Effects of temperature on the parasitoid Eretmocerus warrae

Abstract

Eretmocerus warrae (Hymenoptera: Aphelinidae) is a parasitoid of the greenhouse whitefly，Trialeurodes vaporariorum (Hemiptera: Aleyrodidae). It is suspected to be a better biological control agent at high temperatures than Encarsia formosa (Hymenoptera: Aphelinidae), a wasp which is widely sold for control of T. vaporariorum. Experiments were conducted to investigate how temperature affects the development and behaviour of E. warrae. It attained the highest estimated developmental rate at 31.5 °C and the optimum temperature for oviposition was 30.5 °C. Developmental times of E. warrae at fluctuating temperatures that simulate night-day patterns were similar to those predicted based on constant temperatures. Above the optimum temperature, E. warrae tolerated higher temperatures than En. formosa during development and as adults. Using a ramping temperature approach, the critical thermal maxima of E. warrae was 42.7 ± 0.5 °C, 0.6 °C higher than that of En. formosa. Thus, E. warrae is better adapted to high temperatures than En. formosa, and it could be a complementary or superior biological control agent during summer months in hot regions. The fitness of parasitoids are substantially been influenced by temperature. Although the body-size of both this parasitoid and its host were influenced by temperature, E. warrae gained relatively more fitness in body-size at 20 °C. The predicted body volume index increment of body size from 32 to 20 °C of the parasitoids was 274.1 %, compared with its host at 91.1 %. The life-time oviposition of female adult of E. warrae that grew at immature developmental temperature of 20 °C was 86 ± 22, more than that of 26 and 32 °C at 66 ± 11 and 65 ± 23. Besides the influence on fecundity, temperature also influences the oviposition behaviour at the adult stage, more eggs were oviposited at 20 and 26 °C than those parasitoids at 32 °C. The relationship of life-time oviposition with immature and adult temperature was modelled, both immature and adult temperature play a role on life-time oviposition. Temperature negatively affected longevity in both adult stage and immature developmental stages. An adult female could live 8.9 ± 1.8 days at 20 °C, compared with 5.7 ± 1.7 days at 32 °C. Moreover, E. warrae conducts a quicker oviposition at 26 °C than at 20 °C. More eggs were laid in the first two days at 26 °C, while after two days, females oviposited more eggs at 20 °C. The body-size has a positive effect on both the life-time oviposition and longevity. Experiments were conducted to investigate the effects of (1) host stage on the behaviour and development of parasitoids, and (2) the relative body-size of parasitoids on the host stage choice. In both no choice and choice tests, the parasitoid E. warrae preferred the second instar of T. vaporariorum for parasitism. The behavioural patterns of the parasitoid were observed when handling different instars of hosts. A significantly larger proportion of behaviour was devoted to stinging (40.7 %) when the second instar hosts were provided to parasitoids. The acceptance (84.1 ± 8.9 %) and emergence rates (83.2 ± 7.8 %) from second instar hosts were also the highest, which indicated that second instars are the preferred stage to attack. The body size of E. warrae is affected by temperature during immature development. Wasps that develop at 20 oC are larger than those that develop at 32 oC. The relative bodysize of the parasitoid compared to its host was shown to affect the host stage choice of the parasitoids. Smaller wasps preferred younger host instars while third instar hosts appeared to be too large to be parasitised by them. Larger wasps preferred older instars, while the relatively small first instar hosts were not attacked by larger wasps. However, both small and large wasps preferred second instar hosts. The behavioural patterns of small and large wasps handling second instar hosts were compared. Smaller wasps spent longer periods on antennating hosts than larger ones. Despite displaying similar proportions of the sting behaviour, the frequencies of stinging behaviour of larger parasitoids (35.4 ± 4.1 / h) were much higher than those of smaller ones (17.4 ± 6.0 / h), which reflected the greater fecundity and oviposition activity of larger wasps. In addition, experience was shown to affect the efficiency stinging behaviour of E. warrae.