A case of polyploid utility in biocontrol: Reproductively-impaired triploid Nasonia vitripennis have high host-killing ability

BACKGROUND: Intentionally impairing the fecundity of mass-reared insects has important utility in controlling pest species. Typically, sterilized individuals are competed against wild counterparts, reducing pest population size. A novel consideration is creating biocontrol agents with lower reproductive capacity that are less likely to establish permanently or admix with wild populations, which are both emerging as legal barriers. Hymenopterans have diploid females, but archetypically infertile polyploid triploid females occur for various parasitoid species. As a first test of polyploid utility for these biocontrol concerns, we assessed the species with the best characterized polyploid biology, the gregarious idiobiont Nasonia vitripennis, for triploid female host-killing ability on pupal blowfly hosts (Calliphora vomitoria). RESULTS: We examined four polyploid lines: the old Whiting polyploid line (WPL) derived from a spontaneous mutation, and new polyploid lines made through RNAi knockdown of sex determination genes transformer, transformer-2 and wasp-overruler-of-masculinization. For diploid and triploid females of each polyploid line, and control diploids of the STDR and oyster lines used to maintain them, we measured lifetime number of hosts killed; lifetime number of hosts that produced at least one offspring; the percentage of the hosts killed and the percentage of hosts that produced offspring out of those offered; and lifespan. For all lines, triploids produced viable offspring in far fewer hosts than their diploid counterparts (≤70% less). Surprisingly though, they killed as many or more hosts than diploids over similar lifespans. The offspring production ability of the WPL triploid was half that of the other lines, but lines varied only slightly in the number of hosts killed (±10) among the polyploids. CONCLUSION: The ability of reproductively impaired triploids to kill as many hosts as fertile diploids demonstrate high biocontrol utility for polyploidized females, and downstream potential for reducing ecological risk.