Host Stage Selection and Sex Allocation by Gyranusoidea tebygi (Hymenoptera: Encyrtidae), a Parasitoid of the Mango Mealybug, Rastrococcus invadens (Homoptera: Pseudococcidae)

Host stage selection and sex allocation by Gyranusoidea tebygi Noyes (Hym,, Encyrtidae) were studied in choice and no-choice experiments in the laboratory. The parasitoid reproduced on first, second, and third instars of the mango mealybug, Rastrococcus invadens Williams (Hem., Pseudococcidae), and it avoided hosts that were already parasitized. Host feeding was occasionally observed. Sex ratios of the offspring produced by individual wasps were highly biased in favor of females, whereas the sex ratio of groups of wasps foraging under crowded conditions varied from male biased in smaller hosts to female biased in larger hosts. Females had longer developmental times than males, developed faster in larger mealybugs than in smaller ones, and were always larger than males emerging from the same host instar. Their size increased with the instar of the host at oviposition. About 90% of all ovipositions in second and third instar nymphs resulted from an attack with multiple stings, starting with a sting in the head of the host for the most part. The function of these head stings is either to assess quality of the host or to subdue hosts prior to oviposition. Encounter rates, number of attacks, and number of stings during one attack increased, while ovipositions decreased with host instar. Time investment per oviposition and time spent preening increased with increasing host age because older hosts defended themselves more vigorously than younger ones. Thus, while fitness of the parasitoid increased with host size, fitness returns per time decreased. The implications of this host selection behavior for the biological control of the mango mealybug are discussed.     

Host stage selection of the mealybug parasitoid Anagyrus spec. nov near sinope

The mealybug parasitoid Anagyrus spec. nov near sinope (Hymenoptera: Encyrtidae) is an undescribed parasitoid of the Madeira mealybug, Phenacoccus madeirensis Green (Homoptera: Pseudococcidae). We investigated the preference of Anagyrus spec. nov near sinope for six developmental stadia (first‐ and second‐instar nymphs, third‐instar immature females, third‐ or fourth‐instar immature males, pre‐reproductive adult females, and ovipositing adult females) of P. madeirensis and the fitness consequences of the host stage selection behavior. In the no‐choice test, Anagyrus spec. nov near sinope parasitized and completed development in all host stadia except third‐instar immature males. When all host stadia were offered simultaneously, the parasitoids preferred third‐instar immature and pre‐reproductive adult females. Dissection of the stung mealybugs revealed that the clutch size (number of eggs per host) was approximately four and three in the third‐instar and pre‐reproductive females, respectively, and one egg per first‐instar nymph. Parasitoids emerged from P. madeirensis parasitized at third‐instar or pre‐reproductive adult female completed development in the shortest duration, achieved a higher progeny survival rate, larger brood and body size, and the lowest proportion of males. We showed that the continued development of mealybugs had significant influence on the fitness of the parasitoids. Although deposited as eggs in first‐ or second‐instar nymphs, parasitoids emerged from mummies that had attained third‐instar or adult development achieved similar progeny survival rate, brood size, body size, and sex ratio as those parasitoids deposited and developed in third‐instar or adult mealybugs. By delaying larval development in young mealybugs, Anagyrus spec. nov near sinope achieved higher fitness by allowing the parasitized mealybugs to grow and accumulate body size and resources. We suggest that the fitness consequence of host stage selection of a koinobiont parasitoid should be evaluated on both the time of parasitism and the time of mummification.     

Immune response of the hibiscus mealybug, Maconellicoccus hirsutus Green (Homoptera: Pseudococcidae), to oviposition of the parasitoid Anagyrus kamali Moursi (Hymenoptera: Encyrtidae)

Anagyrus kamali Moursi has been recently introduced into the Caribbean as a biological agent against the hibiscus mealybug, Maconellicoccus hirsutus Green. This host has a cellular defense reaction that involves encapsulation and melanization of the endoparasitoid egg. The impact of this immune response on the parasitoid progeny was assessed, as well as the response of the parasitoid countermeasures to overcome it. Under laboratory conditions, significant differences in the immune response were found for different developmental stages of M. hirsutus. The intensity of the immune response varied between second instar, third instar and adult mealybugs. After 30 h, the level of encapsulation was the highest for eggs oviposited in adults: 58% of eggs were encapsulated, followed by third (32%) and second (4%) instars. Three days after oviposition 23, 44 and 86% of the parasitoid eggs oviposited, respectively, in adult, third and second instars were not encapsulated. The unencapsulated parasitoid eggs could hatch and continue their development. Adult mealybugs required 30 h to encapsulate 50% of the eggs, whereas in second and third instars, 50% level encapsulation was never reached. Superparasitism had a saturating effect on the immune system; reduced levels of encapsulation occurred when more than 10 eggs were oviposited in a single mealybug. Wasp larvae were never encapsulated by M. hirsutus.     

Production of Anagrus epos Girault (Hymenoptera: Mymaridae) on Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae) eggs

Anagrus epos Girault (Hymenoptera: Mymaridae) is a natural enemy candidate for a classical biological control program targeting the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae), in California. Little is known about the biology or ecology of A. epos when it utilizes GWSS eggs as a host. Here, we report the results of laboratory studies that describe the host age preference for oviposition, longevity of A. epos adults provided with different food sources, and developmental rates at six different constant temperature regimes. Anagrus epos is a gregarious parasitoid in GWSS eggs with up to 14 adults emerging from each GWSS egg. In choice and no-choice tests for oviposition, A. epos females successfully parasitized all developmental ages of GWSS eggs (1–8 days old). In choice tests, parasitism rates were significantly higher in 1-, 3-, 4-, and 5-day-old GWSS eggs than in 2-, 6-, 7-, and 8-day-old eggs. If provided with honey and water, honey only, water only, or no food or water, A. epos females lived on average 8.2, 4.7, 2.6, and 1.6 days, respectively. Anagrus epos required 294.1 degree-days above a lower temperature threshold of 12.4 °C to develop from egg to adult (eclosion). Our results provide baseline information useful in the development of an efficient parasitoid mass rearing program for A. epos release and evaluation in California.     

Performance of Microplitis tuberculifer (Hymenoptera: Braconidae) parasitizing Mythimna separata (Lepidoptera: Noctuidae) in different larval instars

The solitary parasitoid Microplitis tuberculifer (Wesmael) is an important biological control agent of various lepidopteran pests in Asia. We examined the preference of M. tuberculifer for different instars of its common host, Mythimna separata (Walker), host instar effects on parasitoid development, and the consequences of parasitism in different stages for growth and consumption of host larvae. The wasp successfully parasitized the first four larval instars of M. separata, but not the fifth, which appeared to be behaviorally resistant. First and second instars were parasitized at higher rates compared to thirds and fourths in no-choice situations, ostensibly due to longer handling times for the latter, but second instars were most preferred in a choice test that presented all stages simultaneously. Although later instar hosts yielded heavier cocoons, the fastest parasitoid development was obtained in second instars. Lower sex ratios were obtained from first instars as females appeared to lay a smaller proportion of fertilized eggs in small hosts. Both weight gain and food consumption of parasitized larvae were reduced significantly within 24 h of parasitism, regardless of the stage parasitized, and final body weights were less than 10% those of unparasitized larvae. Thus, M. tuberculifer has good potential as a biological control agent of M. separata, successfully parasitizing the first four larval instars and dramatically reducing plant consumption by the host in all cases.     

Host stage preference and parasitism behaviour of Aenasius bambawalei an encyrtid parasitoid of Phenacoccus solenopsis

In Pakistan, the cotton mealybug, Phenacoccus solenopsis Tinsley (Sternorrhyncha (Homoptera): Pseudococcidae), is a serious pest of many cultivated plants. A parasitoid, Aenasius bambawalei Hayat (Hymenoptera: Encyrtidae), is associated with P. solenopsis. In order to mass rear A. bambawalei for a biological control programme, it is important to investigate the parasitoid’s host stage preference and its parasitism behaviour for P. solenopsis in order to optimise production. The present study showed that under both choice and no-choice conditions, the parasitoid preferred third instar and pre-reproductive host stage mealybugs for parasitism. Parasitoid larva developing inside the host exhibited a greater longevity, shorter developmental period and longer body size in these preferred host stages. Our study also confirmed that A. bambawalei showed no attraction to male mealybugs and no host feeding on any host stage was recorded. The ability of the parasitoid to effectively discriminate between suitable and non-suitable stages means that it is feasible to rear it on a mixed population.     

Host instar susceptibility and selection and interspecific competition of three introduced parasitoids of the mealybug Paracoccus marginatus (Hemiptera: Pseudococcidae)

Three previously introduced parasitoids (Acerophagus papayae Noyes and Schauff, Anagyrus loecki Noyes and Menezes, and Pseudleptomastix mexicana Noyes and Schauff [Hymenoptera: Encyrtidae]) of the mealybug Paracoccus marginatus Williams and Granara de Willink (Hemiptera: Pseudococcidae) were studied for their host instar susceptibility and sex ratio, host instar selection, and interspecific competition in the laboratory. All three parasitoids were able to develop in the second instars, third-instar females, and adult females of P. marginatus. No progeny emerged from first-instar mealybugs. The proportion of female emergence was increased with increasing host size. Parasitoids selected their host instars for oviposition when they had a choice. Between second- and third-instar hosts, A. papayae and P. mexicana had significantly higher parasitism in second-instar mealybugs, whereas A. loecki had higher parasitism in the third-instar mealybugs. When competed with either one or two parasitoid species, A. papayae was significantly more successful in second-instar hosts and A. loecki was significantly more successful in third-instar mealybugs. P. mexicana was significantly less competitive when with A. papayae in both second and third instars, with A. loecki in third instars and with both A. papayae and A. loecki in second and third instars. Overall, A. papayae provided a better control of the host, when present singly or with the other two parasitoids. This information is important in evaluating the efficiency of A. papayae, A. loecki, and P. mexicana and understanding the outcome of their recovery and establishment in field studies conducted in Florida.     

Vine mealybug sex pheromone increases citrus mealybug parasitism by Anagyrus sp. near pseudococci (Girault)

The present study was aimed at elucidating the role of lavandulyl senecioate (LS), the sex pheromone of Planococcus ficus, in host selection of the parasitoid Anagyrus sp. near pseudococci. Field trials were carried out in Portugal, Italy and Israel. The effect of LS on the parasitism rate of the wasp was determined by exposing sentinel mealybugs combined with pheromone dispensers impregnated with LS, in comparison with other baits: lavandulyl isovalerate (LI); planococcyl acetate (PAc); and unbaited control traps. In addition, in order to study the host location behavior of A. sp. near pseudococci, pheromone dispensers were placed at three different distances: inside the trap, 30 or 60 cm away from the trap. The number of parasitoid females inside the traps, the number of parasitized mealybugs, and the number of days required for the first parasitoid emergence were recorded. The response of A. sp. near pseudococci females to different doses of LS (25–1350 μg) was also evaluated using sticky plate traps. The rate of mealybug parasitism by A. sp. near pseudococci was significantly increased by LS in the three parasitoid populations. PAc and LI had no significant effect on the wasp parasitism rate in most of the trials. However, the Italian population of the parasitoid responded to PAc, showing apparently a different behavioral pattern. The number of parasitoid females trapped did not significantly differ between tested doses of LS. The use of LS as an arrestant in host location by the A. sp. near pseudococci female is discussed.     

Host specificity of the egg pupal parasitoid Fopius arisanus (Hymenoptera: Braconidae) in La Reunion

Fopius arisanus is a polyphagous parasitoid of Tephritidae, which has been recently introduced to La Réunion Island as part of a classical biological control programme. We carried out laboratory experiments to assess the host specificity of this parasitoid, initially reared on Bactrocera zonata, and then offered for parasitization the eight local tephritid pest species. Naive or experienced parasitoid females were given tephritid eggs in no choice tests. Fopius arisanus females parasitize all fly species but parasitism varies with host species. No adult wasps emerge from Bactrocera cucurbitae and the survival of this species is only slightly affected by parasitism. Dissections show that the late instars of this fly may eliminate the parasitoid by encapsulation. When developing on Ceratitis capitata, Ceratitis rosa, Dacus ciliatus, Dacus demmerezi, and Neoceratitis cyanescens, parasitoid survival rate ranges from 10 to 25%. Bactrocera zonata and Ceratitis catoirii are the best hosts, yielding parasitoid survival rates of more than 70% with no premature mortality. The egg-larval mortality of C. capitata, C. rosa, D. ciliatus, and N. cyanescens, and the pupal mortality of D. demmerezi, are significantly increased by parasitism. The size of emerging adults is affected by host species and is correlated to pupal weight. Bactrocera zonata would be a favorable host to support routine colonization of F. arisanus for mass production of this parasitoid.     

Suitability of various stages of mealybug,Phenacoccus solenopsis (Homoptera: Pseudococcidae) for development and survival of the solitary endoparasitoid,Aenasius bambawalei (Hymenoptera: Encyrtidae)

Aenasius bambawalei has been recently reported as a solitary endoparasitoid of Phenacoccus solenopsis in India. Laboratory experiments were conducted comparing the parasitoid's preference for the three developmental stages of mealybug and its effect on parasitism, development, progeny fitness and sex ratio of A. bambawalei. The studies revealed third instar mealybugs as the most preferred stage for development of the parasitoid.     

Escape of Lygus hesperus (Heteroptera: Miridae) Eggs from Parasitism by Anaphes iole (Hymenoptera: Mymaridae) in Strawberries: Plant Structure Effects

Herbivorous insects in natural and agricultural systems experience variation in parasitoid attack on different plant species due to direct and indirect plant influences on parasitoids. Lygus hesperus is a native polyphagous mirid that suffers up to 100% parasitism by the native egg parasitoid Anaphes iole in certain weed hosts, but with inundative releases in commercial strawberries, we achieve <65% L. hesperus suppression. We examined L. hesperus egg distribution in individual strawberry plants and parasitism by A. iole of eggs in different strawberry plant structures to determine whether plant-related factors affected parasitoid performance in strawberries. L. hesperus laid more eggs (46.5% of all eggs laid) in the fruit (between the achenes [seeds] in the fleshy receptacle) than in the petiole (23.3%), leaflet (20.3%), peduncle (6.2%), or calyx (3.7%). In a no-choice test, parasitism by A. iole was higher in the petiole (96.7%), calyx (91.9%), and leaflet (85.2%) than in the fruit (51.8%), in which the achenes appeared to hinder parasitoid access to host eggs. In addition, in young fruits in which the interachene distance was minimum, parasitism was considerably lower (25.4%) than in fruits in which receptacle swelling had resulted in interachene distances that were medium (65.7% parasitism) or large (77.1% parasitism). Our results suggest that strawberry fruits can provide refugia from parasitism by A. iole and that maximum protection occurs when the achenes are contiguous. The presence of refugia in strawberries limits the impact of augmentative biological control with A. iole, highlighting the need for its integration with other strategies to effectively suppress L. hesperus in strawberries.     

First study of the biology of Cryptolaemus montrouzieri and its potential to feed on the mealybug Dactylopius opuntiae (Hemiptera: Dactylopiidae) under laboratory conditions in Morocco

Abstract

Dactylopius opuntiae, is known as specific Opuntia cochineal in many countries around the world. This sap-sucking insect was first detected in Morocco in 2014. To address the problem, the feeding potential of different development stages of Cryptolaemus montrouzieri Mulsant, a biological control agent against mealybugs, was investigated on different development stages of D. opuntiae. Fourth instar larvae and adults of C. montrouzieri were the most voracious feeders on different instars of the mealybug. The numbers of mealybug eggs consumed by first, second, third and fourth instar larva and adults of C. montrouzieri were 36.18?±?1.84, 68.08?±?4.17, 280.55?±?5.41, 540.55?±?5.35, 6514.13?±?64.28, respectively. The numbers of mealybug nymphs consumed by the same stages of C. montrouzieri were 35.43?±?1.75, 67.73?±?3.88, 279.85?±?5.58, 539.63?±?5.08 and 6501.7?±?81.94 (first instars) and 34.83?±?1.20, 57.45?±?1.22, 83.80?±?1.92, 213.65?±?3.46 and 6013.23?±?35.46 (second instars), respectively. The corresponding figures for adult female mealybugs were 1.40?±?0.78, 10.65?±?1.83, 18.58?±?1.24, 25.23?±?1.10 and 197.15?±?3.29, respectively. The egg, larval, prepupal, pupal and adult stages occupied 3.36–3.69, 20.21–27.59, 1.31–1.59, 10.62–10.72 and 96.10–102.51?days, respectively when the coccinellid was reared on different stages of D. opuntiae. The results indicate that C. montrouzieri has the potential to be used as a biocontrol agent in Morocco.     

Effects of host instar and size on parasitization efficiency and life history parameters ofOpius dissitus

The braconid parasitoidOpius dissitus Muesebeck (Hymenoptera, Braconidae) produced 1.7 to 3 times more offspring when provided second and third instar leafminers (Liriomyza sativae Blanchard) as compared to first instars. Females arising from parasitization of different instars did not differ significantly in numbers of chorionated eggs in their ovaries at adult eclosion. Development time was prolonged by about two days when parasitoid oviposition occurred in first, as compared to third instar hosts. Parasitoid length was positively correlated with host weight (r²=0.75). Because only 7% of variation in host weight could be explained by host density, parasitoid length varied considerably among hosts reared at the same density. Longevity and lifetime fecundity of parasitoids were inversely related to the weight of their hosts.     

Does superparasitism improve host suitability for parasitoid development? A case study in the Microplitis rufiventris-Spodoptera littoralis system

Superparasitism refers to the oviposition behavior of parasitoid females who lay their eggs in an already parasitized host. Recent studies have shown that allocation of additional eggs to an already parasitized host may be beneficial under certain conditions. In the present work, mortality of Microplitis rufiventris wasps was significantly influenced by both host instar of Spodoptera littoralis larvae at parasitism and level of parasitism. In single parasitization, all host instars (first through sixth) were not equally suitable. Percentage of emergence success of wasp larvae was very high in parasitized first through third (highly suitable hosts), fell to 60% in the fourth instar (moderate suitable) and sharply decreased in the penultimate (5th) instars (marginally suitable). Singly parasitized sixth (last) instar hosts produced no wasp larvae (entirely unsuitable), pupated and eclosed to apparently normal adult moths. The scenario was different under superparasitism, whereas supernumerary individuals in the highly suitable hosts were almost always killed as first instars, superparasitization in unsuitable hosts (4th through 6th) had significant increase in number of emergence success of wasp larvae. Also, significantly greater number of parasitoid larvae successfully developed in unsuitable hosts containing three wasp eggs than counterparts containing two wasp eggs. Moreover, the development of surplus wasp larvae was siblicidal in earlier instars and nonsiblicidal gregarious one in the penultimate and last “sixth” instars. It is suggested that the optimal way for M. rufiventris to deal with high quality hosts (early instars) is to lay a single egg, while the optimal way to deal with low quality hosts (late instars) might be to superparasitize these hosts.     

Parasitoid–Pathogen–Pest Interactions of Chelonus insularis, Campoletis sonorensis, and a Nucleopolyhedrovirus in Spodoptera frugiperda Larvae

In this study we examined interactions between two solitary endoparasitoids, the braconid Chelonus insularis and the ichneumonid Campoletis sonorensis, and a multiple-enveloped nucleopolyhedrovirus infecting Spodoptera frugiperda larvae. We examined whether ovipositing females minimize interference by discriminating amongst hosts and examined the outcome of within-host competition between parasitoid species and between the parasitoids and the virus. The egg–larval parasitoid Ch. insularis did not discriminate between virus-contaminated and uncontaminated S. frugiperda eggs; all S. frugiperda larvae that emerged from surface-contaminated eggs died of viral infection prior to parasitoid emergence. The larval parasitoid C. sonorensis also failed to discriminate between healthy and virus-infected S. frugiperda larvae or between larvae unparasitized or parasitized by Ch. insularis. Host larvae parasitized in the egg stage by Ch. insularis were suitable for the development of C. sonorensis when they were multiparasitized by C. sonorensis as first, second, third, and fourth instars, whereas emergence of Ch. insularis was dramatically reduced (by 85 to 100%) in multiparasitized hosts. Nonspecific host mortality was significantly higher in multiparasitized hosts than in singly parasitized hosts. The development time and sex ratio of C. sonorensis in multiparasitized host larvae were unaffected by the presence of Ch. insularis larval stages. Both Ch. insularis parasitized and nonparasitized larvae of the same instar (second, third, or fourth instars) had a similar quantitative response to a challenge of virus inoculum. All host larvae that ingested a lethal dose of virus were unsuitable for Ch. insularis development. In contrast, C. sonorensis did not survive in hosts that ingested a lethal virus dose immediately after parasitism, but parasitoid survival was possible with a 2-day delay between parasitism and viral infection and the percentage of parasitoid emergence increased significantly as the interval between parasitism and viral infection increased. The development time of C. sonorensis was significantly reduced in virus-infected hosts compared to conspecifics that developed in healthy hosts. C. sonorensis females that oviposited in virus-infected hosts did not transmit the virus to healthy hosts that were parasitized subsequently. Field applications of virus for biocontrol of S. frugiperda may lead to substantial mortality of immature parasitoids, although field experiments have not yet demonstrated such an effect.     

Determination of Homalodisca coagulata (Hemiptera: Cicadellidae) egg ages suitable for oviposition by Gonatocerus ashmeadi, Gonatocerus triguttatus, and Gonatocerus fasciatus (Hymenoptera: Mymaridae)

Homalodisca coagulata (Say) (Hemiptera: Cicadellidae) eggs 1–10 days of age were exposed to Gonatocerus ashmeadi Girault, Gonatocerus triguttatus Girault, and Gonatocerus fasciatus Girault (all Hymenoptera: Mymaridae) in no choice laboratory trials to investigate egg age utilization and to determine which egg ages are vulnerable to attack by these three parasitoids. The H. coagulata egg ages that were most suitable for oviposition by G. ashmeadi, G. triguttatus, and G. fasciatus were eggs 3, 4, and 2 days of age, respectively. Egg ages least suitable for parasitoid development were 6–10 days for G. ashmeadi (resulting in <50% parasitism), 1–2 and 7–10 days for G. triguttatus (resulting in <25% parasitism), and 3–10 days for G. fasciatus (resulting in <11% parasitism). Pooling parasitism data across all egg ages showed that parasitism by G. ashmeadi was 12.9 and 28.5% higher compared with G. triguttatus and G. fasciatus, respectively, and G. triguttatus resulted in 15.6% higher percentage parasitism compared with G. fasciatus. Egg age had a significant effect on the percentage of female G. ashmeadi offspring produced, but this was not significant for G. triguttatus, and low G. fasciatus parasitism prevented statistical analyses for comparisons. Results from tests where females were offered a choice for oviposition between eggs 1, 3, and 5 days of age demonstrated that G. ashmeadi and G. triguttatus showed no significant oviposition preference, while percentage parasitism by G. fasciatus was 29.4 and 7.4% higher when females were presented eggs 1 and 3 days of age, respectively, compared with eggs 5 days of age. Choice tests indicated that an overlap in egg age suitability for oviposition exists between G. ashmeadi, G. triguttatus, and G. fasciatus, and that interspecific competition for eggs 1, 2, and 3 days of age may occur in the field environment.     

Intraguild predation of Planococcus ficus parasitoids Anagyrus pseudococci and Leptomastix dactylopii by Nephus kreissli

Intraguild predation of the mealybug parasitoids Anagyrus pseudococci (Girault), and Leptomastix dactylopii Howard (Hymenoptera: Encrytidae) by Nephus kreissli Fürsch & Uygun (Coleoptera: Coccinellidae) was studied. The latter is a native predator of the important pest Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae) on grapevines in Turkey. For this purpose, P. ficus of different ages parasitised by A. pseudococci or L. dactylopii, or by both A. pseudococci and L. dactylopii, were served to fourth instars and adults of N. kreissli as food. Experiments were conducted using two different treatments: no-choice (served unparasitised or only one stage of parasitised mealybug) and choice (served unparasitised and only one age of parasitised mealybug together), under controlled environmental conditions. Both fourth instars and adult predators were fed on two- and four-day-old mealybugs parasitised by A. pseudococci or on two-, four- and six-day-old mealybugs parasitised by L. dactylopii or by either A. pseudococci or L. dactylopii. The predators could not consume six-day-old mealybugs parasitised by A. pseudococci, eight-day-old mealybugs parasitised by L. dactylopii, or those parasitised by either of these parasitoids which had become mummified. While it was found that the adult predators preferred parasitised mealybugs to unparasitised, the larvae did not show a pronounced preference between parasitised and unparasitised mealybugs (except for mealybugs parasitised by A. pseudococci).     

Bemisia tabaci (Homoptera: Aleyrodidae) instar effects on rate of parasitism by Eretmocerus mundus and Encarsia pergandiella (Hymenoptera: Aphelinidae)

Studies were conducted to compare preference among Bemisia tabaci Gennadius, biotype B instars for parasitization by Eretmocerus mundus Mercet and Encarsia pergandiella Howard when provided one instar only, two different instars, and four different instars simultaneously. In the single‐instar no choice treatment, Er. mundus was more successful in parasitizing the younger host instars, while En. pergandiella parasitized a greater proportion of the older instars. Similar results were observed when parasitoids were provided a choice of two instars in six different pair combinations. When all four instars were provided simultaneously, the numbers of first, second, and third instars parasitized by Er. mundus were not significantly different from each other (range 10.3–16.4%), but all were significantly higher than parasitism of fourth instar nymphs (2.1%). The highest percentage parasitization by En. pergandiella was in third instar (17.2%), and the lowest in first instar (2.8%).     

Host-stage selection by Trybliographa rapae, a parasitoid of the cabbage root fly Delia radicum

Host-stage selection by Trybliographa rapae Westwood (Hymenoptera: Figitidae) was studied in choice and no-choice experiments in the laboratory. The parasitoid was able to reproduce in first, second, and third instars of the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae), but oviposition occurred more frequently in third instars when all three developmental stages were offered simultaneously. Oviposition in third instars increased the rate of development of offspring and their body size, but did not alter sex ratio. Results are discussed in the light of predictions made by the theory of optimal host acceptance.     

Functional response of Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) to Aphis gossypii Glover (Homoptera: Aphididae) in the Laboratory

Stage specific functional response of Harmonia axyridis (Pallas) to varying densities of Aphis gossypii Glover was examined in a simplified cucumber leaf arena under laboratory conditions. All stages of H. axyridis were isolated individually for 24 h with different prey densities at 25 °C and a photoperiod of 16:8 (L:D) h. The number of prey consumed by the predator was checked at 3, 6, 12, and 24 h. All stages of H. axyridis showed a Type II functional response. Based on the random predator equation, estimated attack rates of H. axyridis at 24 h were 0.0037, 0.0442, 0.3590, 0.3228, and 0.1456, and estimated handling times were 4.1001, 2.4575, 0.7500, 0.2132, and 0.1708 h for the first, second, third, and fourth instars, and female adult, respectively.