Sympatric Parasitoids Research Articles

Negative effect of global warming on biological control is mitigated by direct competition between sympatric parasitoids

Parasitoids are among the most important and successful groups of natural enemies used in the biological control of insect pests. In most systems, several parasitoid species can parasitize the same pest. The coexistence of parasitoids in agroecosystems and their efficacy as biological control agents may be disrupted by global warming. An increase of approximately 3 °C is predicted by the end of the twenty-first century in the Mediterranean basin (IPCC, 2014). In this context, we compared the present and future performance of two sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae), which control the armoured scale Aonidiella aurantii (Hemiptera: Diaspididae) in Mediterranean citrus, either alone or in combination. The net reproductive rate (R0) of the introduced Aphytis melinus DeBach was higher than that of its competitor, the native Aphytis chrysomphali (Mercet), under current conditions. The two parasitoids responded differently to higher temperature and competition. The R0 of A. chrysomphali decreased by 50% when both parasitoids competed in the same patch, but was not affected by the temperature increase. The R0 of A. melinus decreased approximately 46% with the increase in temperature because the proportion of females was reduced. However, the presence of A. chrysomphali competing in the same patch mitigated the negative effect of the increase in temperature on A. melinus (R0 decreased by only 16%). Overall, our results suggest that global warming will have a negative effect on the biological control of A. aurantii and that this effect will be higher in areas, such as southern Spain, where A. melinus has displaced A. chrysomphali.

Effect of host instar on host discrimination of heterospecific‐parasitised hosts by sympatric parasitoids

1. Interspecific competition among hymenopteran parasitoids may shape their behavioural strategies for host resource exploitation. In order to reduce or prevent competition, many parasitoid species have evolved the ability to discriminate between unparasitised hosts and hosts parasitised by another parasitoid species (i.e. heterospecific host discrimination). However, discriminatory ability might be affected by host instar.2. This study reports the first results on whether host instar can influence the use of heterospecific‐parasitised hosts by sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae).3. Aphytis melinus and Aphytis chrysomphali discriminated between unparasitised and heterospecific‐parasitised hosts when they found a third‐instar host (high quality), with a tendency to multi‐parasitise. However, this discrimination was not observed in the second instar (lower size).4. The behavioural strategies adopted towards multi‐parasitise third‐instar hosts varied between both species. Aphytis chrysomphali reduced its clutch size in heterospecific‐parasitised hosts, whereas A. melinus tended to probe them for longer than healthy hosts.5. Overall, our results highlight the importance of host instar in the study of intrinsic competition between parasitoids.

Does host quality dictate the outcome of interference competition between sympatric parasitoids? Effects on their coexistence

The suitability and quality of herbivorous insect hosts for hymenopteran parasitoids is dynamic, varying with host development. Generally, within a host species, large hosts (i.e. older instars) are considered of higher quality for parasitoid development. Studies of interspecific competition between parasitoids have considered the effect of host instar on indirect competition but its effect on interference competition remains unknown. Here, we report the first results on whether the quality of host instars might dictate the outcome of interference competition between sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae) when they attack low-quality (second) and high-quality (third) instars of their common host Aonidiella aurantii (Hemiptera: Diaspididae). Oviposition behaviour (host acceptance and clutch size) in low- and high-quality host instars was similar for both Aphytis species in the absence of competition. When they found heterospecific parasitized hosts of high quality, Aphytis melinus laid more eggs and accepted significantly more hosts than Aphytis chrysomphali, whereas there were no significant differences in the low-quality instar. This result suggests that interference competition is mediated by host quality. However, the progeny proportion of both parasitoids in multiparasitized hosts (outcome of competition) was independent of host quality and A. melinus always emerged at higher rates. Therefore, the result of interference competition between these sympatric parasitoids was not affected by host quality and this competition will contribute to the displacement of the native A. chrysomphali by the introduced A. melinus, which has been observed in some areas of the Mediterranean basin.

Niche separation of pollen beetle parasitoids

Species with similar resource requirements are commonly assumed to competitively exclude each other, unless they differentiate their ecological niches. Hence, parasitoid wasps that use the same host species need to find some way to avoid competition. The aim of this study was to identify the role of volatile cues from oilseed rape plants and the larval host in niche separation between three coexisting parasitoid species. We examined how Phradis interstitialis, Phradis morionellus and Tersilochus heterocerus, sympatric parasitoids of Brassicogethes aeneus, differ in their abundances, distribution on buds and flowers, and oviposition behavior in the field. Furthermore, we tested their preferences for odours from uninfested and infested oilseed rape plants in the bud and flowering stage, and their preferences for odours from three developmental stages of pollen beetle larvae in a two-choice olfactometer bioassay. P. interstitialis was active in the field early in the season, preferred odours of infested buds versus uninfested, and oviposited into buds which contained only pollen beetle eggs, while P. morionellus was active late in the season, preferred odours of infested buds as well as odours of infested flowers over uninfested, and oviposited into buds which contained only larvae. T. heterocerus was active throughout the season, and preferred odours of infested flowers over uninfested. Neither Phradis species were attracted to larval odours, whereas T. heterocerus was attracted to odours from first-instar pollen beetle larvae both in the absence of plant odours, and when presented simultaneously with uninfested plant odour. This suggests that the two Phradis species are separated on a temporal scale and that they parasitize different host stages, while the larval parasitoids P. morionellus and T. heterocerus are separated by choice of microhabitat. The former oviposits into larvae in buds, and the latter in flowers.

Differential thermal biology may explain the coexistence of Platygaster matsutama and Inostemma seoulis (Hymenoptera: Platygastridae) attacking Thecodiplosis japonensis (Diptera: Cecidomyiidae)

Two dominant parasitoids, Platygaster matsutama Yoshida and Inostemma seoulis Ko, often coexist in the host-parasitoid community of the pine needle gall midge, Thecodiplosis japonensis Uchida et Inouye. Subtle differences in parasitoid phenology appear to play a key role in reducing interspecific competition, but the specific thermal conditions for development of each species have not been defined. We examined the thermal biology of two sympatric parasitoids during postdiapause development at seven constant temperatures (12, 15, 18, 21, 24, 27, and 30°C) to determine species-specific developmental parameters. The lower developmental threshold was estimated to be 4.2°C and 8.4°C, and the thermal constant was 741.2 and 946.1 degree days (DD) for P. matsutama and I. seoulis, respectively, using the linear model. The nonlinear model determined that the optimal temperature was 24.8 and 26.5°C for P. matsutama and I. seoulis, respectively, but with an upper threshold temperature (30°C) that was the same for both species. The development rate of P. matsutama was higher than that of I. seoulis over the entire thermal range, and the difference was highest at 21.5°C. Despite the lower developmental rate, I. seoulis was more heat tolerant and its optimal temperature was closer to that of host insect than those of P. matsutama. The results showed clear differences in thermal biology between P. matsutama and I. seoulis. Potential implications are discussed with respect to coexistence of two parasitoids on a single host and biological control of T. japonensis.

Odour‐mediated long‐range avoidance of interspecific competition by a solitary endoparasitoid: a time‐saving foraging strategy

1. In studies on optimal foraging strategies, long-range decisions in the pursuit of resource are rarely considered. This is also the case for sympatric parasitoids, which may be confronted with the decision to accept or reject host larvae that are already parasitized by a competing species. They can be expected to reject already parasitized hosts if it is likely that they will lose the resulting intrinsic competition. However, examples of such interspecific host discrimination are rare. 2. We propose that parasitoids that are not egg-limited should reject inferior hosts only if it saves them time, and that this will be achieved mainly when the parasitoids are able to detect competitors from a distance. We tested this hypothesis using the sympatric parasitoids Cotesia marginiventris (Cresson) and Campoletis sonorensis (Cameron). 3. C. sonorensis was found to be the superior intrinsic competitor but, upon contact with a host larva, both wasps readily accepted hosts that had already been parasitized by the other species. However, in an olfactometer experiment, C. marginiventris females were found to strongly avoid the odour of their superior competitor. 4. These results are in accordance with a time optimization scenario, whereby the inferior competitor accepts competition if it costs only an egg, but avoids competition if it may save time that can be allocated to the search for more profitable hosts. 5. Models on host discrimination strategies in parasitoids had not yet considered discrimination from a distance. Long-range foraging decisions can also be expected for other organisms that have to choose between resources of varying suitability and profitability.

Habitat Partitioning by Telenomus podisi and Trissolcus euschisti (Hymenoptera: Scelionidae) Between Herbaceous and Woody Host Plants

Telenomus podisi Ashmead and Trissolcus euschisti (Ashmead)are sympatric parasitoids of stink bug eggs. In field experiments, T. podisi parasitized significantly more egg masses on alfalfa, Medicago sativa L., than onhackberry, Celtis occidentalis L., and T. euschisti parasitized egg masses only on hackberry. There was no significant difference in the number of host egg masses parasitized 0–1 and 3–4.2 m above the ground by T. euschisti in black cherry, Prunus serotina Ehrhart, and hackberry trees. This is one of the few examples where sympatric solitary Hymenoptera that share a host resource subdivide their environment by host plant type. We suggest that the host plant type be considered when importing scelionids for stink bug control in classical biological control programs.