Infection of a nucleopolyhedrovirus to Neodiprion zhejiangenis Zhou & Xiao (Hymenoptera: Diprionidae)

Neodiprion zhejiangenis Zhou &; Xiao 1981 (Hymenoptera: Diprionidae) nuclear polyhedrosis virus (NezhNPV) is a biocontrol factor with high development potential and application prospects. To study the insecticidal activity of NezhNPV against the larvae of N. zhejiangenis, the susceptibility of larvae at different instars to different NezhNPV concentrations was determined based on laboratory bioactivity. At 2.0?×?10⁷ polyhedron inclusion body (PIB)/mL, the median lethal time required to kill 50% of the instar 2 and 3 larvae was approximately 5 d, whereas the median lethal time of the instar 4 and 5 larvae was 8–9 d. Interestingly, at the NezhNPV concentrations of 2.0?×?10³ and 2.0?×?10⁴ PIB/mL, the feeding ability of the surviving larvae was decreased by 57.4% and 76.4%, respectively, compared with the controls; the pupal weight at both concentrations was decreased by approximately 27%, and the adult emergence rate was decreased by 27.4% and 50.9%, respectively, with a significantly higher proportion of males than of females. The results showed that younger instar larvae of N. zhejiangenis were more susceptible to NezhNPV infection than older instar larvae. The larval mortality rate was dependent on the larval instar and NezhNPV concentration.     

Opposing effects of mortality factors on progeny operational sex ratio may thwart adaptive manipulation of primary sex ratio

Despite extensive research on mechanisms generating biases in sex ratios, the capacity of natural enemies to shift or further skew operational sex ratios following sex allocation and parental care remains largely unstudied in natural populations. Male cocoons of the sawfly Neodiprion abietis (Hymenoptera: Diprionidae) are consistently smaller than those of females, with very little overlap, and thus, we were able to use cocoon size to sex cocoons. We studied three consecutive cohorts of N. abietis in six forest stands to detect cocoon volume‐associated biases in the attack of predators, pathogens, and parasitoids and examine how the combined effect of natural enemies shapes the realized operational sex ratio. Neodiprion abietis mortality during the cocoon stage was sex‐biased, being 1.6 times greater for males than females. Greater net mortality in males occurred because male‐biased mortality caused by a pteromalid parasitic wasp and a baculovirus was greater and more skewed than female‐biased mortality caused by ichneumonid parasitic wasps. Variation in the susceptibility of each sex to each family of parasitoids was associated with differences in size and life histories of male and female hosts. A simulation based on the data indicated that shifts in the nature of differential mortality have different effects on the sex ratio and fitness of survivors. Because previous work has indicated that reduced host plant foliage quality induces female‐biased mortality in this species, bottom‐up and top‐down factors acting on populations can affect operational sex ratios in similar or opposite ways. Shifts in ecological conditions therefore have the potential to alter progeny fitness and produce extreme sex ratio skews, even in the absence of unbalanced sex allocation. This would limit the capacity of females to anticipate the operational sex ratio and reliably predict the reproductive success of each gender at sex allocation.     

Transmission agents of the nuclear polyhedrosis virus ofNeodiprion sertifer [Hym.: Diprionidae]

Colonies ofNeodiprion sertifer (Geoffroy) larvae infected by nuclear polyhedrosis virus were visited by parasitoids, predators, and scavengers. Six transmission pathways were studied in detail. Viable virus polyhedra (PIB) were isolated from external surfaces of parasitic wasps and faeces of tachinid flies, coccinellid larvae, and scavenging flies. Faeces of the scavenging flies remained infectious 9 months on foliage in the field. A bird fed on earthworms from contaminated soil passed infectious faeces. Raindrops on foliage below diseased larval colonies contained more than 10⁸ PIB/ml. It was concluded that although there are numerous transmission pathways, only a few may be available at any one time. The stochastic nature of many pathways implies that the overall transmission rate is highly variable. These results are consistent with frequent observations of low transmission levels between larvae of the same generation.      

Secondary transmission of a nuclear polyhedrosis virus ofNeodiprion taedae linearis [Hym.: Diprionidae] between larval colonies on loblolly pine

Transmission of a nuclear polyhedrosis virus (NtNPV) of the loblolly pine sawfly,Neodiprion taedae linearis Ross was examined in larval populations on loblolly pine. NtNPV infection of larvae in one colony resulted in transmission to other colonies in the tree. Mortality in untreated colonies was highest when the treated and untreated colonies were located in the upper canopy and when small larvae were treated with virus (42.6–61.9%). Levels of mortality from NtNPV in the untreated colonies were low (2.8–25.0%) when the treated colony was located low in the canopy. Larvae from untreated colonies in the tree were usually large at death. Mortality in untreated larval colonies collected from nearby trees was much less (0.0–27.7%) than that in the trees containing the treated colony. Published with the approval of the Director, Arkansas Agricultural Experiment Station. Use of a trade name does not imply endorsement or guarantee of the product or the exclusion of other products of similar nature.