application of parasitoid wasps in pest management strategies.