Parasitoid-host Ratio Research Articles

Preference for host larval stages and density-dependent parasitism of Psyttalia parasitoid species in Bactrocera fruit flies

The study examined host preferences among three Psyttalia parasitoids (P. ponerophaga, P. concolor, and P. fletcheri) regarding the larval stages of two Bactrocera fruit flies (B. cucurbitae and B. oleae). All parasitoids preferred the third instar larvae, followed by the second and first instars. Psyttalia fletcheri had the highest parasitism rate on B. cucurbitae, while P. ponerophaga was most effective on B. oleae. The highest parasitism rates were observed at the host-parasitoid ratio (400:20) for both P. fletcheri on B. cucurbitae and P. ponerophaga on B. oleae. These results highlight the importance of understanding host-parasitoid dynamics for effective fruit fly management.

An efficient method for mass rearing Dastarcus helophoroides (Coleoptera: Bothrideridae), an important natural enemy of longhorned beetles

Dastarcus helophoroides (Coleoptera: Bothrideridae) is an important biocontrol agent of wood-boring longhorned beetles, but artificial rearing of the parasitoid on the factitious host Zophobas morio (Coleoptera: Tenebrionidae) is laborious. High rearing costs are the greatest impediment to biocontrol applications of this parasitoid in China. In this study, a rearing tube was designed and natural infestation methods with (NwT) and without (NnT) tubes were developed to improve on the traditional artificial infestation method (AM) for mass rearing the parasitoid. Fitness parameters of the parasitoid were compared among these three methods, including parasitism rate, emergence rate, the total numbers of cocoons and emerging adults, total developmental time, and the labor required, using four different parasitoid-host ratios. The cost of producing 10,000 parasitoid adults with each method was then calculated. The results showed that the NwT method significantly increased numbers of cocoons and emerging adults per rearing box compared with either AM or NnT, and total developmental time was significantly shortened. Parasitism rate in the NwT treatment was significantly higher than that in the AM treatment. The labor required for the NwT and NnT treatments was significantly less than that for the AM treatment and the cost of producing 10,000 adults was reduced by 75–87% in NwT, whereas, it was reduced by 35–77% in NnT. The study showed that the NwT treatment enhanced the efficiency of mass-rearing D. helophoroides and reduced costs, and that the highest efficiency was achieved at a parasitoid-host ratio of 10:1.

The Parasitic Potential of Bracon brevicornis Wesmael and Chelonus blackburni (Cameron) (Hymenoptera: Braconidae) on the Age of Earias vittella (Fabricius) (Lepidoptera: Nolidae) and Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) in Laboratory Conditions

Aims: The current study was carried out to investigate the parasitic potential of density of two important parasitoids against the age of two important bhendi fruit borer pests.
 Study Design: Completely Random Design (CRD).
 Place and Duration of Study: The study was conducted during 2019-20 and 2020-21 at bio control laboratory of National Bureau of Agricultural Insect Resources (ICAR) Hebbal, Bengaluru (Latitude: 13.097221 Longitude: 77.568291) by adopting standard methodology and materials.
 Methodology: An experiment was designed to examine the effects of parasitoid densities of Bracon brevicornis and Chelonus blackburni on the host age (different instars) larvae of fruit borers two important fruit borers by checking the the percent parasitization. The second, third, and fourth instar larvae of E. vittella were introduced separately at different ratios of Bracon brevicornis parasitoids and host viz., 1:10, 2:10, 3:10, 4:10, and 5:10. Mated female parasitoids were utilised in all cases, and the experiment was carried out in a glass jar of 18 x 12 cm, using the sandwich approach. [1] with four replications. After 24 h, the larvae were observedand the percent parasitization was calculated. The same parasitoids were used for thesecond, third, fourth, and fifth instars larvae of H. armigera. 
 To examine the parasitic potential of C. blackburni, another experiment was conducted with various ratios of its adults and eggs of E. vittella and H. armigera by adopting the procedure of Swamiappan and Balasubramanian [2]. The parasitoid and host eggs were maintained at a ratio of 1:100, 2:100, 3:100, 4:100, and 5:100 with four replications. The observation of percent parasitization was recorded after the egg incubation period.
 Results: The parasitic potential studies showed that the parasitoid host ratio of 5:10 (100%) was the best for E. vittella (II, III and IV instar larvae) with the highest parasitization followed by 4:10 (96.83 %), 3:10 (85.08 %), 2:10 (61.71 %) and least parasitization was observed in 1:10 parasitoid host ratio (52.71 %). Similarly, for H. armigera (II, III, IV and V instar larvae) parasitoid host ratio of 5:10 was recorded the highest parasitization (96.75 %), followed by 4:10 (93.69 %), 3:10 (84.11 %), 2:10 (64.75 %) and least parasitization was observed in 1:10 parasitoid host ratio (45.07 %)
 The parasitic potential of C. blackburni revealed that the 5:100 parasitoid host ratio resulted in the maximum parasitization of 64.75% against E. vittella, followed by 4:100 (60.50%), 3:100 (52.55%), 2:100 (38.27%), and 1:100 (25.60%). Similarly, for H. armigera, parasitization was highest at a parasitoid host ratio of 5:100, followed by 4:100 (64.50%), 3:100 (58.25%), 2:100 (43.45%), and 1:100 (28.50%).
 Conclusion: The parasitoids: host ratio of 5:10 for B. brevicornis and 5:100 for C. blackburni is optimal for managing the bhendi fruit borer complex at the IV instar level.

Parasitic Potential of Bracon brevicornis Wesmael (Hymenoptera: Braconidae) on Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera: Noctuidae)

Maize is one of the most important cereal crops and is called ‘the queen of cereals’ because of its high yielding potential than other cereals. Fall armyworm, Spodoptera frugiperda is one of the serious invasive pests found to cause higher yield loss and extensive damage on maize. Considering the ecological and environmental consequences, biological control of insect pests is gaining importance as an alternative to insecticides. However, the success of any biological control programme depends on the appropriate parasitoid: host ratio and the biological stage of the pest susceptible to parasitoid. To fortify the above facts, the present study was aimed to determine the parasitic potential of B. brevicornis on S. frugiperda through free and no choice tests. This study indicated highest parasitization at the parasitoid: host ratio of 5:1 with 68.60 and 64.95 per cent parasitization on the VI instar followed by V instar (57.75 and 56.95 %), IV instar (48.80 and 46.60 %) and III instar (32.40 and 30.67 %) respectively under free and no choice conditions, respectively. Among the parasitoid host ratios tested 5: 1 was found to be the best indicating that the parasitic potential of B. brevicornis was directly proportional to the parasitoid densities. Biological parameters of B. brevicornis studied on different S. frugiperda instars recorded highest number of eggs (32.00), grub (30.75), cocoon (29.75) and adults (28.00) on sixth instar larva of S. frugiperda followed by fifth instar larva of S. frugiperda. The results revealed that the larval size was directly proportional to the progeny development of B. brevicornis. The egg, grub and cocoon period of B. brevicornis on S. frugiperda ranged from 24.20 to 26.20 h, 72.26 to 73.20 h, 96.20 to 98.20 h, respectively. The total life cycle was completed in 192.66 to 195.80 h in all instars and was found to be on par with all the instars studied. The longevity of adults emerged from different instars were found to be on par with each other and it ranged from 362.0 to 362.8 h.

Optimizing Parasitoid and Host Densities for Efficient Rearing of Ontsira mellipes (Hymenoptera: Braconidae) on Asian Longhorned Beetle (Coleoptera: Cerambycidae)

Ontsira mellipes Ashmead is a gregarious larval ectoparasitoid of woodboring cerambycids. It is native to North America but can readily attack the exotic Asian longhorned beetle, Anoplophora glabripennis (Motschulsky). This study aimed to develop an efficient rearing system for this parasitoid, as a potential novel association biocontrol agent for the beetle, by investigating the effects of different densities of host (two, three, or four larvae) and parasitoid (one, two, three, four, five, six, seven, and eight female wasps) on Ontsira's parasitization efficiency and reproductive outcomes. Results showed that overall parasitism and total numbers of parasitized hosts or progeny produced increased with host and/or parasitoid densities, but the number of parasitized hosts or progeny produced per female parasitoid decreased with parasitoid density at each given host density. Nonlinear regression indicated a consistent pattern of mutual interference as parasitoid density increased. Additional experiments showed that superparasitism (indirect interference) did not occur probably because the parasitoid detects hosts through vibration cues from host feeding and attacked (thus paralyzed) hosts are no longer detectable. Thus, the interference probably results from direct or exploitative competition. Interestingly, female parasitoids responded to increased parasitoid density with a significant increase in clutch size. Overall, per capita parasitization efficiency or reproductive outcomes were optimized at a low parasitoid-host ratio but with large group size of hosts and parasitoids. Therefore, an optimal combination of exposing three or four parasitoids to four hosts is proposed for efficient mass-rearing of this parasitoid.

Field evaluation of Spalangia endius Walker in controlling Zeugodacus cucurbitae (Coquillett)

We designed a series of semifield experiments and two types of nylon cages to evaluate the control efficacy of Spalangia endius Walker in the field and to verify the possibility of using S. endius for the control of Zeugodacus cucurbitae (Coquillett) pupae. More hosts were killed by S. endius at higher parasitoid–host ratios. The effectiveness of S. endius was highest at a parasitoid–host ratio of 1:10, which caused 50.33% and 33% mortality in enclosed and semiopen conditions, respectively. The adult parasitoids supplemented with honey solution or water survived to 7 or 5 days at most, respectively. S. endius showed a low control efficacy for Z. cucurbitae pupae in the field. However, combined with the natural mortality of pupae and the effects of other natural enemies, the mortality of Z. cucurbitae pupae exceeded 85%. Thus, S. endius may help control Z. cucurbitae pupae in the field, but other control strategies need to be applied to kill the pupae in the soil which S. endius cannot reach and to control any remaining adult Z. cucurbitae.

Effect of parasitoid density on the timing of parasitism and development duration of progeny in Sclerodermus pupariae (Hymenoptera: Bethylidae)

The gregarious idiobiont ectoparasitoid, Sclerodermus pupariae Yang et Yao, is a natural enemy that parasitizes buprestid and cerambycid beetle larvae in China. In a recent laboratory study of mass–reared female parasitoids on larvae of the substitute host, Thyestilla gebleri, the subsequent offspring were widely released to control target pests. To develop cost–effective techniques for rearing parasitoids, and improve the parasitic efficiency of Sclerodermus pupariae, several parasitoid–host ratios have been investigated. However, the mechanism whereby increasing the density of inoculated female wasps on a host affects female parasitoid fitness (adult performance and progeny developmental duration) remains unclear. In the present study, we examined the influence of parasitoid density (one to eight female wasps per host) on the time to first attack, paralyzing time, pre–oviposition period and oviposition period of foundresses and developmental duration of offspring. We showed that the time to first attack, paralyzing time, pre–oviposition period and oviposition period of foundresses were significantly negatively associated with parasitoid density, but that developmental duration of progeny was only affected at the larval stage. An increase in the parasitoid density to more than three female wasps per host had no significant influence on the investigated parameters. Our results suggest that host nutrition was adequate, and intraspecific interaction enhanced the parasitic efficiency of wasps sharing a single host. Furthermore, it is known that a suitable parasitoid–host ratio can play an important role in promoting mutually beneficial behaviors which enhance adult performance.

Trade‐offs in parasitism efficiency and brood size mediate parasitoid coexistence, with implications for biological control of the invasive emerald ash borer

Summary Parasitoids often are selected for use as biological control agents because of their high host specificity, yet such host specificity can result in strong interspecific competition. Few studies have examined whether and how various extrinsic factors (such as parasitism efficiency, i.e. the ability to optimize host‐finding attack rates) influence the outcome of competition between parasitoids, even though they could have profound effects on the implementation of classical biological control programmes. To determine the potential influence of extrinsic competition and coexistence on host suppression efficacy, we compared parasitism by two larval parasitoids (Tetrastichus planipennisi and Spathius galinae) of the invasive emerald ash borer (EAB) Agrilus planipennis, under different host densities, parasitoid densities, host plant sizes and parasitoid–host ratios. Spathius galinae had significantly higher parasitism efficiency (≈4 times), but significantly lower brood size (>6 times) than that of T. planipennisi. The attack rates of hosts increased significantly with parasitoid density, whereas host density did not significantly affect multiparasitism. The parasitism rate of T. planipennisi on small host logs was significantly higher than that on large logs, while host plant (log) size had no significant impact on S. galinae parasitism. The multiparasitism rate was rather low regardless of host log size and parasitoid/host density, indicating that intrinsic competition between the two species of parasitoids might seldom occur in the field. The two species of parasitoids could therefore coexist in the same habitat, and any adverse effects on the suppression of EAB populations caused by competitive behaviour between the two species of parasitoids would likely be negligible. Synthesis and applications. Our findings suggest that introducing multiple species of parasitic natural enemies could be feasible for management of invasive species, but it is important to examine multiple extrinsic factors simultaneously when evaluating interspecific competition between them. Among these different extrinsic factors, we found that coexistence between parasitoids can be mediated by trade‐offs in their parasitism efficiency and brood sizes. Thus, the differences in life‐history traits of natural enemies could be used to select among biological control agents being considered for releases.

Standardisation of Host-Parasitoid Ratio of Plutella xylostella (Linnaeus) and Trichogrammatoidea bactrae Nagaraja

Host parasitoid ratio for Plutella xylostella and its egg parasitoid Trichogrammatoidea bactrae was worked out under invitro conditions, at 26+1oC and 65% RH. Host to parasitoid ratio of 35:1 was optimum with parasitism and adult emergence of >90%. The ratio was on par with ratio range from 21-30:1. Mass rearing of the parasitoid for would be more economical when reared at the optimum ratio. Effective dose for inundative releases of the parasitoid for suppression of Plutella xylostella could be determined based on the field evaluation of the optimum host – parasitoid ratio (35 host eggs:1 female parasitoid).

Superparasitism ofCeratitis capitataandAnastrepha fraterculus(Diptera: Tephritidae) byDiachasmimorpha longicaudata(Hymenoptera: Braconidae)

Diachasmimorpha longicaudata is the most widely used braconid in biological control programs of fruit flies around the world. The objective of this study was to investigate the influence of different parasitoid-host ratios on D. longicaudata parasitism of Ceratitis capitata and Anastrepah fraterculus. Larvae of these fruit fly species were exposed to 5 different parasitoid-host ratios (4:1, 2:1, 1:1, 1:2 and 1:4). Subsequently, the numbers of oviposition scars and of parasitoid larvae per host were evaluated in the pupal stage. In both tephritid species, a larger number of scars and parasitoid larvae were observed at the 4:1 and 2:1 ratios. Results suggest that females of D. longicaudata can superparasitize both hosts. The differences between the tested parasitoid-host ratios suggest that there is an optimum parasitoid-host ratio for multiplying this natural enemy. These results are important for improving mass rearing programs of D. longicaudata.

Biological Characteristics ofTrichospilus diatraeae(Hymenoptera: Eulophidae) are Influenced by the Number of Females Exposed Per Pupa ofTenebrio molitor(Coleoptera: Tenebrionidae)

Different numbers of parasitoid females confined with a host can variously affect the number, sex ratio and other characteristics of the parasitoid's progeny. The objective of this study was to elucidate the effects of various ratios of Trichospilus diatraeae Cherian & Margabandhu (Hymenoptera: Eulophidae) females to pupae of one its hosts, Tenebrio molitor L. (Coleoptera: Tenebrionidae), primarily on the number of this parasitoid's progeny and their sex ratio. Both the parasitoid and the host used in this study were taken from cultures reared in the laboratory by standard methods. In order to minimize the effects of variations in host weight, 24 h-old T. molitor pupae weighing between 0.110 and 0.140 g were held as a single individuals in glass tubes (2.5 cm Ø × 14 cm L) with 48 h-old T. diatraeae females for 72 h to allow parasitization. After this period, female wasps were removed and the host pupa were transferred, one per glass tube, to a climate-controlled room at 25 ± 2 °C, 70 ± 10% RH and 12:12 h L:D. The experimental design was completely randomized with six treatments (parasitoid-host ratios) and 12 replicates per treatment. The ratios of T. diatraeae females per host used were: 1:1, 7:1, 14:1, 21:1, 28:1 and 32:1. The percentage parasitism of T. diatraeae on T. molitor pupae was 33.33, 82.00 and 83.33% at ratios of 1:1, 7:1 and 14:1, respectively, and 100% at all other ratios. The emergence of T. diatraeae adults from parasitized pupae was 75% at a parasitoid-host ratio of 1:1 and 100% at ratios of 21:1, 28:1 and 32:1. The duration of the parasitoid's life cycle ranged from 21.00 ± 2.22 to 24.00 ± 2.00 days at parasitoid-host ratios of 32:1 and 1:1, respectively. The number of T. diatraeae progeny per T. molitor pupa was highest at a ratio of 21:1 (246.50 ± 50.18). The proportion of T. diatraeae females in the offspring decreased as the parasitoid-host ratio increased, varying between 0.82 ± 0.06 and 0.97 ± 0.01. A parasitoid-host ratio of 21:1 T. diatraeae females per T. molitor pupa is considered the most adequate and appropriate for mass-rearing of this parasitoid. Tenebrio molitor appears to be a suitable alternate host for efficient mass-rearing of T. diatraeae for biological control of lepidopteran pests. At the parasitoid-host ratio of 21:1, each T. molitor pupa supported the production of 246.5 parsitoids of which 88% were females, i.e., 216.9 females and 29.6 males; each T. diatraeae female produced 9.55 ± 0.48 female progeny, and the developmental time from egg to adult was 20.4 days.

Biological control of Plodia interpunctella (Lepidoptera: Pyralidae) by single and double releases of two larval parasitoids in bulk stored wheat

The Indian meal moth, Plodia interpunctella is a world-wide economically important pest of stored-products and processed food commodities including bulk grain. Two larval parasitoids, Habrobracon hebetor and Venturia canescens were studied under laboratory conditions to suppress small populations of P. interpunctella in 30-L polyethylene jar filled with 20 kg wheat. Twenty 3 to 4-week-old moth larvae were released into the jar and were allowed to produce webbings in the grain for 3–4 d. Then 10, 20 or 40 female parasitoids were added. A further batch of parasitoids was added to half of the jars 4 d after the first batch, 7–8 d after the introduction of P. interpunctella.Treatments comprised untreated control, only V. canescens or only H. hebetor released, and the combination of both parasitoids per release. After two weeks, the grains were sieved, the numbers of surviving parental parasitoids recorded and the cocoons of moths and parasitoids were collected from each jar and kept under controlled conditions of 25 °C. The emerged moth adults and parasitoid progeny were recorded until day 42.The highest mortality (93%) was observed with a single release of 20 H. hebetor plus 20 V. canescens (host-parasitoid ratio 1:1 for each parasitoid species). The various numbers of H. hebetor and V. canescens alone or in combination, once or twice released, led to mortalities between 50% and 80% that did not differ significantly. The single release of 10 H. hebetor or 10 V. canescens resulted in the lowest mortalities of P. interpunctella ranging from 34% to 39%. The different treatments affected the number of parasitoid offspring produced. Venturia canescens progeny occurred in all trials with combinations of the two parasitoid species. The single release of 20 H. hebetor resulted in more progeny emerged compared to all other treatments. For application in biological control, our data suggest the combination could be as least as effective as the release of H. hebetor alone.

Parasitic Potential of some Braconid Parasitoids against Okra Fruit Borers

In surveys conducted in Coimbatore district during 2009-10, Okra fruit borers, Earias vittella Fabricius, Earias insulana Boisdual and Helicoverpa armigera (Hübner) were the serious pests causing 50 per cent loss. During the survey, three hymenopteran parasitiods viz., Bassus spp., Brachymeria lasus Walker, Goryphus spp., of the families Braconidae, Chalcididae and Ichneumonidae against Earias spp. and the dipteran parasitoid, Carcelia illota (Tachinidae ) against H. armigera were recorded with 0.00 to 10.00 per cent parasitization. All the parasitoids recorded were not amenable for mass culturing, and hence, other braconid parasitoids viz., Bracon brevicornis Wesmael, Bracon hebetor Say and Chelonus blackburni Cameron were studied for their parasitic potential against fruit borer complex. Cent per cent parasitization was observed with B. brevicornis against all instars of E. vittella and H. armigera, higher parasitizaion of 100 and 97.50 per cent was observed by B. hebetor at a parasitoid-host ratio of 5:10 on second instar larvae of H. armigera and fourth instar larvae of E. vittella respectively. The egg - larval parasitoid, C. blackburni recorded the highest parasitization of 70.54 per cent and 64.58 per cent against E. vittella and H. armigera at a parasitoid host ratio of 5:100. For B. brevicornis and B. hebetor, highest cocoon formation and parasitoid emergence was recorded at 4:10 ratio against third and fourth instars. In C. blackburni, the parasitoid host ratio of 3:100 yielded the highest number of adult parasitoids. B. brevicornis was found to be more effective against fruit borers.

The foraging behavior of Diaeretiella rapae (Hymenoptera: Braconidae) on Diuraphis noxia (Hemiptera: Aphididae)

Host stage preference, functional response and mutual interference of Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae: Aphidiinae) on Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae) were investigated under defined laboratory conditions (20?1?C; 60?5% relative humidity; 16 h light/8 h dark photoperiod). Nicholson?s model and linear regression were used to determine per capita search-efficiency and the interference coefficient, respectively. There was a significant difference between the rates of parasitism on different stages of D. noxia. The highest parasitism percentage was observed on the third instar nymphs of D. noxia in both choice and no-choice preference tests. Results of logistic regression revealed a type II functional response. The estimated values of search-efficiency (a) and handling time (Th) were 0.072 h-1 and 0.723 h, respectively. The maximum attack rate was calculated to be 33.22. The per capita search-efficiency decreased from 0.011 to 0.004 (h-1) as parasitoid densities increased from 1 to 8. Therefore, different host-parasitoid ratios can affect the efficacy of D. rapae.

Laboratory studies on control of the maize weevil Sitophilus zeamais by the parasitoid Anisopteromalus calandrae.

Anisopteromalus calandrae (Howard), a cosmopolitan parasitoid, attacks several beetle pests of stored products, including the maize weevil, Sitophilus zeamais (Motschulsky). Two laboratory experiments were conducted at ambient conditions (25-29 °C, 60-70% RH, natural photoperiod). A short-term experiment evaluated the density of A. calandrae (0, 4, 8, 12, 16, or 20 per box) for the optimal production of parasitoids in 3825-cm 3 boxes containing milled rice. The percentage of parasitoid emergence was highest and the percentages of weevil emergence and parasitoid-induced mortality i.e., parasitoid mortality resulting from superparasitism were relatively low with 16 females per box rather than with smaller (0 to 12) or larger (20) numbers of females added per box. The decline in parasitoid emergence at the highest parasitoid density tested (20 females per box) was probably due to superparasitism. A long-term experiment was conducted to determine the parasitoid densities (0, 2, 4, 6, 8, or 10 per bottle) that most effectively suppress weevils in milled rice during 6 months of storage in 1-l bottles. To simulate repetitive release of parasitoid, the same number of parasitoids was introduced after each monthly sampling. The number of maize weevils decreased as parasitoid density increased. The best control was obtained with 10 female parasitoids per bottle. At this density, the number of emerged weevil remained stable from the third to the sixth month. Based on the short-term experiment, a parasitoid-host ratio of 1:47 will produce the largest number of parasitoids. Based on the long-term experiment, a parasitoid-host ratio of 1:30 will provide the best control of maize weevils in a monthly release program. The current results indicate that large-scale experiments on biological control of the maize weevil with A. calandrae are now needed.

Reproductive performance of Nesolynx thymus (Hymenoptera: Eulophidae) as influenced by host (Musca domestica) size

An investigation was conducted to understand the influence of host size on the reproductive performance of Nesolynx thymus, an ecto-pupal parasitoid of the uzi fly, Exorista bombycis, inflicting 10–20% loss to silkworm crops in the southern silk producing states of India. Two day-old adults of N. thymus were allowed to parasitize pupae of M. domestica, categorized into big and small at a parasitoid-host ratio of 1:20. After parasitoid adult eclosion, data on parasitization rate, developmental duration, male, female and total adult recovery per pupa and per female, sex ratio, adult size and adult longevity were recorded. The performance of first generation adults as influenced by host size was also recorded by allowing the first generation adults to parasitize three day-old pupae of E. bombycis. The parameters female and total progeny production per female, sex ratio, longevity, body length and wing span of males, females, head width of male, and abdomen length of female increased significantly with host size. No impact of host size was observed on performance of first generation adults.

Evaluating the use of nuclear techniques for colonization and production of Trichogramma chilonis in combination with releasing irradiated moths for control of cotton bollworm, Helicoverpa armigera

Gamma radiation was tested as a means of increasing production of the egg parasitoid Trichogramma chilonis Ishii by improving the suitability of host eggs and by stimulating reproduction of the parasitoid females. For manipulation of the host eggs’ suitability, radiation was used to either (a) produce developmentally-inactivated (DI) eggs incapable of hatching, or (b) to produce F1 sterile host eggs. For treatment of the parasitoid females with the intent of stimulating reproduction, parasitoid pupae were exposed to very low dose radiation (250 mGray). For tests on host suitability using radiation-induced DI host eggs, newly-laid (<8 h old) host eggs (Helicoverpa armigera Hubner) were exposed to 300 Gy of 60Co gamma radiation. For tests of F1 sterile host eggs, H. armigera moths were mated with individuals exposed to 250 Gy as pupae. Tests were performed with eggs resulting from all possibilities of normal (N) and sterile (S) ♀ × ♂ matings. Both types of DI host eggs (irradiated or sterile), along with untreated host eggs (controls), were exposed to T. chilonis females, using the following host egg-to-parasitoid ratios: 1:10, 1:30, 1:60 and 1:90. Developmentally-inactivated host eggs exposed to 300 Gy did not differ in suitability from normal host eggs at a 1:10 parasitoid–host ratio, but were significantly more suitable at the higher host–parasitoid ratios. F1 sterile eggs were not significantly different in suitability from normal eggs at a 1:10 host–parasitoid ratio but were marginally better at the higher host–parasitoid ratios. In tests performed using T. chilonis females exposed to low-dose radiation (250 mGy), no effects were observed when cohorts of 5 T. chilonis females were provided with only 50 host eggs, but when more hosts were provided (ratios of 1:30, 1:60 and 1:90), significantly higher rates of parasitization were noted for the parasitoids exposed to low-dose radiation. This effect prevailed using both normal host eggs and DI host eggs exposed to 300 Gy. The stimulatory effect also was noted when F1 sterile host eggs were provided to the irradiated T. chilonis females. These results suggest that release of T. chilonis irradiated with 250 mGy may complement release of irradiated H. armigera moths, which produce sterile F1 eggs that can serve as supplemental hosts in the field and thereby enhance the pest management system.

Reproductive performance of Nesolynx thymus (Hymenoptera: Eulophidae), in relation to age of Musca domestica (Diptera: Muscidae)

An investigation was undertaken to record the influence of host age on the reproductive performance of Nesolynx thymus (Hymenoptera:Eulophidae). This is an indigenous, gregarious, ecto-pupal parasitoid of certain dipteran insects, including the tachinid fly, Exorista bombycis (Louis) which is a well-known endo-larval parasitoid of the silkworm, Bombyx mori. Each gravid N. thymus female was allowed to parasitise 1–4-day-old puparia of Musca domestica L. for 2 days at a parasitoid–host ratio of 1:20. The parasitised host puparia were observed for progeny recovery, sex ratio, female longevity and fitness (adult size). In addition, reproductive performance of the parasitoid progeny was assessed by allowing its females to parasitise for 2 days, 3-day-old puparia of E. bombycis at a parasitoid–host ratio of 1:4. There was a significant negative correlation between host age and parasitisation rate, parasitoid developmental duration, sex ratio and female longevity while the correlation was positive between host age and parasitoid recovery per host puparium. Similarly, negative correlation was observed between host age and morphometric parameters (body length, head width and wing span of male and female and length and width of female abdomen) of the progeny adults. Host age did not have any impact on the reproductive performance of progeny adults when allowed to parasitise puparia of E. bombycis.

Models for integrated pest control and their biological implications

Successful integrated pest management (IPM) control programmes depend on many factors which include host–parasitoid ratios, starting densities, timings of parasitoid releases, dosages and timings of insecticide applications and levels of host-feeding and parasitism. Mathematical models can help us to clarify and predict the effects of such factors on the stability of host–parasitoid systems, which we illustrate here by extending the classical continuous and discrete host–parasitoid models to include an IPM control programme. The results indicate that one of three control methods can maintain the host level below the economic threshold (ET) in relation to different ET levels, initial densities of host and parasitoid populations and host–parasitoid ratios. The effects of host intrinsic growth rate and parasitoid searching efficiency on host mean outbreak period can be calculated numerically from the models presented. The instantaneous pest killing rate of an insecticide application is also estimated from the models. The results imply that the modelling methods described can help in the design of appropriate control strategies and assist management decision-making. The results also indicate that a high initial density of parasitoids (such as in inundative releases) and high parasitoid inter-generational survival rates will lead to more frequent host outbreaks and, therefore, greater economic damage. The biological implications of this counter intuitive result are discussed.

Effect of host density and host-parasitoid ratio on parasitization of Pyrilla perpusilla (Walker) by Epiricania melanoleuca (Fletcher).

Laboratory and field studies were carried out on the influence of host-parasitoid ratio on parasitism of Pyrilla perpusilla (Walker) (Homoptera: Lophopidae) by Epiricania melanoleuca (Fletcher) (Lepidoptera: Epipyropidae). Enhanced parasitism was observed with increased host-parasitoid ratio from 48.1 per cent at 1: 1 to 84.9% at 1: 4 ratios. Enhanced pupation was also observed with increase in the host-parasitoid ratio (46.1 to 79.6%). The host-parasitoid ratio of 1: 4 at 5 pyrilla per leaf was found optimum for mass multiplication of E. melanoleuca in laboratory conditions. Under field condition non-significant effect of life stages (nymphs 80.0 and adults 77.8%) of pyrilla was noticed on their parasitization by E. melanoleuca. In field conditions also, enhanced parasitization was noticed with increase in host-parasitoid ratios from 1: 1 to 1: 7, but 1: 3-1: 5 ratio at a host density of 5 pyrilla per leaf resulted in better increase in field parasitism.