Influence of larval rearing temperature on the quality of cold‐stored Oomyzus sokolowskii Kurdjumov (Hymenoptera: Eulophidae)

Oomyzus sokolowskii, an important parasitoid of Plutella xylostella, has great potential for use in biological control. Storage at suboptimal temperature is valuable for increasing the shelf‐life of insect parasitoids. In this study, O. sokolowskii larvae were reared at 30/25, 25/25 and 25/20°C light/dark (65 ± 5% RH, 16 : 8 h L : D) until pupation. The pupae were then cold‐stored at 4 ± 1°C (60 ± 5% RH, full darkness). The pupae were removed out from the storage at 10, 20, 30 and 40 days after storage (DAS) and maintained at 25 ± 2°C until adults emerged or pupae died. Quality of the emerging adults and their F₁ offspring were assessed. Incidence of parasitism by O. sokolowskii was higher at 30/25°C than at 25/20°C. Cold storage of O. sokolowskii pupae greatly affected the fitness of the parasitoid: adult emergence rates were lower in the 40 DAS treatment than in other treatments; when O. sokolowskii larvae developed at 25/25°C, female proportions of the emerged adults were lower in the 40 DAS treatment than in the 0 and 10 DAS treatments. Larval rearing temperature mildly affected the adult emergence rate, post‐storage developmental time and female proportion with a few exceptions. Number of parasitoids emerged per host pupa, and incidence of parasitism by the females were neither affected by larval rearing temperature nor cold storage duration. Trans‐generational effects on F₁ offspring were evident in adult emergence rate, egg‐adult developmental time and female proportion which were negatively affected by long duration of storage (40 days), but not by larval rearing temperature with a few exceptions. In conclusion, O. sokolowskii pupae could be stored at 4°C for up to 30 days without significant fitness loss.     

Thermal biology and establishment potential in temperate climates of the predatory mirid <Emphasis Type="Italic">Nesidiocoris tenuis</Emphasis>

Nesidiocoris tenuis Reuter (Hemiptera: Miridae) is a polyphagous mirid currently used for the control of leafminers, thrips, whitefly and spider mites in Mediterranean regions to which it is indigenous. This study investigates the establishment potential of N. tenuis in cool temperate climates typical of northern Europe through assessment of its thermal biology and low temperature tolerance in laboratory and field experiments. The developmental threshold of N. tenuis was estimated to be 12.9°C with no indication of ability to diapause. Supercooling points of the acclimated and non-acclimated adults and nymphs of the mirid were between −17.6° and −21.5°C and the LTemp₅₀ was around −12°C, indicating a high level of pre-freeze mortality. The LTime₅₀ at 5°C was nine days and 100% mortality occurred after less than four weeks of winter field exposure. Collectively these data suggest that N. tenuis is unlikely to establish in northern Europe and would therefore have little or no non-target effects on native species in such regions.     

Effects of cold storage on life-history traits of Aphidius matricariae

Aphidius matricariae Haliday (Hymenoptera: Braconidae) is a polyphagous solitary endoparasitoid, attacking more than 40 species of aphids. This parasitoid is an important commercial product of many companies that produce biological control agents. Storage at low temperature increases the shelf life of many biocontrol agents, allowing companies to provide a steady and sufficient supply of insects for biocontrol programs. In the current study, the effects of cold storage of 1-day-old host mummies with A. matricariae for various time periods (5, 10, 15, 20, and 30 days) at 5 °C on the parasitoid’s key life-history traits were investigated. Parameters assessed after storage included adult emergence rate, offspring sex ratio, adult longevity, oviposition period, fecundity, and life-table parameters (R₀, r, λ, T, and DT). Our results showed that the mummies of A. matricariae could be stored at 5 °C for 5 days without loss of quality and for 10–15 days with minimal reduction in quality (e.g., some reduction in adult longevity and R₀). If parasitoids were stored for >15 days, quality was more strongly affected. In conclusion, A. matricariae pupae could be stored at 5 °C for up to 15 days without significant negative post-storage effects on fitness of the parasitoid. These results could be used to improve the planning of mass rearing and mass release of A. matricariae in augmentative biological control programs.     

Pre-release analysis of the overwintering capacity of a classical biological control agent supporting prediction of establishment

This study investigated the effect of temperature on the development and overwintering capacity of the pupal parasitoid, Diadromus pulchellus Wesmael (Hymenoptera: Ichneumonidae), a candidate classical biological control agent against leek moth, Acrolepiopsis assectella (Zeller) (Lepidoptera: Acrolepiidae) in Canada. It was estimated that 256.4 day-degrees, above a lower threshold temperature of 7.3°C, were required for D. pulchellus to complete development, from egg to adult eclosion. Laboratory and field experiments on the immature and mature parasitoids indicated that D. pulchellus overwinters primarily, if not exclusively, in the adult stage. Only adults were able to survive an entire winter under natural outdoor conditions in central Europe. Immature parasitoids developing inside their pupal hosts were capable of withstanding short periods of temperatures as low as −5°C or −10°C, but even much higher temperatures were lethal if sustained for several weeks. Among adults, females demonstrated greater cold hardiness than males. The LTime₅₀ at −12°C, simulating winter temperatures without snow cover, was 4–5 and 6–7 days for males and females, respectively. The LTime₅₀ at −4°C, simulating winter temperatures beneath an insulating snow layer, was 1–2 and 2–3 weeks for males and females, respectively, with maximum survival of eight weeks. It is likely that survival would be even greater in a natural environment where the parasitoids could select optimal overwintering sites and have the option to feed when temperatures rise enough to permit activity. Based on these results, D. pulchellus is expected to survive winters in the targeted release areas of Ontario and Quebec.     

Geographic structure with no evidence for host-associated lineages in European populations of Lysiphlebus testaceipes,an introduced biological control agent

Lysiphlebus testaceipes (Cress.) is an aphidiine parasitoid originally introduced to Europe as a biological control agent of citrus aphids in the Mediterranean. It has rapidly become widespread in coastal areas continuing gradually to expand inland. L. testaceipes exploited a large number of aphids in Europe, including new hosts and significantly changed the relative abundance of the native parasitoids. This behavior may reflect a broad oligophagy of the introduced parasitoid or it may require the evolution of host specialization that results in genetically differentiated subpopulations on different hosts. To address this issue we used the mitochondrial cytochrome oxidase subunit I and seven microsatellite loci to analyze the structure of genetic variation for L. testaceipes samples collected from 12 different aphid hosts across seven European countries, as well as some samples from Benin, Costa Rica, USA, Algeria and Libya for comparison. Only five COI haplotypes with moderate divergence were identified overall. There was no evidence for the association of haplotypes with different aphid hosts in the European samples, but there was geographic structuring in this variation. Haplotype diversity was highest in France, where L. testaceipes was introduced, but only a single haplotype was detected in areas of south-eastern Europe that were invaded subsequently. The analysis of microsatellite variation confirmed the lack of host-associated genetic structure, as well as differentiation between populations from south-western and south-eastern Europe. The parasitoid L. testaceipes in Europe is thus an opportunistic oligophagous species with a population structure shaped by the processes of introduction and expansion rather than by host exploitation.     

Temperature influences the performance and effectiveness of field and laboratory strains of the ichneumonid parasitoid, <Emphasis Type="Italic">Campoletis chlorideae</Emphasis>

To understand the influence of temperature on host–parasitoid interactions as a consequence of climatic change, we studied development, survival, and fecundity of field and laboratory strains of the Helicoverpa armigera larval endoparasitoid, Campoletis chlorideae at five different temperatures under laboratory conditions. Post-embryonic development period and degree-days required for completing the life cycle by both the strains decreased by 2.5 and 1.5 folds at 27°C compared to 18°C. Post embryonic development period showed a negative (r = −0.99, P < 0.001) and the development rate a positive (r = 0.99, P < 0.001) association with an increase in temperature. However, no parasitoid larvae survived in H. armigera larvae reared at 12 and 35°C after parasitization, suggesting that temperatures ≥35°C as a result of global warming will be lethal for development and survival of immature stages of C. chlorideae. Adult longevity was negatively associated (r = −0.91 to −0.96, P < 0.001) with temperatures between 12 and 35°C. The parasitoid adults stored at 12°C survived for longer period and exhibited higher fecundity than those kept at 27°C, but the efficiency of parasitism and adult emergence were quite low. Sex ratio of the progeny at 12°C was highly male-biased than the insects kept at 27°C. Laboratory strain of the parasitoid exhibited better survival, and the adults lived longer than the field strain at 18°C than at 27°C. Therefore, C. chlorideae adults stored at 18°C could be used for parasitism, while the immature stages should be reared at 27°C for mass production of the parasitoid for biological control of H. armigera.     

Assessing the effects of low temperature on the establishment potential in Britain of the non-native biological control agent Eretmocerus eremicus

Abstract. Eretmocerus eremicus is a parasitoid wasp that is not native to Britain. It is a biological control agent of glasshouse whitefly and has recently been released under licence in Britain for the first time. This study assessed the effect of low temperature on the outdoor establishment potential of E. eremicus in Britain. The developmental threshold calculated by three linear methods was between 6.1° and 11.6 °C, with a degree‐day requirement per generation between 256.3 and 366.8° day^?1. The supercooling points of non‐acclimated and acclimated larvae were similar (approximately ?25 °C). Non‐acclimated and acclimated larvae were subject to considerable pre‐freeze mortality, with lethal temperature (LTemp₅₀₎ values of ?16.3 and ?21.3 °C, respectively. Lethal time experiments indicated a similar lack of cold tolerance with 50% mortality of both non‐acclimated and acclimated larvae after 7 days at ?5 °C, 10 days at 0 °C and 13 days at 5 °C. Field trials showed that neither non‐acclimated nor acclimated larvae survived longer than 1 month when exposed to naturally fluctuating winter temperatures. These results suggest that releasing E. eremicus into British greenhouses would pose minimal risk because typical British winter temperatures would be an effective barrier against establishment in the wild.     

Temperature‐dependent development of the double‐spined spruce bark beetle Ips duplicatus (Sahlberg, 1836) (Coleoptera; Curculionidae)

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Host instar suitability of Aphis gossypii (Hemiptera: Aphididae) for Lysiphlebus testaceipes (Hymenoptera: Braconidae) and parasitism effect on aphid life table

Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae, Aphidiinae) has constituted a well-studied parasitoid insect model, but very little is known about the host-instar suitability of aphid for the wasp so far. One of the hosts of L. testaceipes is Aphis gossypii Glover (Hemiptera: Aphididae). The latter is a serious aphid pest to vegetable production in Benin. Therefore, the objectives of our study were to: (1) examine the oviposition behavior of L. testaceipes on A. gossypii; (2) investigate the host-instar suitability of A. gossypii for L. testaceipes; and (3) compare the life table parameters of A. gossypii with aphids parasitized by L. testaceipes and unparasitized aphids (control). The study was conducted in a laboratory at 26 ± 1 °C in petri dishes and revealed that the parasitoid utilized up to seven stabbing stings to handle and oviposit, particularly in older A. gossypii. In aphids parasitized at the third instar, the net reproductive rate R _o as well as the intrinsic rate of natural increase r _m was significantly lower (2.119 ± 0.272 and 0.110 ± 0.018) compared to the control (15.529 ± 1.287 and 0.272 ± 0.008), respectively (p < 0.01).     

Thermal activity thresholds of the parasitic wasp Lysiphlebus testaceipes and its aphid prey: implications for the efficacy of biological control

Adult parasitoids Lysiphlebus testaceipes Cresson (Hymenoptera: Aphidiidae) lose locomotory function and enter chill coma at significantly lower temperatures (?0.1 and ?8.0 °C, respectively) than their second‐instar hosts, the black bean aphid Aphis fabae Scop. (Hemiptera: Aphididae) (5.6 and 2.3 °C, respectively). Parasitoids are also more heat tolerant, stop walking at 41.4 °C, with heat coma at 44.1 °C, than the aphid (39.1 and 43.0 °C, respectively). Furthermore, across a range of temperatures (0–20 °C), L. testaceipes has considerably faster walking speeds than A. fabae. These data are discussed in relation to the climatic conditions under which L. testaceipes would be an effective control agent, and the likelihood of establishment and spread in northern European climates.     

Effects of host stages and temperature on population parameters of Oomyzus sokolowskii, a larval-pupal parasitoid of Plutella xylostella

Oomyzus sokolowskii is alarval-pupal parasitoid of diamondback moth, Plutella xylostella. In a host stage preference test, the parasitoid parasitised all larval and pupal stages, but exhibited a strong preference for larvaeover prepupae or pupae, and did not show a preference among the larval instars. At 25°C, the developmental time, number and sex ratio of offspring per host pupa, and successful parasitism did not differ significantly among parasitoids reared from host larvae of different instars, indicating similar host suitability between larvae of different instars. Mean developmental times from egg to adult at 20, 22.5, 25, 30, 32.5, and 35°C were 26.5,21.0, 16.0, 12.7, 11.9 and 13.4 days, respectively. The favourable temperature range for development, survival, and reproduction of the parasitoid was 20--30°C. However, wasps that developed and emerged at a favourable temperature could parasitise effectively at 32--35°C for 24 hours. Life-fertility table studies at 20, 25, and 30°C showed that each female wasp on average parasitised 3.1, 13.2, 6.8 larvae of diamondback moth and produced 20.5, 92.1, 50.4 offspring, respectively, during her lifetime. The highest intrinsic rate of natural increase (r _m) of 0.263 female/day was reached at 30°C as a result of the short mean generation time at this temperature compared to that at 20 and 25°C, suggesting that the parasitoid had the highest potential for population growth at relatively high temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Establishment potential of the predatory mirid <Emphasis Type="Italic">Dicyphus hesperus</Emphasis> in northern Europe

The predatory mirid Dicyphus hesperus Knight (Hemiptera: Miridae) is native to North America. The species has been used for the control of glasshouse whitefly on aubergine in the Netherlands, and is currently being evaluated for continued and wider release in Europe. Field and laboratory studies were conducted on a population collected from southern California, USA, to assess the cold tolerance and potential for outdoor establishment under prevailing northern European climates. The supercooling points (whole animal freezing temperatures) of nymphal and adult insects were around −20°C. The lethal temperatures (LTemp₅₀) of non-diapausing nymphs and adults and diapausing adults were close to their respective freezing temperatures at −17.6, −17.6 and −19.2°C. At 5°C, the LTime₅₀ was 54, 101.7 and 117.5 days for fed nymphs, non-diapausing and diapausing adults respectively. When first instar nymphs were placed in the field in winter, starved samples died out after 70 days, but 5% of the fed nymphs survived until the end of winter (140 days) and developed to adult on return to the laboratory. After a similar 5-month field exposure, 50% of fed diapausing adults and 15% of fed non-diapausing adults were still alive at the end of winter, whereas starved diapausing adults died after 140 days. On return to the laboratory after 5 months in the field, both diapausing and non-diapausing adults mated and laid eggs, forming viable populations. Overall, the field and laboratory experiments indicate that this population of D. hesperus is able to enter diapause and that winter temperatures are not a barrier to establishment in northern Europe.     

Life history and coldhardiness ofAthrycia cinerea (Dipt.: Tachinidae) in western Canada

Athrycia cinerea Coq. is a univoltine parasitoid of the bertha armyworm,Mamestra configurata Walker (Lepidoptera: Noctuidae), in western Canada. This parasitoid overwinters as a pupa in the soil. These pupae are less sensitive than their host to increased cold stress caused by lower temperatures or danger durations of exposure. Exposure to −7.5°C for 140 days did not significantly reduce survival. Survival decreased with exposure to temperatures from −10 to −20°C, but survival was 48% even after 40 days exposure to −20°C. The frequency distribution ofA. cinerea puparia per host is highly contagious. Other aspects of the life history are described. Contribution No. 1403 Winnipeg Research Station.     

Effects of acclimation and diapause on the cold tolerance of Trogoderma granarium

Khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae), is a pest of stored grain in Africa, Asia, and Europe. It is a quarantine insect for much of the rest of the world. Control of T. granarium can be achieved with methyl bromide, but this fumigant is an ozone‐depleting substance and is being phased out worldwide. Thus, there is an urgent need to find new methods of control, including the use of low temperatures. Here, we assess the effects of diapause and cold acclimation on the cold tolerance of T. granarium. The percentage of larvae in diapause increased with larval density, reaching 57.3% when reared at a density of 73 larvae g^?1 diet. The cold tolerance of T. granarium was assessed by the supercooling points (SCPs) of various life stages. The SCP of non‐acclimated insects ranged from ?26.2 ± 0.2 °C (mean ± SEM) for eggs to ?14.4 ± 0.4 °C for larvae. The lowest SCP for larvae, ?24.3 ± 0.3 °C, was obtained for diapausing‐acclimated larvae. Based on mean LT₅₀ values, the most cold‐tolerant stage at ?10 °C was the diapausing‐acclimated larvae (87 days) followed by non‐diapausing‐acclimated larvae (51 days), diapausing non‐acclimated larvae (19 days), adults (4 days), non‐diapausing non‐acclimated larvae (2 days), pupae (0.4 days), and eggs (0.2 days). The estimated times to obtain 99.9968% mortality (Probit 9) for diapausing‐acclimated larvae are 999, 442, 347, 84, and 15 days at 0, ?5, ?10, ?15, and ?20 °C, respectively. Probit 9 is an estimated value used by quarantine experts to estimate conditions that are required to kill all insects. In light of the long exposure time needed to control T. granarium even at ?20 °C, cooling to below ?27 °C (i.e., below the SCP of eggs) will quickly kill all life stages and may be the best way to control this insect with low temperatures.     

Freeze tolerance in larvae of the winter-active Diamesa mendotae Muttkowski (Diptera: Chironomidae): a contrast to adult strategy for survival at low temperatures

The winter-active Diamesa mendotae Muttkowski (Diptera: Chironomidae) is freeze intolerant in the adult stage with a low mean supercooling point (SCP) of ~−20 °C. However, cold-hardiness strategies for immatures of this species are unknown. In this study, we measured SCP values for D. mendotae larvae, pupae and adults using surface-contact thermometry. In addition, the lower lethal temperature (LLT) was determined for the larval stage. The mean SCPs for larvae (−7.4 °C) and pupae (−9.1 °C) were relatively high compared to adults (−19.7 °C). Our results indicate that the larvae of D. mendotae are freeze tolerant with a LLT₉₉ (−25.4 °C), ~−10 °C lower than their minimum SCP (−15.6 °C). Freeze tolerance in these larvae may be a strategy to provide protection from short-term exposures to ice crystals or to permit diapause within frozen substrates. The change in cold-hardiness strategy from freeze tolerant to freeze intolerant between the larval and adult stages of this species is likely a result of the different habitats occupied by these two life stages.     

Biology of Lysiphlebus fabarum following cold storage of larvae and pupae

Cold storage is one means of preserving parasitoids prior to release in augmentation biological control programs. This study examined the feasibility of storing larval and pupal stages of a sexual population of Lysiphlebus fabarum Marshall (Hymenoptera: Braconidae: Aphidiinae) at 6 ± 1 and 8 ± 1 °C, 50–60% r.h., and L14:D10 photoperiod. These life stages were stored for periods of 1, 2, and 3 weeks under fluctuating thermal regimes (2 h daily at 21 ± 1 °C). Generally, pupae gave better results than larvae, and 6 °C was better than 8 °C, considering wasp survival, wasp size (tibial and antennal lengths), egg load, and egg size. The best results were obtained with pupae stored for 2 weeks under a fluctuating temperature regime at 6 °C. Females emerging from this treatment did not differ from controls (developing directly at 21 °C) in body size, egg size, or progeny sex ratio, and suffered less than 20% mortality. Egg loads were reduced in these wasps, but the reductions were substantially less than occurred in other 2‐week‐storage treatments. Wasps stored in this manner successfully parasitized similar numbers of aphids as controls and produced similar progeny sex ratios. These results reveal a suitable set of low‐temperature conditions that can be used to delay the development of L. fabarum for 2 weeks with minimal impact on wasp fitness.     

Morphology and development of <Emphasis Type="Italic">Pteromalus cerealellae</Emphasis> (Ashmead) (Hymenoptera: Pteromalidae) on <Emphasis Type="Italic">Callosobruchus maculatus</Emphasis> (F.) (Coleoptera: Chrysomelidae)

Pteromalus cerealellae (Ashmead) (Hymenoptera: Pteromalidae) is an ectoparasitoid of several stored-product insect pests. Very little information has been published on its biology and development in host larvae, which typically are concealed within seeds. We documented the development of P. cerealellae within fourth instar larvae of its concealed host, Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) infesting cowpea seeds. The preimaginal life stages of the parasitoid were characterized for the first time using morphological structures revealed by microscopic techniques including scanning electron microscopy. Pteromalus cerealellae produces hymenopteriform eggs and larvae. Eggs hatch into 13-segmented first instar larvae with peripneustic condition of spiracles. The larvae have simple, tusk-like mandibles, whereas the mandibles of the pupae and the adults are of the conventional toothed types. Using statistical analyses of the sizes of the larval mandibles and head capsules in conjunction with reliable characters such as the number of exuviae on the body of parasitoid larvae, cuticular folding, and excretion of the meconium, we recorded four larval instars for P. cerealellae. The data showed significant positive correlations between larval mandible lengths and widths of larval head capsules, as well as between mandible lengths and larval instars, suggesting that mandible length is a good predictor of the number of instars in P. cerealellae. Developmental time from egg to adult emergence was ∼12 d for females and ∼11 days for males at 30 ± 1°C, 70 ± 5% r.h. and 12L:12D photoperiod.     

Parasitism of the house fly parasitoid <Emphasis Type="Italic">Spalangia cameroni</Emphasis> on Norwegian pig farms: local effect of release method

Mass release of parasitoids (Hymentoptera: Pteromalidae) is one possible control method of house flies (Musca domestica L.) on livestock farms. To improve the success of this method, however, there is a need for more detailed recommendations. In the present study, parasitism was evaluated in and around pens following release of the parasitoid Spalangia cameroni Perkins by hand and from containers. The study was conducted at conventional Norwegian pig farms with scattered breeding grounds for house flies. The experiment was carried out twice, with a total of seven trials of each release method followed by four weeks of monitoring parasitism by house fly sentinel pupae. No significant difference was found between the two release methods. Parasitism decreased with temperature (range 18–23°C) and was low on farms with few sites for the parasitoids to hide.     

Effect of parasitism on flight behavior of the soybean aphid, Aphis glycines

Many aphid species possess wingless (apterous) and winged (alate) stages, both of which can harbor parasitoids at various developmental stages. Alates can either be parasitized directly or can bear parasitoids eggs or larvae resulting from prior parasitism of alatoid nymphs. Winged aphids bearing parasitoid eggs or young larvae eventually still engage in long-distance flights, thereby facilitating parasitoid dispersal. This may have a number of important implications for biological control of aphids by parasitoids. In this study, we determined the effect of parasitism by Aphelinus varipes (Hymenoptera: Aphelinidae) on wing development and flight of the soybean aphid, Aphis glycines (Hemiptera: Aphididae). We also quantified the influence of aphid flight distance on subsequent A. varipes development. Parasitism by A. varipes was allowed at different A. glycines developmental stages (i.e., alatoid 3rd and 4th-instar nymphs, alates) and subsequent aphid flight was measured using a computer-monitored flight mill. Only 35% of aphids parasitized as L3 alatoid nymphs produced normal winged adults compared to 100% of L4 alatoids. Flight performance of aphids parasitized as 4th-instar alatoid nymphs 24 or 48 h prior to testing was similar to that of un-parasitized alates of identical age, but declined sharply for alates that had been parasitized as 4th-instar alatoid nymphs 72 and 96 h prior to testing. Flight performance of aphids parasitized as alate adults for 24 h was not significantly different from un-parasitized alates of comparable ages. Flight distance did not affect parasitoid larval or pupal development times, or the percent mummification of parasitized aphids. Our results have implications for natural biological control of A. glycines in Asia and classical biological control of the soybean aphid in North America.     

Assessment of Ceratitis capitata (Diptera,Tephritidae) pupae killed by heat or cold as hosts for rearing Spalangia cameroni (Hymenoptera: Pteromalidae)

In this work, we study the suitability of using dead medfly Ceratitis capitata pupae, killed by heat- or cold-shock, for the mass rearing of Spalangia cameroni, a pupal parasitoid of key pests. 100% mortality of medfly pupae could be accomplished with cold-shock at –20°C for 60 min or with heat-shock at 55°C for 30 min. Neither parasitism percentage nor sex ratio of the offspring differed significantly among heat-shocked, cold-shocked and untreated pupae. In addition, there was no significant difference in the percentage of parasitoids that aborted (♂♂ or ♀♀) among pupal treatments. Some of the pupae were covered with peat because the third larval instar of the medfly buries itself before pupation. However, the buried pupae were not parasitised at a greater or lesser rate than those not covered with peat. The percentage of parasitism was also unaffected by whether the pupae had been killed recently or had been stored at between 4°C and 6°C over 15 or 30 days. The use of dead hosts and later storage permitted the following: (a) the use of hosts over long periods of time; (b) a rapid increase in parasitoid numbers and (c) the availability of pupae killed at the most suitable postpupation times for the production of parasitoids. Furthermore, in biological control projects, the use of dead parasitised pupae in the field avoids the risk of enhancing the pest and allows an increase in parasitism in the field through the use of pupae treated with cold- or heat-shock.