Egg allocation by Bracon hylobii Ratz., the principal parasitoid of the large pine weevil (Hylobius abietis L.), and implications for host suppression

1 The braconid parasitoid Bracon hylobii Ratz. is one of the few specialist natural enemies of the large pine weevil, Hylobius abietis L., a destructive pest of conifer transplants. An assessment of its role as an agent of biological control requires a detailed knowledge of the allocation of its reproductive effort. 2 Parasitoid females were continuously observed in laboratory culture with individually reared host larvae in bark discs. The outcome of sequential parasitoid–host encounters was recorded by subsequent examination of hosts and by rearing all parasitoids. 3 Parasitoids avoided ovipositing on host larvae < 100 mg fresh weight, even though such larvae represented sufficient biomass for complete parasitoid development. All larger larvae were vulnerable to attack, which leaves a window of vulnerability for parasitoids of about 90% of weevil larval life. 4 Parasitoids presented with a range of host sizes showed no preference above 100 mg for the size of host first attacked, but allocated more eggs and a greater total handling time to larger hosts. 5 Most eggs were deposited on the first host attacked, with progressively fewer allocated to subsequent hosts. However, oviposition experience did not affect the time spent on the next host. 6 From these results it is anticipated that when weevil larval size is reduced by less favourable feeding substrates, fewer parasitoid eggs will be allocated to each but more host larvae will ultimately be attacked. 7 Generation time, host finding, oviposition rate, clutch size, life expectancy and diapause induction are strongly affected by temperature. Life expectancy is substantially shorter for parasitoids deprived of non‐host food supplement. At 15 and 20 °C the number of hosts attacked and the number of eggs deposited decreased with female age. 8 Bracon hylobii is inevitably poorly synchronized with a variable life‐cycle host; it is egg‐limited and can enter diapause at a relatively high field temperature. None of these characteristics suggest that it could stabilize the abundance of its host below an economically acceptable threshold density. However, the reproductive potential of the parasitoid suggests that it could make a significant contribution to larval mortality and suppress adult recruitment, thus complementing other control strategies.     

Stability in the host-parasitoid relationship of Dialeurodes citri and Encarsia lahorensis in the citrus orchard

The citrus whitefly, Dialeurodes citri (Ashmead), a citrus pest, has been prevalent in Israel since 1975. The parasitic wasp Encarsia lahorensis (Howard) has been successfully used for its biological control since 1980 and thehost-parasitoid system is stable. This studyuses field data from four and a half years inorder to examine whether heterogeneity ofparasitism and risk aggregation can explain thestability. After establishing that theprobability of parasitism is not constant overpatches, we examined the question of parasitoidaggregation, dependent or independent of thehost, at different patch levels. At the treelevel we found an inverse relationship betweenthe proportion of parasitism and host density.At the leaf level, taking the tree effect intoconsideration, the host density dependence wasweak and non-significant. At the leaf level, acombined examination of both types ofheterogeneity in parasite distribution – hostdensity dependent heterogeneity (direct orindirect) and host density independentheterogeneity, was performed using the (CV)² > 1 criterion. The mean (CV)² value over different sampling dateswas greater than one. Host density independentheterogeneity had the greater contribution tostability. The (CV)² findings atleaf level in the plot, combining leaf and treeeffects, represent both aggregation at the treelevel (inverse density dependence) andaggregation at the leaf level (host densityindependence). The heterogeneity findings forparasitoid distribution, together with thestability, which was directly observed, supportour hypothesis that aggregation of risk is animportant mechanism in the stabilisinginteraction between the citrus whitefly and itsspecific parasitoid, E. lahorensis.     

The effect of varying Bemisia argentifolii and Eretmocerus mundus ratios on parasitism

We investigated the effects of different host: parasitoid ratios on the efficacy of the parasitoid Eretmocerus mundus attacking the silverleaf whitefly, Bemisia argentifolii. When host density was held constant (100 second instars) and parasitoid density was decreased from 15 to 1 females, the percentage of total host mortality was significantly lower at low parasitoid densities. The number of host nymphs killed, and the number of female parasitoid progeny per female, increased 3.6 and 20.4 times, respectively. The emergence rate, sex ratio, longevity, and body lengths of progeny were significantly larger at the lowest parasitoid density while developmental time was significantly shorter. When the number of hosts was increased from 5 to 250 and parasitoid density was held constant (5 females), the percentage of nymphal mortality decreased 1.6 times. The percentage of desiccated nymphs was significantly highest (65.7%) at the lowest host density, while percentage parasitism (34.3%) was significantly lowest at the lowest host density. The data could be described using a Type I functional response curve. We propose a generalized index of efficacy (GIE) to summarize and compare the total effects of parasitoid--host ratios. This index showed that the most efficient ratio was one parasitoid female per ten second instar host nymphs.     

Evolutionary character changes and population responses in an insect host-parasitoid experimental system

In an insect host (the cowpea weevilCallosobruchus maculatus)- parasitoidHeterospilus prosopidis) experimental system, the population densities of the component species oscillated for the first 20 generations and then abruptly stabilized as the parasitoid density decreased. Examination of the host and parasitoid after the 40th generation in the long-term experiment showed that (1) host larvae exhibited contest-type competition (killing other larvae inhabiting the same bean), in contrast to the founder population being scramble-type competitors and (2) the parasitoid attack rate on the host did not change. There was also an evolutionary trade-off between body size and the rates of larval survival and development, suggesting a cost of contest competition on larval survivorship and development. I tested model predictions (Tuda and Iwasa 1998) that (1) host equilibrium population size should gradually decrease as the proportion of the contest type increases and that (2) random attacks of the parasitoid on the host should reduce the rate of increase in proportion of the contest type, and the effect should become manifest especially during the first 20 generations. Two of three host-only replicates showed significant decrease in population sizes. Although the density of emerging adults per bean did not differ between replicates of the host-only and host-parasitoid systems, comparison of the host body size between them on day 270 (at the 13th generation) showed that the host was more contest-type in the host-only system than in the host-parasitoid system, as the model predicted, and later on day 650 the effect of the parasitoid had disappeared.     

Predicting the impacts of climate change on the biological control of Plutella xylostella by Diadegma semiclausum

1. The parasitoid Diadegma semiclausum Hellén (Hymenoptera: Ichneumonidae) has been widely adopted as a biological control agent for Plutella xylostella L. (Lepidoptera: Plutellidae) over the last 80 years. Earlier studies have found differential responses to temperature between the host and its parasitoid and demonstrated the multiple ways in which the parasitoid is more susceptible to elevated temperatures.
2. Using data from experimental studies, the modelling package CLIMEX was used to investigate the suitability of current climates for the host and its parasitoid and the effects on their potential global geographical distributions. The study was then extended to investigate possible changes to these distributions that might result under different climate change scenarios by 2080. The models predict that the global distributions of both the host and parasitoid will be reduced. These changes will not be proportionate and many areas in tropical, sub-tropical, and temperate regions that are currently suitable for D. semiclausum are predicted to become unsuitable for the parasitoid, whereas retaining suitability for P. xylostella. The seasonal dynamics of both the host and parasitoid are also predicted to be significantly reshaped under climate change.
3. Analysis of associations between annual P. xylostella outbreaks and weather conditions in three provinces in China with field data collected between 1995 and 2017 indicated significant effects of temperature on P. xylostella outbreaks at the beginning of the peak season in warmer provinces where P. xylostella can overwinter; such associations were not found in the colder provinces where it is unable to survive harsh winters.