Behavioural response of the woodwasp Sirex noctilio to volatile emissions of its fungal symbiont

The wood‐boring wasp, Sirex noctilio, is a global invasive pest that infects and kills pine trees by inoculating spores of a symbiotic fungus (Amylostereum areolatum) at oviposition. Wasp larvae depend on fungal growth to feed, while the fungus relies on female wasps to initially condition the pine tree by inoculating a phytotoxic venom and for dispersal. Wasp larvae use the fungus as an external gut for the digestion of lignocellulosic compounds resulting in a strong correlation between fungal growth inside the wood and wasp fitness. This study explores the hypothesis that female wasps will use fungal volatiles as a synomone in the process of locating suitable oviposition areas (i.e. trees). Using a Y‐tube olfactometer, adult female wasp behaviour was assessed towards fungal and pine tree volatiles (i.e. positive control). Our results are the first to demonstrate attraction of female S. noctilio towards volatiles of their fungal symbiont. Furthermore, the positive response towards these volatiles seems to be stronger than the response to a known attractant (i.e. pine volatiles). These results could be an important contribution to improving baits for monitoring and control purposes. Further work is needed, mainly oriented towards the identification of the volatiles that trigger the observed attraction response and their possible synergistic effects with tree volatiles.     

Lethal and sublethal effects of spinosad-based GF-120 bait on the tephritid parasitoid Diachasmimorpha longicaudata (Hymenoptera: Braconidae)

Experiments were performed to determine the likely compatibility of spinosad-based GF-120 applications and mass-releases of the braconid parasitoid Diachasmimorpha longicaudata (Ashmead) for control of tephritid fruit flies. Severe effects on D. longicaudata survival and reproduction in the laboratory contrasted with mild effects in semi-field studies. Topical application and contact with dried residues on plastic surfaces in the laboratory generally resulted in high mortality of wasps and significantly reduced survival times compared to controls. Survival times of both sexes were more severely affected when exposure to dried residues on mango leaves compared to wasps that were offered GF-120 mixed with honey. Brief (24 h) exposure to GF-120 in honey or dried residues on mango leaves did not affect total progeny production by wasps, whereas continuous exposure over a 10 d period resulted in reductions in female survival, progeny production and net fecundity (∑L_xM_x) compared to honey-fed controls. During the rainy season, wasp survival times in field cages with young mango trees that had been sprayed with GF-120 or treated with GF-120 mixed with honey were only reduced by 1 or 2 days, respectively, compared to average 10.1 d survival of honey-fed controls. During the dry season, wasp mortality was not significantly affected by exposure to GF-120 residues on mango leaves that had been subjected to natural weathering in the field, compared to controls. Our results suggest that applications of GF-120 are unlikely to adversely influence the effectiveness of mass-releases of D. longicaudata. This prediction requires validation from comparative field studies on parasitoid populations in treated and untreated areas.     

Effect of wasp size, physiological state, and prior host experience on host-searching behavior in a parasitoid wasp (Hymenoptera: Ichneumonidae)

Host-searching behavior in insects generally varies among individuals. A number of physiological and environmental factors can be involved in such individual variation. Here, a series of behavioral observations were made to highlight the importance of physiological state (i.e., number of mature eggs a female carries, amount of nutrient reserves, etc.) and learning state (i.e., prior host experience) on host-approaching behavior of parasitoids. Itoplectis naranyae (Hymenoptera: Ichneumonidae), a solitary endoparasitoid wasp attacking lepidopteran pupae and prepupae, was used as a test insect. The results show that female wasps with experience ovipositing on hosts 2 days before the test found hosts more quickly than did naïve wasps. Prior experience of host odor itself did not affect host-finding behavior, however. A single oviposition was enough for wasps to shorten time to find a host; additional experience had no significant effect on the efficiency of searching. The number of mature eggs a female carried had no effect on the time required to find a host regardless of prior host experience. The size of wasps, instead, was a significant factor when wasps had no prior host experience, and larger wasps found hosts more rapidly than did smaller wasps. Searching activity was not affected by how many hosts a female wasp had fed on before testing.     

A new biological method to control Anobium punctatum,by using the parasitoid wasp Spathius exarator

Biological pest control by means of beneficial organisms is for long part of agriculture and Integrated Pest Management (IPM). A new and efficient strategy to control the most common timber pest species in churches and museums, the furniture beetle Anobium punctatum (De Geer) (Coleoptera: Anobiidae), is based on the parasitoid wasp species Spathius exarator (Linnaeus) (Hymenoptera: Braconidae). Once this braconid wasp detects its host species beneath the surface, it pierces the wood with its ovipositor to lay one single egg onto the beetle larva. After hatching, the wasp larva feeds on the beetle larva thereby killing it. Afterwards, it pupates and emerges through a self-gnawed hole as an adult wasp. The tiny, 0.5-mm-wide exit hole can easily be distinguished from the 1- to 2-mm-wide exit hole of A. punctatum. Laboratory tests revealed that female wasps have an average life span of 85 days and produce a total of 24 offspring, when nutrition is provided. Between 2012 and 2019, braconid wasps were introduced into 54 different A. punctatum infested buildings. Treatment success was monitored by examining exit holes of new beetles and wasps thereby calculating corresponding parasitism rates. After the first year of treatment, parasitism rates were significantly higher with a mean value of 0.15 when compared to untreated objects with a natural parasitism and a mean value of 0.08 (n = 54). Following treatment of three objects over a period of eight years, parasitism rates continuously increased from 0.02 up to 0.31. In a church organ, which was treated during this period, the monitoring revealed a complete suppression of pest activity. These data prove that this biological method of pest control is an efficient, sustainable and non-toxic option to manage the common furniture beetle.     

Oviposition Marking Behavior of <Emphasis Type="Italic">Diachasma alloeum</Emphasis>, (Hymenoptera: Braconidae), Parasitizing <Emphasis Type="Italic">Rhagoletis pomonella</Emphasis>, (Diptera: Tephritidae)

Diachasma alloeum (Muesebeck) (Hymenoptera: Braconidae) is a solitary larval endoparasitoid attacking Rhagoletis (Diptera: Tephritidae) species. Rhagoletis pomonella (Walsh) mark the surface of fruit after oviposition with an oviposition marking pheromone (OMP) which deters conspecific female flies. Herein we demonstrate that female D. alloeum wasps reared from either apple or hawthorn race R. pomenella larvae also deposit an OMP that reduces oviposition by conspecific female wasps. Significantly fewer wasps accepted fruit that had received prior wasp oviposition and OMP or OMP alone without oviposition compared with control fruit for a minimum of 7 days on both fruit types. Rinsing fruit with a 50% ethanol solution appeared to remove the OMP rendering fruit more acceptable for oviposition than marked fruit that was not rinsed. Wasps of each host race were able to detect and avoid the OMP of the sister race and fruit substrate type did not affect wasp response to the pheromone. The possibility of an internal marker deposited during oviposition is also discussed.     

Effect of the <Emphasis Type="Italic">Drosophila</Emphasis> endosymbiont <Emphasis Type="Italic">Spiroplasma</Emphasis> on parasitoid wasp development and on the reproductive fitness of wasp-attacked fly survivors

In a previous study, we showed that Spiroplasma, a maternally transmitted endosymbiotic bacterium of Drosophila hydei, enhances larval to adult survival of its host when exposed to oviposition attack by the parasitoid wasp Leptopilina heterotoma. The mechanism by which Spiroplasma enhances host survival has not been elucidated. To better understand this mechanism, we compared the growth of wasp larvae in Spiroplasma-infected and uninfected hosts. Our results indicate that wasp embryos in Spiroplasma-infected hosts hatch and grow normally for ~2 days, after which their growth is severely impaired, compared to wasps developing in uninfected hosts. Thus, despite their reduced ability to complete development in Spiroplasma-infected hosts, developing wasps may exert fitness costs on their hosts that are manifested after host emergence. The severity of these costs will influence the degree to which this protective mechanism contributes to the long-term persistence of Spiroplasma in D. hydei. We therefore examined survival to 10-day-old adult stage and fecundity of Spiroplasma-infected flies surviving a wasp treatment. Our results suggest detrimental effects of wasp attack on longevity of Spiroplasma-infected adult flies. However, compared to Spiroplasma-free flies exposed to wasps, Spiroplasma-infected flies exposed to wasps have ~5 times greater survival from larva to 10 day-adult. The relative fecundity of wasp-attacked Spiroplasma-infected females was ~71% that of un-attacked Spiroplasma-free females. Our combined survival and female fecundity results suggest that under high wasp parasitism, the reproductive fitness of Spiroplasma-infected flies may be ~3.5 times greater than that of uninfected females, so it is potentially relevant to the persistence of Spiroplasma in natural populations of D. hydei. Interestingly, Spiroplasma-infected males surviving a wasp attack were effectively sterile during the 3-day period examined. This observation is consistent with the expectation that, as a maternally transmitted symbiont, there is little selective pressure on Spiroplasma to enhance the reproductive fitness of its male hosts.     

Determination of the optimal parasitoid‐to‐host ratio for efficient mass‐rearing of the parasitoid,Sclerodermus pupariae (Hymenoptera: Bethylidae)

Sclerodermus pupariae Yang et Yao (Hymenoptera: Bethylidae) is used as a potential biocontrol agent for several buprestid and cerambycid larvae. This study aimed to enhance the efficiency of mass‐rearing of this parasitoid by investigating the fitness gain of this bethylid wasp, including the proportion of successful parasitism and development, brood size, sex ratio, proportion of winged female offspring, body size and longevity of female offspring, under eight different maternal parasitoid density treatments using Thyestilla gebleri Faldermann as host in the laboratory. The results indicated that the foundress densities did not affect the parasitism or emergence rate of this parasitoid. Brood size of the parasitoids increased significantly when the number of maternal wasps ranged from one to four. However, further increases in foundress number did not affect the parasitoid brood size. The sex ratios of S. pupariae were always female‐biased. The proportions of male in the progeny colonies were <10% throughout all experimental treatments. The percentage of winged female progeny was not significantly influenced by the density of adult maternal parasitoids. Body sizes of parasitoids significantly declined with increasing maternal parasitoid densities. Although the parasitoid body size reduced when maternal wasp number was higher, it could be compromised by the relatively higher number of female offspring produced. Further, more than 70% of the parasitoids remained alive when they were stored at 12°C for four months throughout the experiments. These findings suggest that exposure of four female wasps to a single host larva would result in the highest fitness of S. pupariae. Our findings might provide a new approach to enhance the efficiency of mass‐rearing of this bethylid wasp.     

<Superscript>15</Superscript>N/<Superscript>14</Superscript>N ratios of amino acids as a tool for studying terrestrial food webs: a case study of terrestrial insects (bees,wasps, and hornets)

Compound-specific stable isotope analysis (CSIA) of amino acids is a new method that enables estimates of trophic position for consumers in food webs. We examined the nitrogen isotopic composition (δ¹⁵N) of amino acids of Japanese social insects (three bee, three wasp, and four hornet species) to evaluate the potential of CSIA of amino acids in studies of terrestrial food webs. For wasps, we also examined samples at different growth stages (ranging from egg to adult) to assess the effect of metamorphosis on CSIA estimates of trophic position. The δ¹⁵N values of bulk tissues for Japanese social insects are only weakly correlated with the biologically expected trophic positions. In contrast, the trophic positions estimated from the δ¹⁵N values of amino acids (yielding values of between 2.0 and 2.3 for bees, between 2.8 and 3.3 for wasps, and between 3.5 and 4.1 for hornets) are consistent with the biologically expected trophic positions for these insects (i.e., 2.0 for bees, 3.0 for wasps, and 3.0–4.0 for hornets). Although large variability is observed among the δ¹⁵N values of individual amino acids (e.g., ranging from 3.0 to 14.9‰ for phenylalanine), no significant change is observed in the trophic position during wasp metamorphosis. Thus, the CSIA of amino acids is a powerful tool for investigating not only aquatic food webs but also terrestrial food webs with predatory insects.     

Use of herbivore‐induced plant volatiles as search cues by Tiphia vernalis and Tiphia popilliavora to locate their below‐ground scarabaeid hosts

Japanese beetle, Popillia japonica Newman, and oriental beetle, Anomala orientalis (Waterhouse) (both Coleoptera: Scarabaeidae) are considered invasive species and have been reported as key pests of urban landscapes in the Northeastern USA. Tiphia vernalis Rohwer and Tiphia popilliavora Rohwer (Hymenoptera: Tiphiidae) were introduced as biocontrol agents against these beetles. These parasitic wasps burrow into the soil and search for grubs. When a host is found, the wasp attaches an egg in a location that is specific for the wasp species. It is unknown if these wasps can detect patches of concealed hosts from a distance above ground and what role, if any, herbivore‐induced plant volatiles play in their host location. This study evaluated the responses of female T. vernalis and T. popilliavora to grub‐infested and healthy plants in Y‐tube olfactometer bioassays. Also the effect of root herbivory on the composition of turfgrass (Poaceae) volatile profiles was investigated by collecting volatiles from healthy and grub‐infested grasses. Tiphia wasps were highly attracted to volatiles emitted by grub‐infested tall fescue (Festuca arundinacea Schreb.) and Kentucky bluegrass (Poa pratensis L.) over healthy grasses. In contrast, wasps did not exhibit a significant preference for grub‐infested perennial ryegrass (Lolium perenne L.) as compared with the control plants. The terpene levels emitted by grub‐infested Kentucky bluegrass and tall fescue were greater than that of control plants. Low levels of terpenes were observed for both test and control perennial ryegrass. The elevated levels of terpenes emitted by grub‐infested Kentucky bluegrass and tall fescue coincided with the attractiveness to the tiphiid wasps. Here, we provide evidence that plant exposure to root‐feeding insects P. japonica and A. orientalis resulted in an increase in terpenoid levels in turfgrasses, which strongly attracts their above‐ground parasitoids.     

Biased oviposition and biased survival together help resolve a fig–wasp conflict

We report evidence that helps resolve two competing explanations for stability in the mutualism between Ficus racemosa fig trees and the Ceratosolen fusciceps wasps that pollinate them. The wasps lay eggs in the tree's ovules, with each wasp larva developing at the expense of a fig seed. Upon maturity, the female wasps collect pollen and disperse to a new tree, continuing the cycle. Fig fitness is increased by producing both seeds and female wasps, whereas short‐term wasp fitness increases only with more wasps, thereby resulting in a conflict of interests. We show experimentally that wasps exploit the inner layers of ovules first (the biased oviposition explanation), which is consistent with optimal‐foraging theory. As oviposition increases, seeds in the middle layer are replaced on a one‐to‐one basis by pollinator offspring, which is also consistent with biased oviposition. Finally, in the outer layer of ovules, seeds disappear but are only partially replaced by pollinator offspring, which suggests high wasp mortality (the biased survival or ‘unbeatable seeds’ explanation). Our results therefore suggest that both biased oviposition and biased survival ensure seed production, thereby stabilizing the mutualism. We further argue that biased oviposition can maintain biased survival by selecting against wasp traits to overcome fig defenses. Finally, we report evidence suggesting that F. racemosa balances seed and wasp production at the level of the tree. Because figs are probably selected to allocate equally to male and female function, a 1:1 seed:wasp ratio suggests that fig trees are in control of the mutualism.     

Host acceptance behavior of the Japanese aquatic wasp,Agriotypus gracilis (Hymenoptera: Ichneumonidae) toward the caddisfly host,Goera japonica (Trichoptera: Limnephilidae)

The host acceptance behavior of the Japanese aquatic wasp,Agriotypus gracilis Waterston, an ectoparasitoid of the sand case building caddisfly,Goera japonica Banks was investigated in the laboratory. Female wasps were observed to enter the water by walking down a stone protruding from the water surface. Antennae were held backward and not utilized in searching for hosts under the water. Female wasps examined hosts from the outside of their cases by 2 consecutive steps, “turning” and “probing”. Turning behavior, in which female wasps move between the anterior and posterior ends of host cases, may be related to the measurement of case size. Host stages are considered to be discriminated by probing, in which females probe host cases with their sheathing ovipositors. Female wasps most frequently accepted and oviposited on pupal and prepupal hosts.     

Finding hidden females in a crowd: Mate recognition in fig wasps

Multi-species mating aggregations are crowded environments within which mate recognition must occur. Mating aggregations of fig wasps can consist of thousands of individuals of many species that attain sexual maturity simultaneously and mate in the same microenvironment, i.e, in syntopy, within the close confines of an enclosed globular inflorescence called a syconium – a system that has many signalling constraints such as darkness and crowding. All wasps develop within individual galled flowers. Since mating mostly occurs when females are still confined within their galls, male wasps have the additional burden of detecting conspecific females that are “hidden” behind barriers consisting of gall walls. In Ficus racemosa, we investigated signals used by pollinating fig wasp males to differentiate conspecific females from females of other syntopic fig wasp species. Male Ceratosolen fusciceps could detect conspecific females using cues from galls containing females, empty galls, as well as cues from gall volatiles and gall surface hydrocarbons.In many figs, syconia are pollinated by single foundress wasps, leading to high levels of wasp inbreeding due to sibmating. In F. racemosa, as most syconia contain many foundresses, we expected male pollinators to prefer non-sib females to female siblings to reduce inbreeding. We used galls containing females from non-natal figs as a proxy for non-sibs and those from natal figs as a proxy for sibling females. We found that males preferred galls of female pollinators from natal figs. However, males were undecided when given a choice between galls containing non-pollinator females from natal syconia and pollinator females from non-natal syconia, suggesting olfactory imprinting by the natal syconial environment.     

SEX-RATIO MANIPULATION IN RESPONSE TO HOST SIZE BY THE PARASITOID WASP SPALANGIA CAMERONI: A LABORATORY STUDY

The prediction of Charnov et al.'s (1981) host-size model that there should be a negative relationship between host size and wasp sex ratio (proportion sons) was supported for Spalangia cameroni, a solitary parasitoid wasp. The relationship was shown to be a result of offspring sex manipulation by females in response to host size rather than a result of differential mortality of the sexes. A major assumption of the host-size model is that host size has a greater effect on the ultimate reproductive success of emerging female wasps than of males. This assumption was not supported. Host size had a positive effect on the size of both male and female S. cameroni. However, neither host size nor wasp size affected longevity, production of offspring by females, or ability of males to compete for mates. Host size may differentially affect the reproductive success of female and male wasps through effects on other aspects of reproductive success. Tests of the assumptions of offspring sex-ratio manipulation hypotheses are scarce but critical, not only for parasitoid wasps, but also for other organisms.     

Deep mtDNA divergences indicate cryptic species in a fig-pollinating wasp

Background  

Figs and fig-pollinating wasps are obligate mutualists that have coevolved for ca 90 million years. They have radiated together, but do not show strict cospeciation. In particular, it is now clear that many fig species host two wasp species, so there is more wasp speciation than fig speciation. However, little is known about how fig wasps speciate.     

Effects of parental age at mating on the fecundity and progeny sex ratio of <Emphasis Type="Italic">Campoletis chlorideae</Emphasis>, an endolarval parasitoid of the pod borer, <Emphasis Type="Italic">Helicoverpa armigera</Emphasis>

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), commonly called gram pod borer, is a serious pest of pulse crops in India and damages chickpea (Cicer arietinum L). On average a 30% crop loss is reported. One of the potential natural enemies reported for its biological control is Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae), an idiobiont parasitic wasp that attacks second instar larvae of the pod borer. Male-biased sex ratios hinder efforts to mass release parasitic Hymenoptera for biological control by making the production of females costly. Parental age at time of mating is known to affect the progeny sex ratio in some Braconidae. In this view, we examined the reproduction and survival of the parasitoid C. chlorideae in the laboratory [22 ± 4°C, 70 ± 10% R.H. and 10:14 h light:dark photoperiod]. All the nine (i.e. 3 × 3) combinations of possible mating were made for males and females, being each of different age groups (0–12, 48–60 and 96–108 h old). Second instar larvae of the pod borer were reared on chickpea and exposed to mated females of different mating age group combinations. Results revealed that (a) newly emerged females (0–12 h old) mated to aged males yielded more progeny contrasted to the older females inseminated by newly emerged males (0–12 h old). This implies that increased maternal age at mating caused greater reduction in the progeny yield than that of paternal age; (b) the progeny sex ratio was lowest (0.344 ± 0.057) (mean ± SD) when parents mated just after emergence (0–12 h old). It became more and more male biased as parents were deprived of mate and was highest (0.666 ± 0.701) for 96–108 h old parents. The analysis of variance of the data shows that mate deprived males caused greater contribution in the production of sons, resulting a male-biased progeny sex ratio. Our conclusion indicate that newly emerged parasitoids (0–12 h old) are most fecund and should be used in rearing practices in order to enhance the progeny yield, particularly with respect to daughters. Handling Editor: Torsten Meiners     

Middle aged wasps mate through most of the year, without regard to body size, ovarian development and nestmateship: a laboratory study of the primitively eusocial wasp Ropalidia marginata

We studied the mating behaviour of the primi-tively eusocial wasp Ropalidia marginata and the factors that may influence sperm transfer. By introducing a male and a female R. marginata into ventilated transparent plastic boxes, we were able to observe mating behaviour, and it involved mounting and short or long conjugation of the wasps. Dissection of female wasps after the observation indicated that long conjugation is a good behavioural predictor of sperm transfer. This finding makes it possible to obtain mated females without dissecting them every time. We tested the effect of age, season, relatedness, body size and female’s ovarian status on mating. Under laboratory conditions, mating success declined rapidly below and above the ages 5–20 days. Within this age range mating success was significantly low in December compared to other months tested. There was no nestmate discrimination, and there was no influence of male and female body size or of the ovarian state of the female on the probability of sperm transfer.     

Maize plants sprayed with either jasmonic acid or its precursor,methyl linolenate,attract armyworm parasitoids,but the composition of attractants differs

Treatment of both uninfested and armyworm‐infested maize plants with jasmonic acid (JA) is known to attract the parasitic wasp, Cotesia kariyai Watanabe (Hymenoptera: Braconidae). Here, we show that treatment with a methyl ester of a JA precursor, methyl linolenate (MeLin), also causes maize plants to attract this wasp, yet does not cause elevated levels of endogenous JA. The volatile chemicals emitted from either infested or uninfested maize plants treated with MeLin were qualitatively and quantitatively different from those emitted from JA‐treated plants. Among compounds emitted from MeLin‐treated plants, α‐pinene and menthol attracted wasps in pure form in a two‐choice test using a choice chamber. A mixture of methyl salicylate, α‐copaene, and β‐myrcene also attracted wasps. In contrast, (Z)‐3‐hexenyl acetate was among the main attractants for C. kariyai in JA‐treated plants. These data show that in addition to JA, MeLin also has the potential to increase the host‐finding ability of C. kariyai, but that the composition of attractants they induce differs.     

Sex allocation of three solitary ectoparasitic wasp species on bean weevil larvae: Sex ratio change with host quality and local mate competition

Sex ratio manipulation by ovipositing females was surveyed in 3 solitary ectoparastic wasp species,Dinarmus basalis (Pteromalidae),Anisopteromalus calanrae (Pteromalidae), andHeterospilus prosopidis (Braconidae), that parasitize azuki bean weevil (Callosobruchus chinensis (L) (Coleoptera: Buruchidae)) larvae within azuki beans (Vigna angularis). Variables were local mate competition (LMC) and host quality (HQ). We used host age as a measure of host quality (from 9-to 16-day-old hosts), changed the number of ovipositing females to control the level of local mate competition (1 female and 10 females), and examined oviposition patterns of the wasps. The offspring sex ratios (proportion of females) of the 3 wasp species respond qualitatively same to HQ and LMC. The common qualitative tendency among the 3 species is an increase of sex ratios increase with host age. In the process of changing the sex ratio (9–13-day-old) 3 wasp species respond only to HQ. In the hosts that end development in size (14–16-day-old) wasps respond to LMC. The response of sex ratio change to LMC in the old host ageclasses are different among the 3 species. In the situation that there exists LMC (10 females) sex ratios are the same among the 3 wasps. However, the sex ratios in no LMC (single female) are heterogeneous among the 3 wasps.     

Restructuring of Lepidoptera communities by introduced Vespula wasps in a New Zealand beech forest

Introduced social wasps (Vespula vulgaris) reach high densities in some New Zealand beech forests, because honeydew provides an abundant high-energy food source. We manipulated wasp density to estimate an “ecological damage threshold” for large, free-living Lepidoptera larvae. There will be a continuum of ecological damage thresholds for wasp density depending on the prey species or habitat. Experimentally placed small caterpillars had a significantly higher survival rate than large caterpillars, and the survival rate of both groups decreased with increasing wasp density. Spring-occurring caterpillars have a probability of surviving of 0.90–0.95, assuming wasps are the only source of mortality. However, at the peak of the wasp season we predict caterpillars would have virtually no chance (probability of 10⁻⁷⁸ to 10⁻⁴⁰) of surviving to adults. Wasp abundance must be reduced by at least 88% to conserve the more vulnerable species of free-living caterpillars at wasp densities similar to those observed in our study sites. This equates to a damage threshold of 2.7 wasps per Malaise trap per day. It was exceeded for about 5 months of the year in non-poisoned sites. There are currently no biological or chemical control techniques available in New Zealand that will reduce wasp abundance below this damage threshold throughout the year. Our models show that most Lepidoptera with spring caterpillars will be able to persist, but species with caterpillars occurring in the peak wasp season will be eliminated. Received: 5 January 1998 / Accepted: 10 February 1999     

Host-feeding and acceptance by a parasitic wasp (Hymenoptera: Ichneumonidae) as influenced by egg load and experience in a patch

Optimal host selection models based on dynamic programming predict that the physiological state of a foraging insect, i.e. egg load, energy reserves etc., influences behavioral decisions. To test this prediction, the effect of physiological state on host acceptance of the ectoparasitic wasp Agrothereutes lanceolatus was investigated. Female wasps in plastic cups (regarded as patches) were presented with hosts, and their responses to the hosts were continuously observed. After observations, the wasps were dissected and the number of mature and immature eggs they carried were counted. The results showed that behavioral decisions by the female wasps were influenced by mature egg load, but not by wasp size or immature egg load. Hence the wasps with higher egg loads were more likely to oviposit. The number of hosts previously encountered in a patch (i.e. wasp experience) also had an independent effect on females' host acceptance, indicating that female informational state was updated during foraging in that patch. Female wasps host-fed only when mature egg load approached zero. Concurrent host-feeding was not observed. Parasitoid survival was almost zero when parasitoid eggs were transferred onto hosts that were fed upon, indicating that concurrent host-feeding could cause a high degree of offspring mortality. These three results supported the assumption and prediction of optimal host-feeding models. Parasitoid host selection and host-feeding are discussed in the context of recent models.