Response of the braconid parasitoid Cotesia (= Apanteles) glomerata to volatile infochemicals: effects of bioassay set-up, parasitoid age and experience and barometric flux

Upon initiating a research project on the role of volatile infochemicals in the tritrophic system Cotesia (= Apanteles) glomerata (L.)-Pieris brassicae (L.)-cabbage, a bioassay was developed to investigate the response of C. glomerata. The bioassay should be effective in terms of high responsiveness and minimum variability and constructed through a comparative approach. Twenty seven treatments, organized in a factorial randomized block design, compared the effect of three bioassay set-ups (glasshouse flight chamber, wind-tunnel and Y-tube olfactometer), three parasitoid age groups (1–2, 4–5 and 8–9 days old females), three pre-treatment experiences (naive, damage experienced and oviposition experienced wasps) and the day-to-day effect on response of C. glomerata to clean cabbage (CC) and planthost complex (PHC) in a dual choice test.The best results with regard to the strength and consistency of response to the PHC were obtained in the glasshouse flight-chamber by 4–5 days old female wasps with either damage or oviposition experience (94 and 90%, respectively). It is therefore recommended as a suitable bioassay for studying the role of volatile infochemicals in host-habitat location by C. glomerata.A day-to-day variation in response was found in the glasshouse and wind-tunnel. It was correlated with the direction of change in barometric pressure within the time period of the experiment, showing that steadily increasing atmospheric pressure yields a significantly higher response than steadily decreasing or fluctuating barometric flux. To control for the day effect it is suggested to conduct further experiments in a block design, having day as a block. Several aspects of the infochemical ecology of C. glomerata are discussed.     

Coexistence and niche segregation by field populations of the parasitoids Cotesia glomerata and C. rubecula in the Netherlands: predicting field performance from laboratory data

Field experiments with foraging parasitoids are essential to validate the conclusions from laboratory studies and to interpret differences in searching and host selection behaviour of parasitoid species. Furthermore, field experiments can indicate whether the parameters measured in the laboratory are relevant to elucidation of the ecological processes under study, such as adaptation or species interactions. In previous extensive laboratory studies we studied plant- and host-searching behaviour, host acceptance, host suitability; host plant preference, and learning of two congeneric parasitoids of Pieris caterpillars: the generalist Cotesia glomerata, which has been reported to attack several Pieridae species, and C. rubecula, a specialist of the small cabbage white Pieris rapae. In the present field study our aim was to verify the importance of these previous laboratory findings for explaining the performance of these two species in the field. We investigated experimentally whether parasitism on three Pieris species varied with parasitoid species and with food plant of the caterpillars. We exposed different types of host plants, infested with different Pieris species, to parasitism by natural populations of Cotesia species, by setting the experimental plants out in Brussels sprouts cabbage fields. Furthermore we made direct observations of parasitoid foraging in the field. In general, the field results confirmed our predictions on the range of host plant and host species used in the field. The two Cotesia species appear to coexist through niche segregation, since C. glomerata was mainly recovered from P. brassicae and C. rubecula from P. rapae. Although C. glomerata is a generalist at the species level, it can be a specialist at the population level under certain ecological circumstances. Our study shows the importance of variation in host plant attraction and host species acceptance in restricting host plant and host diet in the field. Furthermore the results suggest that, at least in the Netherlands, specialisation of C. glomerata on P. brassicae may occur as a result of C. rubecula outcompeting C. glomerata in P. rapae larvae. Received: 8 July 1999 / Accepted: 31 January 2000     

Responses of a generalist and a specialist parasitoid (Hymenoptera: Eucoilidae) to Drosophilid larval kairomones

Foraging parasitoids are thought to need more specific information than generalists on the presence, identity, availability, and suitability of their insect host species. In the present paper, we compare responses to host kairomones by two phylogenetically related parasitoid species that attack Drosophilidae and that differ in the width of their host range. As predicted, the behavioral response of the parasitoids to host kairomones reflected their difference in host range. The response of the specialist parasitoid Leptopilina boulardiwas restricted to contact kairomones from its natural hosts and one closely related species. In contrast, the generalist parasitoid Leptopilina heterotomaresponded to contact kairomones of a variety of Drosophilidae species.     

Optimal resource allocation to survival and reproduction in parasitic wasps foraging in fragmented habitats

Expansion and intensification of human land use represents the major cause of habitat fragmentation. Such fragmentation can have dramatic consequences on species richness and trophic interactions within food webs. Although the associated ecological consequences have been studied by several authors, the evolutionary effects on interacting species have received little research attention. Using a genetic algorithm, we quantified how habitat fragmentation and environmental variability affect the optimal reproductive strategies of parasitic wasps foraging for hosts. As observed in real animal species, the model is based on the existence of a negative trade-off between survival and reproduction resulting from competitive allocation of resources to either somatic maintenance or egg production. We also asked to what degree plasticity along this trade-off would be optimal, when plasticity is costly. We found that habitat fragmentation can indeed have strong effects on the reproductive strategies adopted by parasitoids. With increasing habitat fragmentation animals should invest in greater longevity with lower fecundity; yet, especially in unpredictable environments, some level of phenotypic plasticity should be selected for. Other consequences in terms of learning ability of foraging animals were also observed. The evolutionary consequences of these results are discussed.     

Unrewarding experiences and their effect on foraging in the parasitic waspLeptopilina heterotoma (Hymenoptera: Eucoilidae)

The host-foraging behavior of female entomophagous parasitoids is commonly modified by positive associative learning. Typically, a rewarding experience (e.g., successful oviposition in a host) increases a female's foraging effort in a host microhabitat of the type associated with that experience. Less well understood are the effects of unrewarding experiences (i.e., unsuccessful foraging). The influence of unrewarding experience on microhabitat choice and residence time within a microhabitat was examined for the eucoilid parasitoid,Leptopilina heterotoma, in laboratory and greenhouse assays. As determined previously, females which oviposited successfully in either of two microhabitat types (fermenting apple or decaying mushroom) strongly preferred to forage subsequently on that microhabitat type. However, failure to find hosts in the formerly rewarding microhabitat caused females to reverse their preference in favor of a novel microhabitat type. The effect, though striking, was transient: within 1–2 h, the original learned preference was nearly fully restored. Similar effects of unrewarding experiences were observed with respect to the length of time spent foraging in a microhabitat. As determined previously, oviposition experience in a particular microhabitat type increased the time spent foraging in a patch of that microhabitat type. However, failure to find hosts in the patch caused the time a wasp spent in the next unoccupied patch of that type to decrease to almost nothing. In addition, there was a tendency for an unrewarding experience on a formerly rewarding microhabitat type to extend the time spent in a patch of a novel type. The function of the observed effects of unrewarding experiences is discussed.     

Comparisons of watershed sulfur budgets in southeast Canada and northeast US: new approaches and implications

Most of eastern North America receives elevated levels of atmospheric deposition of sulfur (S) that result from anthropogenic SO₂ emissions from fossil fuel combustion. Atmospheric S deposition has acidified sensitive terrestrial and aquatic ecosystems in this region; however, deposition has been declining since the 1970s, resulting in some recovery in previously acidified aquatic ecosystems. Accurate watershed S mass balances help to evaluate the extent to which atmospheric S deposition is retained within ecosystems, and whether internal cycling sources and biogeochemical processes may be affecting the rate of recovery from decreasing S atmospheric loads. This study evaluated S mass balances for 15 sites with watersheds in southeastern Canada and northeastern US for the period 1985 to 2002. These 15 sites included nine in Canada (Turkey Lakes, ON; Harp Lake, ON; Plastic Lake, ON; Hermine, QC; Lake Laflamme, QC; Lake Clair, QC; Lake Tirasse, QC; Mersey, NS; Moosepit, NS) and six in the US (Arbutus Lake, NY; Biscuit Brook, NY; Sleepers River, VT; Hubbard Brook Experimental Forest, NH; Cone Pond, NH; Bear Brook Watershed, ME). Annual S wet deposition inputs were derived from measured bulk or wet-only deposition and stream export was obtained by combining drainage water fluxes with SO₄ ^2? concentrations. Dry deposition has the greatest uncertainty of any of the mass flux calculations necessary to develop accurate watershed balances, and here we developed a new method to calculate this quantity. We utilized historical information from both the US National Emissions Inventory and the US (CASTNET) and the Canadian (CAPMoN) dry deposition networks to develop a formulation that predicted SO₂ concentrations as a function of SO₂ emissions, latitude and longitude. The SO₂ concentrations were used to predict dry deposition using relationships between concentrations and deposition flux derived from the CASTNET or CAPMoN networks. For the year 2002, we compared the SO₂ concentrations and deposition predictions with the predictions of two continental-scale air quality models, the Community Multiscale Air Quality (CMAQ) model and A Unified Regional Air-quality Modeling System (AURAMS) that utilize complete inventories of emissions and chemical budgets. The results of this comparison indicated that the predictive relationship provides an accurate representation of SO₂ concentrations and S deposition for the region that is generally consistent with these models, and thus provides confidence that our approach could be used to develop accurate watershed S budgets for these 15 sites. Most watersheds showed large net losses of SO₄ ^2? on an annual basis, and the watershed mass balances were grouped into five categories based on the relative value of mean annual net losses or net gains. The net annual fluxes of SO₄ ^2? showed a strong relationship with hydrology; the largest net annual negative fluxes were associated with years of greatest precipitation amount and highest discharge. The important role of catchment hydrology on S budgets suggests implications for future predicted climate change as it affects patterns of precipitation and drought. The sensitivity of S budgets is likely to be greatest in watersheds with the greatest wetland area, which are particularly sensitive to drying and wetting cycles. A small number of the watersheds in this analysis were shown to have substantial S sources from mineral weathering, but most showed evidence of an internal source of SO₄ ^2?, which is likely from the mineralization of organic S stored from decades of increased S deposition. Mobilization of this internal S appears to contribute about 1?C6 kg S ha^?1 year^?1 to stream fluxes at these sites and is affecting the rate and extent of recovery from acidification as S deposition rates have declined in recent years. This internal S source should be considered when developing critical deposition loads that will promote ecosystem recovery from acidification and the depletion of nutrient cations in the northeastern US and southeastern Canada.     

Development of the parasitoid,Cotesia rubecula (Hymenoptera: Braconidae) in Pieris rapae and Pieris brassicae (Lepidoptera: Pieridae): evidence for host regulation

Several recent models examining the developmental strategies of parasitoids attacking hosts which continue feeding and growing after parasitism (=koinobiont parasitoids) assume that host quality is a non-linear function of host size at oviposition. We tested this assumption by comparing the growth and development of males of the solitary koinobiont endoparasitoid, Cotesia rubecula, in first (L1) to third (L3) larval instars of its preferred host, Pieris rapae and in a less preferred host, Pieris brassicae. Beginning 3 days after parasitism, hosts were dissected daily, and both host and parasitoid dry mass was determined. Using data on parasitoid dry mass, we measured the mean relative growth rate of C. rubecula, and compared the trajectories of larval growth of the parasitoid during the larval and pupal stages using non-linear equations. Parasitoids generally survived better, completed development faster, and grew larger in earlier than in later instars of both host species, and adult wasps emerging from P. rapae were significantly larger than wasps emerging from all corresponding instars of P. brassicae. During their early larval stages, parasitoids grew most slowly in L1 P. rapae, whereas in all other host classes of both host species growth to pupation proceeded fairly uniformly. The growth of both host species was markedly reduced after parasitism compared with controls, with the development of P. brassicae arrested at an earlier stage, and at a smaller body mass, than P. rapae. Our results suggest that C. rubecula regulates certain biochemical processes more effectively in P. rapae than in P. brassicae, in accordance with its own nutritional and physiological requirements. Furthermore, we propose that, for parasitoids such as C. rubecula, which do not consume all host tissues prior to pupation, that parasitoid size and host quality may vary independently of host size at oviposition and at larval parasitoid egression.     

The Functional Role of Balinese Water Temples: A Response to Critics

In earlier publications we have proposed a model to explain the functional role of water temple networks in the agro-ecology of wet rice irrigation on the island of Bali. We argued that the key ecological effects of temple networks are best understood as emergent properties of a complex adaptive system. This argument implies that important aspects of the temple system are largely opaque from the perspective of conventional social science. We proposed that the Green Revolution created a real-world test of our model, by effectively removing the temple networks from their functional role in ecological management. The idea that water temple networks represent a hitherto-unknown kind of institution has been met with appropriate skepticism by other social scientists, who have run a fine-toothed comb through our evidence. Here we evaluate their critiques.