Abstract
Destructive host-feeding is common in hymenopteran parasitoids. Such feeding may be restricted to host stages not preferred for oviposition. However, whether this is a fixed strategy or can vary according to resource levels or parasitoid needs is less clear. We tested the trade-off between host feeding and oviposition on two whitefly parasitoids under varying host densities. Females of two aphelinid parasitoids, Eretmocerus hayati and Encarsia sophia were exposed to nine different densities of their whitefly host, Bemisia tabaci, in single-instar tests to identify their functional response. Mixed-instar host choice tests were also conducted by exposing whiteflies at four densities to the parasitoids. We hypothesized that the parasitoid females can detect different host densities, and decide on oviposition vs. host-feeding accordingly. The results showed that both Er. hayati and En. sophia females tended to increase both oviposition and host-feeding with increased host density within a certain range. Oviposition reached a plateau at lower host density than host-feeding in Er. hayati, while En. sophia reached its oviposition plateau at higher densities. At low densities, Er. hayati parasitized most on first and second (the optimal ones), and fed most on third nymphal instars (the suboptimal one) of the whitefly host as theory predicts, while at high densities, both parasitism and host-feeding occurred on first and second instars which are preferred for oviposition. En. sophia parasitized most on third and fourth (the optimal ones), while fed on first instars (the suboptimal one) at low densities, and utilized third and fourth instars for both at high densities. In conclusion, oviposition vs. host-feeding strategy of parasitoid females was found to vary at different host densities. The balance between reserving optimal hosts for oviposition or using them for host-feeding depended on parasitoid life history and the availability of host resources.
Highlights
Parasitoids have attracted considerable attention because of their importance in biological control of pests, and due to their value as experimental models in investigating the evolution of reproductive strategies
(Hymenoptera: Aphelinidae) and Eretmocerus hayati (Zolnerowich & Rose) (Hymenoptera: Aphelinidae), are good candidates to be used as biological control agents for Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) Middle East-Asia Minor 1 which is one of the most important invasive insect pests in China [14,15,16,17]
Single-instar no choice tests The number of hosts parasitized by Er. hayati significantly increased with increased density of B. tabaci nymphs (F8, 81 = 103.70, P,0.0001) and reached a plateau at 11.2 (SE = 0.5) at 40 2nd instar nymphs per cage (Figure 1A)
Summary
Parasitoids have attracted considerable attention because of their importance in biological control of pests, and due to their value as experimental models in investigating the evolution of reproductive strategies. Optimal foraging models predict that host-feeding parasitoid females, when attacking different types of hosts of varying quality, should oviposit on hosts of high quality, while feeding on ones of lower quality [3,10], resulting in more efficient use of host resources [11]. Such behaviour is reported in several species, including Encarsia formosa (Gahan) (Hymenoptera: Aphelinidae) [12], Aphytis lingnanensis (Compere) (Hymenoptera: Aphelinidae) [6], and Diadromus subtilicornis (Gravenhorst) (Hymenoptera: Ichneumonidae) [13]. When host resource is limited (at low host density and/or quality), a parasitoid female would refrain from host feeding on the optimal host instar for oviposition, and feed on alternative host instars
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