Parasitism,longevity and progeny production of six indigenous Kenyan trichogrammatid egg parasitoids (Hymenoptera: Trichogrammatidae) at different temperature and relative humidity regimes

Six native Kenyan species/strains of Trichogramma and Trichogrammatoidea, recovered from Helicoverpa armigera were evaluated at six different temperatures (10, 15, 20, 25, 30 and 35°C) and two relative humidity levels (40–50 and 70–80%) with the aim of selecting strains adapted to warmer temperature regimes. The species/strains were collected from low (<700 m), medium (between 700 and 1200 m) and high altitude (>1200 m) locations and were evaluated for parasitism, adult longevity, progeny production and progeny sex ratio at the different environmental regimes. Eggs of the factitious host, Corcyra cephalonica Stainton (Lepidoptera: Pyralidae) were used in the investigations. Temperature and humidity interactions affected parasitism and progeny production. The highest parasitism at the two humidity levels was at 25 and 30°C for all the strains evaluated. Adult longevity was also significantly affected by the interaction of temperature and relative humidity and was longer at the lower than higher relative humidity. Survival followed a type I survivorship curve at lower temperatures and a type III survivorship curve at the higher temperatures. Trichogramma sp. nr. mwanzai from low altitude, Trichogramma sp. nr. mwanzai from medium altitude and Trichogrammatoidea sp. nr. lutea also from medium altitude lived longer than other strains at all the temperatures and relative humidity levels evaluated, including the warmest regimes of 30 and 35°C. These strains appear promising as candidates for augmentative biocontrol of H. armigera in Kenya.     

Performance of Trichogrammatoidea sp. nr lutea Girault (Hymenoptera: Trichogrammatidae) collected from Mussidia spp. in Kenya on eggs of six lepidopteran hosts

Mussidia nigrivenella Ragonot (Lepidoptera: Pyralidae), an important pest of maize ears in West Africa, has never been reported to attack crops in East and southern Africa (ESA), though it was found on various wild host plants in these regions. It was suggested that in ESA M. nigrivenella might be under natural control. In Kenya, exploration for natural enemies associated with Mussidia spp. yielded several parasitoids including a trichogrammatid egg parasitoid, Trichogrammatoidea sp. nr lutea Girault. The ability of T. sp. nr lutea to attack the eggs of several lepidopteran species found in Kenya was studied. The lepidopterans included the noctuids Busseola fusca (Fuller) and Sesamia calamistis (Hampson), the pyralids Eldana saccharina Walker, Mussidia fiorii Cecconi and de Joannis and Mussidia‘madagascariensis’, and the crambid Chilo partellus (Swinhoe). The former three species also infest cereals in West Africa. Trichogrammatoidea sp. nr lutea successfully attacked and developed in eggs of all six species indicating its potential to exploit other lepidopteran pests of maize in West Africa. Busseola fusca and S. calamistis were the most suitable hosts and had the largest number of eggs parasitized and progeny per female wasp where E. saccharina and C. partellus were the poorest hosts. The host species used to rear the parasitoid and the age of egg also significantly affected the total number of host eggs parasitized by the parasitoid. It was concluded that the ability of T. sp. nr lutea to exploit lepidopterans that are also pests of maize in West Africa may enhance biological control of M. nigrivenella and it should be considered for translocation to that area from Kenya.     

Trichogramma parasitoids for control of Lepidopteran borers in Taiwan: species,life‐history traits and Wolbachia infections

Trichogramma, polyphagous endoparasitoids of lepidopteran eggs, are used against a variety of crop pests throughout the world including those of sugar cane and corn in south‐eastern Asia. Their ability to be easily and economically reared on factitious hosts and their wide host range have contributed to their widespread use in pest control. The overall aim of this study was to select strains for eventual release in crop areas for control of lepidopteran borer pests of sugar cane and corn. To this end, we identified common Trichogramma species emerging from corn borer egg masses throughout south‐western Taiwan, compared their life‐history characteristics, assessed their thermal limits and identified the Wolbachia infection status of collected Trichogramma parasitoids. Trichogramma ostriniae was the most commonly collected species on corn, with occasional detection of T. chilonis and an unidentified species designated as T. sp. y. Although the sex ratio varied widely between sites, Wolbachia infection was detected only at a single site in one species (T. ostriniae). Wolbachia‐infected T. ostriniae were tolerant to high temperature stress. Trichogramma chilonis had lowest fecundity of the three species tested, and a Wolbachia‐infected T. ostriniae strain had lower fecundity than an uninfected strain. Given the limited availability of distribution and historical data for Trichogramma species in Taiwan, the current study provides a baseline for future work and also highlights the importance of accurately identifying species when establishing colonies of natural enemies for biocontrol.     

Assessment ofTrichogramma species for biological control of forest lepidopteran defoliators

In a laboratory study, we determined the potential of threeTrichogramma (Hymenoptera: Trichogrammatidae) species,T. brassicae Bezdenko,T. minutum Riley andT. nr.sibiricum Sorokina, for biological control against six species of forest lepidopteran pests, black army cutworm, hemlock looper, eastern spruce budworm, western spruce budworm, white-marked tussock moth, and gypsy moth. Females of each parasitoid species were offered eggs from each of the six host species. Parasitization and the effect of the host species on the emerging progeny were examined and recorded.Trichogramma minutum had the broadest host range and successfully parasitized four host species out of the six offered.Trichogramma nr.sibiricum had the narrowest host range and parasitized only two species of hosts. Of the six host species, black army cutworm was the most preferred by all threeTrichogramma species; white-marked tussock moth and gypsy moth were not parasitized by any parasitoids. There was a positive correlation between the size of female offspring and their corresponding egg complement in all three parasitoid species. The developmental time of parasitoids from egg to adult was influenced by both the parasitoid and host species. Our results suggest thatT. minutum has the greatest potential for biological control against various forest lepidopteran pests and that the black army cutworm may be the best target candidate for further study.     

Evaluation of persistence and relative toxicity of some pest control products to adults of two native trichogrammatid species in Kenya

The utilization of native trichogrammatids for biocontrol of Helicoverpa armigera (Hbn.) and their potential integration with pesticide use are currently receiving attention. In this study the interaction of adults of Trichogramma sp. nr. mwanzai and Trichogrammatoidea sp. nr. lutea with commonly used pesticide products was investigated. The toxicity of eight pest control products commonly used in vegetable crops in Kenya, were evaluated by exposing the adults of the two species to detached potted tomato leaves at different intervals after spraying. Two biologically derived products, Achook^? (neem-based) and Dipel^? (Bt ssp. kurstaki)—were found to be harmless and had no persistent toxicity on both the trichogrammatid species. Two organophosphate products tested, dimethoate (Rogor^?) and malathion (Malathion^?) were found to be ‘slightly harmful’ and ‘moderately persistent’ respectively. Three other synthetic insecticides, lambdacyhalothrin (Karate^?), bifenthrin (Brigade^?) and alpha-cypermentrin (Fastac^?) were found to be ‘moderately harmful’ and ‘persistent’ respectively. Polytrin^? (mixture of cypermethrin and profenofos) was also found to be persistent but only ‘slightly harmful’. On the basis of five concentrations tested (0.032, 0.016, 0.008, 0.004 and 0.002 of the recommended field rates) the LC₅₀ values for adult T. sp. nr. mwanzai were estimated as 285, 411, 435 and 1,916 (μg active ingredient (a.i) ml^?1) for dimethoate; malathion; lambdacyhalothrin as well as cypermethrin?+?profenofos respectively. The corresponding values for T. sp. nr. lutea were 247, 251, 278 and 697 respectively. Further, Karate^? appeared to be the least toxic among the four products, across all the respective concentrations. The study was an attempt to adopt the methodologies of the IOBC (International Organization for Biological Control) on non-target risk assessment for pest control products to cater for the local needs of integrating the use of the trichogrammatids along with other pest control products.     

Suitability assessment of three Trichogramma species in the control of Mythimna separata (Lepidoptera: Noctuidae)

Mythimna separata (Walker) is a serious cosmopolitan pest. Trichogramma species have been used worldwide as biological control agents of lepidopteran pests. However, no data on the control efficiency of Trichogramma species on this pest have been reported. We evaluated the suitability of M. separata eggs for three Trichogramma species, Trichogramma dendrolimi Matsumura, T. ostriniae Pang et Chen and T. japonicum Ashmead, and compared the suitability with that of Corcyra cephalonica (Stainton) eggs. The parasitism rate, development time, emergence rate, female proportion, body size and egg load of female offspring indicated that the M. separata egg is a suitable host for the three Trichogramma species. On the other hand, the non‐emergence rate and the relationship between egg load and body size reflected that M. separata eggs are less suitable for the three Trichogramma species than C. cephalonica eggs. Egg resorption occurred in T. japonicum 4 days after emergence.     

Natural occurrence of Wolbachia-infected and uninfected Trichogramma species in tomato fields in Portugal

Minute egg parasitoids of the genus Trichogramma (Hymenoptera; Trichogrammatidae) are promising candidates for biological control of lepidopteran pests in tomato in Portugal. This certainly applies to native Trichogramma strains that have thelytokous reproduction, i.e., produce only daughters. In Trichogramma wasps, thelytoky is mostly induced by the intracellular bacterium Wolbachia. In this study, we carried out a field survey of native Trichogramma species in four locations in Ribatejo, the main processing tomato region of Portugal, and determined the prevalence of Wolbachia in those species. Five Trichogramma species were found to emerge from lepidopteran eggs collected in the field, namely Trichogramma bourarache, Trichogramma cordubensis, Trichogramma evanescens, Trichogramma pintoi, and Trichogramma turkestanica. T. evanescens and T. pintoi were by far the dominating species representing, respectively, 64.9 and 26.4% of the trichogrammatids collected. Total natural parasitism rates of the collected lepidopteran eggs by Trichogramma wasps ranged from 28.2 to 64.6%. Three Trichogramma species were found to be infected with Wolbachia, namely T. cordubensis, T. evanescens, and T. turkestanica. All the wasp broods belonging to T. cordubensis were infected, whereas low infection rates were found in T. evanescens (0.9% of the broods) and T. turkestanica (4.5% of the broods). The latter represents the first record of a Wolbachia infection in T. turkestanica. Sequencing of the Wolbachia surface protein, wsp, revealed this Wolbachia infection to be related to other Wolbachia infections in Trichogramma wasps. As Wolbachia-infected thelytokous strains exist for T. evanescens, the most abundant Trichogramma species naturally occurring in the tomato fields of the Ribatejo region, this species offers interesting and powerful options for biological control of lepidopteran pests in processing tomato in this region.     

Inbreeding and incompatibility in Trichogramma nr. brassicae: evidence and implications for quality control

Inbreeding effects and incompatibility relationships were examined in strains of the egg parasitoid Trichogramma nr brassicae (Hymenoptera: Trichogrammatidae) from southeastern Australia. Crosses between strains provided weak evidence of incompatibility in a few cases. However sex ratio in crosses within strains tended to be more female-biased than in crosses between strains. Inbreeding was imposed for four generations (F>0.59) of sib mating. The fitness of inbred strains was compared to that of outbred strains generated by crossing the inbred strains. No effects of inbreeding were found for any of the four female traits examined (fecundity, body length, head width and hind tibia length), indicating that T. nr. brassicae is not subjected to inbreeding depression. Inbreeding effects were also not found for male mating success as expected for the haploid sex. There were differences among strains for all traits apart from fecundity, indicating heritable variation. Strain differences for fitness measures were uncorrelated with wasp size. The potential use of inbreeding in the quality control of Trichogramma for mass-release is discussed. Inbreeding may be a useful tool in minimising the effects of laboratory adaptation, thereby extending the useful life of a strain.     

Scope for genetic enhancement of the parasitisation potential of four native strains of Trichogrammatoidea sp. nr. lutea Girault (Hymenoptera: Trichogrammatidae) in Kenya

In response to emerging interest in commercial mass production of Trichogramma for Helicoverpa armigera biocontrol in eastern Africa, laboratory experiments were undertaken to assess the scope for genetic enhancement of the parasitisation potential of native strains of the local common trichogrammatid species, Trichogrammatoidea sp. nr. lutea. Four promising strains (ex-Kilifi – Kilifi District, ex-Kwa Chai – Kibwezi District, ex-Rarieda – Bondo District and ex-Ebuhayi, Kakamega District) were tested for cross-mating in reciprocal combinations with focus on fecundity and progeny female ratio. While all the crosses resulted in F1 progeny of both sexes, significant differences were observed between homogamic and reciprocal heterogamic crosses in fecundity, progeny production, proportion of female progeny and adult longevity. Among all the crosses, the cross between ex-Rarieda strain females and ex-Kilifi strain males resulted in progeny that was significantly superior in fecundity and progeny female ratio. Conversely, Kilifi strain females crossed to males from ex-Rarieda strain gave rise to progeny with relatively low fecundity and female ratio. There were significant differences between homogamic crosses and most reciprocal heterogamic crosses in the major biological attributes. Genotypic and phenotypic variance-covariance matrices generated for six life-history traits showed high positive correlations for most traits in both inbred (P<0.05) and reciprocal heterogamic crosses (P<0.05 and P<0.001). Fecundity and number of female offspring were the most important factors in the heterogamic crosses. The results confirmed the scope for genetic enhancement through inter-strain crossing for improving the field impact potential of T. sp. nr. lutea being targeted for commercial mass production.     

Iranian <Emphasis Type="Italic">Trichogramma</Emphasis>: ITS2 DNA characterization and natural <Emphasis Type="Italic">Wolbachia</Emphasis> infection

Egg parasitoids of the genus Trichogramma (Hymenoptera: Trichogrammatidae) are important natural enemies of many lepidopterous pests in agricultural and forest areas. Because the use of indigenous Trichogramma populations/species can significantly increase the biological control efficiency, the characterization of endemic populations and precise species identification is important. In this study, Trichogramma wasps were collected from parasitized eggs of several lepidopteran pests in Northern and Central Iran. Seven Trichogramma species were identified based on the morphology and the nucleotide sequence, size and restriction profile (PCR-RFLP) of the internal transcribed spacer 2 (ITS2) region of the rDNA of T. brassicae Bezdenko, T. cacoeciae Marchal, T. embryophagum Hartig, T. evanescens Westwood, T. euproctidis Girault, T. pintoi Voegelé and T. tshumakovae Sorokina. This is the first report of T. euproctidis in Iran. Additionally, natural populations were surveyed for the prevalence of Wolbachia. Two populations of T. brassicae were found to be infected by a single Wolbachia strain which was identified by using a multi-locus sequencing approach.     

Temperature-dependent Development of Four Egg Parasitoid Trichogramma Species (Hymenoptera: Trichogrammatidae)

Trichogramma species have been successfully utilized for biocontrol of several lepidopteran pests worldwide. The development, survival and progeny production of two Kenyan species' Trichogramma bournieri Pintureau & Babault and Trichogramma sp. nr. mwanzai Schulten & Feijen (collected from Kenya), Trichogramma evanescens Westwood (Germany) and Trichogramma chilonis Ishii (India) was studied at four constant temperatures (13, 18, 25 and 34°C) with the 011 aim of assessing the relative biotic potential of the two native species for 011 biocontrol of Helicoverpa armigera and Plutella xylostella in Kenya. The study was conducted at the Institute 011 for Biological Control (BBA), Darmstadt, Germany. The Trichogramma species tested showed variations 011 in fertility, developmental time, percent emergence, progeny production and sex ratio 011 at the four temperature regimes. Fertility decreased as temperature increased from 25 to 34°C. 011 T. chilonis and T. evanescens completed development at all temperatures tested, but T. 011 bournieri and T. sp. nr. mwanzai failed to complete development at 13°C. The developmental 011 period for all the species decreased as the temperature increased. The duration of development from 011 oviposition to adult emergence varied from 8 days to 12 weeks shorter at 34°C than at 011 13°C for T. chilonis and T. evanescens . For the various temperatures tested, a linear model was 011 satisfactory for egg to adult development at P = 0.05 for T. chilonis and T. evanescens . The 011 lower temperature thresholds for development and duration in degree-days were 8.83°C and 188 for 011 T. chilonis and 9.23°C and 192 for T. evanescens , respectively. For all temperatures tested, 011 T. sp. nr. mwanzai had the highest preimaginal survivorship. Adult emergence was lower at 13°C and 34°C than at 011 18 and 25°C. The highest fertility (mean ±SE) (50.37 ±2.32 adult 011 female -1 ) and progeny production (44.03 ±2.02 adult female -1 ) was recorded at 25°C for 011 T. evanescens . Sex ratio was biased towards female at all temperatures in T. bournieri and T. chilonis . 011 At all temperatures tested, T . sp. nr. mwanzai produced more males than females. For all species tested, 011 favourable parasitism was between 18 and 25°C. The results from this study will be useful for mass rearing purposes as well as for future field release programmes.     

Phoresy in the field: natural occurrence of Trichogramma egg parasitoids on butterflies and moths

Phoretic insects utilize other animals to disperse to new environments. We recently discovered how egg parasitoids use an exciting phoretic strategy to reach egg-laying sites of their butterfly hosts. In the laboratory, female Trichogramma wasps detect and mount mated female cabbage white butterflies that emit an anti-aphrodisiac pheromone. Hardly any information exists about the natural occurrence of phoresy in wasps of this genus. Therefore, we monitored the presence of phoretic Trichogramma wasps on lepidopteran hosts in the field. Only female wasps were found at low prevalence on six lepidopteran species. Wasps were mostly found on female hosts and mainly on abundant solitary host species. This is the first report of phoretic Trichogramma wasps on butterflies in nature. We suggest that phoresy is only one of several strategies used by these polyphagous egg parasitoids. The evolution of phoresy is discussed in relation to the nutritional ecology of egg parasitoids.     

Variation in biological parameters of Trichogramma spp. purchased from commercial suppliers in the United States

The efficacy of biological controlobtained through releases of commerciallysupplied Trichogramma is influenced by bothabiotic conditions in the field during andfollowing release and an array of biologicalattributes of the released Trichogramma. Thisstudy was undertaken to assess the degree ofvariation that existed among commerciallysupplied Trichogramma in an array of biologicalparameters that potentially may influence thelevel of control that can be expected in aTrichogramma release program. Shipments ofTrichogramma were obtained from 12 commercialsuppliers of beneficial insects over a 2-yearperiod and the following parameters weremeasured: percentage of emergence duringshipping, total percentage of emergence,percentage of females, percentage ofbrachypterous females, percentage ofbrachypterous males, adult female longevity,and species composition. In addition, weestimated the percentage of macropterousfemales that had not emerged prior to the timethe shipment was received, based on the firstfour parameters. Results indicate a high levelof variation in each of these parameters amongsuppliers of Trichogramma and among shipmentsby individual suppliers. The proportion ofnon-emerged macropterous female wasps was lowfor all suppliers (range 2.5–27.7%). Thiswas primarily attributable to the highincidence of brachyptery among female wasps. However, variation among suppliers in theincidence of emergence during shipping and theproportion of female wasps in a shipment alsocontributed to the low proportion ofnon-emerged macropterous female wasps. Inaddition, in 11 of 24 shipments received duringthe 2-year study, the Trichogramma speciesdiffered from that specified by the shipper orthe shipment contained a mixture ofTrichogramma species.     

Nocturnal parasitism of moth eggs by Trichogramma wasps

Parasitoid wasps of the genus Trichogramma are used worldwide as biological control agents against lepidopteran pests. Trichogramma wasps develop inside eggs of a wide range of host species, most of them moths. They are generally considered as diurnal insects. Here, we investigated whether Trichogramma wasps can also successfully parasitise host eggs at night under controlled laboratory conditions. Eggs of the moth Ephestia kuehniella were offered under dark conditions (scotophase) to females of Trichogramma brassicae and Trichogramma evanescens either from 9:00 PM to 9:00 AM or from 11:00 AM to 5:00 PM at four different temperatures (5°C, 10°C, 15°C and 20°C). Both species are known to parasitise E. kuehniella eggs in the photophase during daytime. The results show that T. brassicae did not parasitise eggs in the scotophase at night and only very few in the artificially induced scotophase during daytime from 10°C to 20°C. In contrast, T. evanescens parasitised more eggs in the dark both at night and artificially induced scotophase during daytime. Parasitism in the scotophase already started at 5°C, with more eggs being parasitised and more offspring being produced at higher temperatures. T. evanescens displayed higher parasitism activity in the induced scotophase during daytime than in the scotophase at night. The present study suggests that Trichogramma are capable of successfully parasitising host eggs at night, even at low temperatures, but that nocturnal activity with respect to parasitism varies between wasp species.     

Molecular identification of two Trichogramma species (Hymenoptera: Trichogrammatidae) in Korea

Trichogramma wasps parasitize eggs of various insect species. Several Trichogramma species have been commercialized to manage Lepidopteran agricultural pests. Correct species identification is essential for successful biological control. However, the microscopic size and morphological similarity of Trichogramma species makes correct identification very difficult. Internal transcribed spacer (ITS)-2 gene sequences have been used to identify Trichogramma species. Using this molecular marker, Trichogramma species that are commercially available in Korea were identified as Trichogramma ostriniae and Trichogramma brassicae. The latter has not previously been reported in Korea. Additionally, Cadra cautella and Sitotroga cereallela eggs (host eggs of the commercialized Trichogramma species in Korea) were infected with Wolbachia. However, Trichogramma were not infected with the bacterium. This indicates that horizontal transfer of Wolbachia does not occur from host to wasp.     

IndigenousTrichogramma (Hym.: Trichogrammatidae) parasitizing eggs ofRhopobota naevana (Lep.: Tortricidae) on cranberries in British Columbia

Two indigenous species ofTrichogramma were reared from natural populations of the blackheaded fireworm,Rhopobota naevana (Hübner) infesting cranberries in British Columbia, Canada. One was identified asTrichogramma sp. nr.sibericum Sorokina and the other wasTrichogramma minutum Riley. Percentage of parasitized hosts increased during the season in an abandoned field, but decreased in a commercial field where pesticides were used. The potential ofTrichogramma as a biological control agent against the fireworm is discussed.     

The relationship between egg load and fecundity among Trichogramma parasitoids

1. The relationships between parasitoid egg load, size, and age (3–72 h) for Trichogramma minutum, T. platneri, and T. pretiosum, reared from two factitious hosts, Ephestia kuehniella and Sitotroga cerealella, were evaluated to test the hypothesis that 24‐h egg load can be used to estimate the fecundity of Trichogramma parasitoids. 2. Egg load increased in relation to female age over the first 3 days of adult life for all three Trichogramma species to a mean egg storage capacity of 46.7 eggs for T. minutum, 41.1 for T. pretiosum, and 35.7 for T. platneri. At 24 h of age, T. minutum had matured enough eggs to fill 67% of its storage capacity, in comparison with 74% for T. pretiosum and 91% for T. platneri. There was a positive relationship between egg load and parasitoid size for all ages of the three Trichogramma species reared from both hosts (with the exception of T. platneri at 3 h post emergence), accounting for 14–69% of the variance in egg load. 3. The potential fecundity, realised (3 day cumulative) fecundity, and oviposition rate (potential fecundity/longevity) of T. platneri were all related linearly to size‐dependent variation in 24‐h egg load, but only the realised fecundity of T. pretiosum, and none of the reproductive characteristics of T. minutum. It is suggested that 24‐h egg load may not be an accurate measure of egg storage capacity in parasitoids and should be used cautiously to represent fecundity. 4. The potential fecundity of seven Trichogramma species reared from E. kuehniella varied from 55 to 150, but neither potential fecundity nor oviposition rate was related significantly to egg load (represented by eggs laid during first 24 h). Selection to avoid egg depletion in the attack of gregarious hosts appears most likely to account for the variation in potential fecundity among Trichogramma species.     

The effect of temperature and humidity on the bionomics of six African egg parasitoids (Hymenoptera: Trichogrammatidae)

The life table statistics of six native Kenyan species/strains of Trichogramma and Trichogrammatoidea were established using a factitious host Corcyra cephalonica, Stainton (Lepidoptera: Pyralidae), at eight different temperatures (10, 15, 20, 25, 28, 30, 32 and 35 degrees C) and two humidity levels (40-50 and 70-80%). The objective was to select insects with superior attributes for augmentative release against lepidopteran pests in horticultural crops. Both temperature and humidity affected developmental time and life table parameters of the parasitoids but temperature played a more critical role. Developmental time was inversely related to temperature. The intrinsic and finite rates of increase increased with temperature up to 30 degrees C. Both net reproduction rate and intrinsic rate of increase were higher at the lower humidity. Temperature inversely affected generation time of parasitoid strains regardless of the relative humidity. Two strains of Trichogramma sp. nr. mwanzai collected from both low and medium altitudes and Trichogrammatoidea sp. nr. lutea from the mid-altitudes, were better adapted to both low and high temperatures than the other strains, as indicated by the high intrinsic and net reproductive rates, at both humidity levels. These three strains appear to be promising candidates for augmentation biocontrol against the African bollworm Helicoverpa armigera in Kenya.     

Maternal thermal effect on diapause in Trichogramma species (Hymenoptera: Trichogrammatidae)

Both direct thermal and maternal photoperiodic effects on diapause induction have been thoroughly investigated in many insect species, while maternal thermal effects have been infrequently studied. We studied the effect of temperature during development of maternal generation on the proportion of diapausing progeny in four species of the genus Trichogramma Westw., minute egg parasitoids, widely used for biological control of lepidopteran pests. The maternal generations were reared at day lengths of 12 and 18 h and temperatures of 17, 20, 25 and 30°C, and their progeny developed under day length of 12 h and temperatures of 13 and 14°C. In T. evanescens and T. piceum, the proportion of diapausing progeny decreased with increasing temperature under all tested photoperiods and thermal regimes of progeny development; the high temperature of 30°C totally averted diapause of progeny. In T. buesi and T. principium, low temperatures of 17 and 20°C resulted in relatively high proportion of diapausing progeny only when the maternal generation developed under short‐day conditions. The threshold of the maternal thermal response varied from 17–18 to 22–23°C. Under field conditions, Trichogramma females are exposed to such high temperatures only during summer, when diapause in their progeny is in any case prevented by the maternal photoperiodic response and by the thermal response of the larvae. We conclude that the maternal thermal effect on diapause induction, although to a different extent, is inherent to Trichogramma species but, at least as suggested by laboratory experiments, it does not play any role in the regulation of seasonal development under natural conditions. However, during mass rearing of Trichogramma wasps, it should be taken into account that high temperature, even when combined with short photoperiod, can avert diapause in the next generation.     

Behavioural variations among strains of Trichogramma spp.: Host-age selection

Host-age selection by several Trichogramma spp. strains was examined in laboratory choice-tests, in order to use this as a criterion to select a candidate strain for inundative releases against lepidopterous pests of cabbage in the Netherlands. The behaviour of individual females toward combinations of host eggs of two different ages (zero vs 2 and 1 vs 3-day-old) of the host species Mamestra brassicae, Pieris brassicae and P. rapae was observed directly. Host age generally did not appear to affect contact or acceptance of eggs of either host species, except that oviposition was generally lower in the oldest M. brassicae eggs. The duration of the oviposition behaviour was sometimes influenced by host-age. All strains were able to discriminate between parasitized and unparasitized hosts by external or internal examation. The lack of variation among the strains suggests that the host-age selection pattern of a given host-Trichogramma combination is predominantly dependent on the host species. The results are discussed in relation to optimal foraging theory.

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Résumé Afin de sélectionner une souche candidate de Trichogramma sp. pour des lâchés inondatifs contre les Lépidoptères, pestes du chou aux Pays-Bas, la sélection hôte-âge de plusieurs souches de Trichogramma spp., issues d'un éventail de 60 souches, a été examinée en laboratoire par des tests de choix. Le comportement de femelles individuelles, mises en présence de combinaisons d'oeufs-hôtes d'âges différents (zero ou 2 jours et 1 ou 3 jours) des espèces-hôtes Mamestra brassicae, Pieris brassicae et P. rapae, a été observé directement. En général, l'âge de l'hôte ne semble pas affecter le contact et l'acceptation des oeufs quelque soit l'espèce-hôte. La ponte diminue généralement dans les oeufs les plus âgés de M. brassicae, mais pas chez les autres espèces. La durée du comportement de ponte est parfois influencée par l'âge de l'hôte. Toutes les souches ont montré la capacité de discriminer entre les hôtes parasités et nonparasités, a l'examen externe ou interne et ceci, indépendamment de l'âge de l'hôte. Le manque de variation parmi les souces suggère que le mode de sélection hôte-âge est surtout caractéristique de l'espèce-hôte. Les resultats sont discutés en relation avec la théorie de fourrage optimal.