How effective is Psyllaephagus bliteus (Hymenoptera: Encyrtidae) in controlling Glycaspis brimblecombei (Hemiptera: Psylloidea)?

• Effects of parasitism and climate in the abundance of G. brimblecombei were studied.
• Climate and the host plant were the major factors controlling G. brimblecombei.
• Parasitism by P. bliteus did not prevent the host’s population build-up.
• Parasitism was higher in cooler coastal sites than in the hotter inland sites.

The parasitoid Psyllaephagus bliteus (Hymenoptera: Encyrtidae) was fortuitously introduced in Portugal with its invasive host Glycaspis brimblecombei (Hemiptera: Psylloidea). Its impact on the psyllid was investigated on Eucalyptus camaldulensis during two years by population dynamics studies, parasitoid exclusion experiments, and by comparing parasitism levels in cool and in hot climatic zones. The breeding season of the psyllid is compressed into a 5-month period, between spring and summer, when temperatures are warm and new leaves for oviposition are abundant. At peak egg density, the psyllid females laid significantly more eggs/leaf in soft new leaves (32.0 ± 4.2) than in older ones (20.9 ± 3.5). The psyllid population collapses in mid-summer (July-August), before the rains resume, when maximum air temperatures can exceed 30 °C and plant growth slows. The population of P. bliteus followed the host population densities with a 3-week delay, but did not prevent the host population build-up. Low to moderate mortality by parasitism from P. bliteus, ranging from 0 to 48%, was observed in field samples. In two exclusion experiments the mortality from undetermined factors in unprotected psyllid colonies was far more important (90 and 93%) than the mortality caused by parasitism (24 and 7%). The climate affected the parasitoid performance, since the parasitism rates were significantly higher in cooler coastal areas (43% ± 3.0) than in inland sites (12% ± 2.0) with average temperatures 4 °C higher. The results show that under Mediterranean climates the contribution of P. bliteus to the psyllid population’s regulation is minor and that climate has a major impact.

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