Vulnerability of Bemisia tabaci immatures to phytoseiid predators: Consequences for oviposition and influence of alternative food

An earlier study showed that two phytoseiid species, Euseius scutalis (Athias‐Henriot) and Typhlodromips swirskii (Athias‐Henriot) (Acari: Phytoseiidae), are capable of suppressing populations of Bemisia tabaci (Gennadius) (Hemiptera: Aleurodidae) on isolated cucumber plants supplied with Typha latifolia L. pollen. However, the predators did not exterminate their prey, and this may be caused by the existence of invulnerable B. tabaci stages. Little is known of the differential vulnerability of the immature B. tabaci stages. Here, we quantified their vulnerability by assessing the rate of predation on each of the immature stages when offered alone at a density high enough to allow for a maximal predation rate. All immature stages of B. tabaci were vulnerable to predation by each of the two predator species. However, the per capita predation rates, the oviposition rates of phytoseiids, as well as the percentage of predators feeding and the percentage ovipositing decreased with increasing stages of B. tabaci. Compared to that of eggs and 1st instars, the vulnerability of 2nd and later B. tabaci instars is an order of magnitude lower. To investigate how the presence of alternative food changes the rates of predation, we added pollen to a diet of 1st instars, one of the most vulnerable instars. This resulted in a decrease in the predation rate of E. scutalis, but not of T. swirskii, while the oviposition rate of both phytoseiid species remained equally high. The decreased predation of the 1st instars probably resulted from E. scutalis switching to pollen feeding.     

Phytoseiid predators as potential biological control agents for Bemisia tabaci

Mites of the family Phytoseiidae are known to be predators of whiteflies in several agroecosystems, especially of Bemisia tabaci Gennadius, a pest with high resistance to chemical insecticides that occurs in greenhouses in temperate regions. We collected predatory mites that were found to co-occur with whiteflies in the Middle East for control of B. tabaci: Typhlodromus athiasae (Porath and Swirski), Neoseiulus barkeri Hughes, Typhlodromips swirskii (Athias-Henriot), Euseius scutalis (Athias-Henriot), Phytoseius finitimus Ribaga. As a first step in the evaluation of these species as biological control agents, we measured their life-history traits when feeding on whiteflies. The intrinsic rates of increase (r _m) of the predatory mite species ranged between 0.131 and 0.215 per day and E. scutalis had the highest r _m estimated. Comparisons with the r _m of B. tabaci indicate that some of the species should be capable of suppressing local populations of whitefly. The ability of predators to use alternative food was also tested, since it facilitates the setup of mass cultures and it can promote their persistence in the crop, even if the prey is scarce. All predatory mite species tested were able to survive and reproduce on a diet of broad bean pollen.     

Evaluation of phytoseiid predators for control of western flower thrips on greenhouse cucumber

Ten predatory mite species, all phytoseiids, were evaluated for control of western flower thrips (WFT), Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), on greenhouse cucumber. This study was done to further improve biological control of thrips on this crop. Neoseiulus cucumeris (Oudemans) is at present used for biological control of thrips in greenhouses. Compared to this species, Typhlodromalus limonicus (Garman & McGregor), Typhlodromips swirskii (Athias-Henriot) and Euseius ovalis (Evans) reached much higher population levels resulting in a significantly better control of thrips. T. limonicus was clearly the best predator of WFT. Also Euseius scutalis (Athias-Henriot) increased to higher populations levels than N. cucumeris, but without controlling the thrips, probably because of an unequal distribution of this predator on the plant. Iphiseius degenerans (Berlese), Neoseiulus barkeri (Hughes), Euseius finlandicus (Oudemans) and Typhlodromus pyri (Scheuten) did not establish better than N. cucumeris. A non-diapausing exotic strain of N. cucumeris did not differ from the North European strain. The best performers in this study were all of sub-tropical origin. T. limonicus, T. swirskii and E. ovalis have good potentials for controlling not only thrips but also whiteflies. Factors affecting the efficacy of phytoseiids on greenhouse cucumbers are discussed.     

利用植绥螨防治烟粉虱的研究进展

对目前利用植绥螨防治烟粉虱Bemisia tabaci(Gennadius)的研究现状进行综述。主要介绍植绥螨作为生防作用物的应用历史、替代食物在利用植绥螨进行生物防治中的作用,并对代表种Typhlodromips(Amblyseius)swirskiiAthias-Henriot等作为烟粉虱生防作用物的最新研究进展、产业化现状、应用前景及可能存在的问题进行了分析,以期为这些植绥螨的引进和利用提供参考。     

Herbivore host plant selection: whitefly learns to avoid host plants that harbour predators of her offspring

Evidence is accumulating that herbivorous arthropods do not simply select host plants based on their quality, but also on the predation risk associated with different host plants. It has been suggested that herbivores exclude plant species with high predation risk from their host range. This assumes a constant, predictable predation risk as well as a rather static behaviour on the part of the herbivore; plants are ignored irrespective of the actual predation risk. We show that adult females of a small herbivore, the whitefly Bemisia tabaci, can learn to avoid plants with predatory mites that attack only juvenile whiteflies, while they accept host plants of the same species without predators. Predatory mites disperse more slowly than whiteflies; they cannot fly and walk from plant to plant. Hence, by avoiding plants with predators, the whiteflies create a temporary refuge for their offspring. We suggest that the experience of arthropod herbivores with risks associated with host plants plays an important role in their host plant selection.     

不同季节黄瓜上烟粉虱自然种群生命表研究

1引言自从Morris等[10]应用生命表技术研究云杉卷叶蛾(Choristoneura fumiferana)自然种群动态以来,生命表技术成为昆虫种群动态的重要研究方法.庞雄飞等[11~13]在Mor-ris和Miller生命表的基础上进行了补充和改进,提出组建由作用因子组配的生命表和相应的分析方法.烟粉虱(Bemisia tabaci)属同翅目粉虱科.我国早在1949年就有烟粉虱的记载[27].长期以来,烟粉虱不是我国的主要害虫,近年来,由于一系列原因使烟粉虱成为蔬菜、园林植物上的重要害虫[1,5,9,16,17,21,22,25].有学者应用生命表方法评价各种措施对烟粉虱种群的控制作用[19,20].本研…     

Seasonal occurrence of specialist and generalist insect predators of spider mites and their response to volatiles from spider-mite-infested plants in Japanese pear orchards

In two adjacent Japanese pear orchards (orchards 1 and 2), we studied the seasonal occurrence of the Kanzawa spider mite, Tetranychus kanzawai, and its predators. Also the response of these predators to the volatiles from kidney bean plants infested with T. kanzawai was investigated using trap boxes in orchard 1. The mite density in orchard 1 was unimodal, with one peak at the end of August. In this orchard, population development of the specialist insect predators, Scolothrips takahashii, Oligota kashmirica benefica and Stethorus japonicus, was almost synchronized with that of the spider mites. These predators disappeared when the density of their prey became very low in mid-September. Both S. takahashii and O. kashmirica benefica abruptly increased in number in orchard 2 when the spider mite population in orchard 1 decreased. These results suggested that some of the predators migrated from orchard 1 to orchard 2. In this period, predator-traps with T. kanzawai-infested bean plants attracted significantly more S. takahashii than traps with uninfested plants. Very few individuals of S. japonicus and O. kashimirica benefica were found in the traps, despite their abundance in orchard 1. The generalist insect predator, Orius sp., was attracted to the traps throughout the experimental period irrespective of the density of spider mites, although this predator was never observed inside the orchards.     

Pollen availability and abundance of predatory phytoseiid mites on natural and secondary hedgerows

Mites occurring on a number ofnatural and secondary (i.e., planted) hedgerowslocated in north-eastern Italy were monitoredwith particular attention being paid tophytoseiids (Acari: Phytoseiidae). Pollendiversity and abundance were analysed tohighlight potential relationships betweenpollen availability and phytoseiids. Secondaryhedgerows were structured in modules containinga single species (field maple, hornbeam,dogwood, hazel, and elderberry) andmodules containing a mixture of theabove-mentioned species. These hedgerows werelocated in an experimental farm where they werecontiguous to a vineyard. Natural hedgerowswere located in the same area. The pollen ofPoaceae was the most important component ofnon-arboreal pollen (NAP), and that ofelderberry of arboreal pollen (AP). Pollendiversity was higher on natural than onsecondary hedgerows. Generally, pollendensities reached relatively high levels fromlate April to late June and decreased insummer. Pollen was more abundant on naturalthan on secondary hedgerows and phytoseiidabundance showed a similar trend. On elderberryand hornbeam in natural hedgerows a potentialrelationship between pollen availability andthe abundance of phytoseiids, in particular ofEuseius finlandicus (Oudemans) was found.Experimental pollen applications increased thefecundity and the abundance of this species.     

烟粉虱在不同寄主作物上的种群动态及化学防治

20 0 1年在河北省廊坊市研究了烟粉虱Bemisiatabaci(Gennadius)在主要寄主作物棉花、大豆、花生和玉米上的种群动态和化学防治方法。研究结果表明 ,烟粉虱的种群数量在寄主的生育期内持续增长 ,在 8月 2 2日左右达到高峰 ,之后逐渐减少。烟粉虱虽然寄主范围广泛 ,但在每种寄主上的发生量并不一致 ,调查结果表明烟粉虱在 4种不同寄主上的种群数量比较为 :棉花 >大豆 >花生 ,玉米上没有发现烟粉虱的卵及若虫 ,只有极少量的成虫在植株上逗留。在药效试验中 1 0 %吡虫啉WP、2 5 %扑虱灵WP和 1 0 %高效氯氰菊酯LC在厂家推荐浓度下对烟粉虱成虫的防治效果分别为 2 0 0 7%～ 3 3 1 9% ,2 0 62 %～ 44 88%和 4 0 5 %～ 6 78% ;对若虫的防治效果分别为 5 1 91 %～ 64 46% ,5 4 3 0 %～ 63 99%和 1 6 73 %～ 2 6 97%。     

Variation between laboratory populations of Encarsia formosa in their parasitization behavior on the host Bemisia tabaci

We tested the hypothesis that two populations of the parasitoid Encarsia formosa Gahan (Hymenoptera: Aphelinidae) differed in their behavioral interactions with the whitefly host Bemisia tabaci Gennadius (Homoptera: Aleyrodidae). The two wasp populations were studied because previous work suggested large differences between the populations in performance on this host. In this study the populations differed behaviorally in both the number of hosts encountered and their reactions to hosts once encountered. The population reared for many years on Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae) that previously performed more poorly on B. tabaci had a higher host encounter rate but rejected hosts more frequently. The population reared for a number of years on B. tabaci encountered fewer hosts but accepted a higher percentage of hosts for oviposition. The number of parasitized hosts did not differ between the two populations, however. These data demonstrate that there are heritable differences between these two populations of asexual wasps in host-associated behavioral traits. These behavioral differences in host acceptance do not explain performance differences seen in the earlier study, however, possibly due to different conditions between the two experiments.     

Population dynamics of the whitefly,Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), in Java,Indonesia, with special reference to spatio-temporal changes in the quantity of food resources

Six monitoring plots were established in the northern part of West Java, Indonesia, to clarify the factors that influence population fluctuations of the whitefly,Bemisia tabaci. Yellow sticky traps were used to monitor adult populations. To examine the relationship between the population fluctuations and the quantity of food resources, the quantity of food resources ofB. tabaci (i.e. soybean and mung bean) was also investigated routinely in each subdistrict where the experimental plot was located. The maximum number of adults per trap in each sowing season showed a similar noticeable seasonal trend in all experimental plots. Climatic factors did not have a major role to play in population fluctuations. The number of adults per trap in each experimental plot tended to increase when the quantity of food resources in each subdistrict increased. The changes of the quantity of food resources seemed to influence the population fluctuations. It is suggested that the operation of regulatory processes in population density is influenced largely by both the distance between habitat patches and the amplitude of temporal fluctuations of the quantity of food resources. An explanation for why the time lag between fluctuations of the quantity of food resources and the number of individuals ofB. tabaci occurred is discussed. Based on their flight ability and intrinsic rate of natural increase,B. tabaci is considered not to be a serious pest in the environment where the host plants are grown discontinuously in time and space.     

Variation between laboratory populations in the performance of the parasitoid Encarsia formosa on two host species, Bemisia tabaci and Trialeurodes vaporariorum

We tested the hypothesis that populations of the parthenogenetic parasitic wasp Encarsia formosa Gahan (Hymenoptera: Aphelinidae) differed in their ability to use two different host species, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae). Of the three wasp populations tested, two populations had been reared for many generations on B. tabaci and one population had been reared for many years on T. vaporariorum. Performance was measured by the number of whitefly nymphs that were successfully parasitized by individual wasps, and performance on either host was measured in separate experiments. There was variation between wasp populations in their performance on the host B. tabaci, with one wasp population reared for many years on this host performing considerably better than the other two populations. There were no significant differences between populations in their use of the preferred host, T. vaporariorum. The experiments were conducted in such a way that we could distinguish heritable differences between populations from environmentally-induced conditioning differences due to the immediate host from which an individual wasp enclosed. In either experiment there were no significant effects of conditioning, although there was a trend within each population for wasps conditioned on T. vaporariorum to have higher performance than those conditioned on B. tabaci. Thirdly, we conducted a selection experiment, initiated with wasps from a single population historically reared on T. vaporariorum, to measure the effect of laboratory rearing on different hosts for 17 generations. We did not see any difference in the performance of wasps on B. tabaci after this period of rearing on either of the two hosts. In summary, populations of E. formosa do differ in their relative performance on B. tabaci. The one population that was tested further did not show any response to selection by rearing, but the ability to respond to selection on performance may not be equal for all populations. The possibility that wasp populations have differential performance on particular hosts may affect the use of this species as a biological control agent.     

Quantifying the effects of predators on aphid populations

Four methods of analysis based on a simple population model and applicable to aphid and predator field density data are described. Two of the analyses determine the extent to which predators accounted for an observed change in aphid densities, and the other two estimate by how much the aphid population would have increased if the predators had not been present. The methods are illustrated with examples taken from cereal aphid research, and their advantages and limitations are discussed.

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Résumé Quatre méthodes d'analyse, basées sur un modèle de population simple, applicables aux données concernant les densités de pucerons et de prédateurs dans la nature, sont décrites. Deux des analyses déterminent l'importance de l'intervention des prédateurs lorsque l'on observe une modification de la densité de pucerons; les deux autres estiment de combien la population de pucerons aurait dû s'accroître en l'absence de prédateurs. Ces méthodes sont illustrées d'exemples tirés des recherches sur les pucerons des céréales. Possibilités et limites de ces méthodes sont discutées.

     

How larvae of Thrips tabaci reduce the attack success of phytoseiid predators

Neoseiulus barkeri (= Amblyseius mckenziei) and Amblyseius cucumeris (Acari:Phytoseiidae) are used as control agents of Thrips tabaci (Insecta:Thripidae) in greenhouse crops. Their success in capturing prey larval stages is related to both the feeding state of the predators and to the size of the larvae. When starved, predators are more successful in seizing larvae. Upon contact with a starved predator second stage prey larvae incur a lower death risk than first stage larvae. The larvae of T. tabaci reduce the attack success of their predators by jerking the abdomen and by producing a drop of rectal fluid. When this defensive behaviour is prevented by anaesthetising the larvae with CO₂, predator attack success increases. Anaesthesia does not, however, level out the difference in death risk of the two larval stages. Conceivable causes for this discrepancy are discussed.Availability of suitable prey is dependent on the dynamics of the age structure of the prey population and, hence, may be lower than total thrips density suggests. If so, alternative food sources may be important to maintain the predator population.

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Zusammenfassung Neoseiulus barkeri (= Amblyseius mckenziei) und Amblyseius cucumeris (Acari: Phytoseiidae) werden zur Bekämpfung von Thrips tabaci (Insecta: Thripidae) in Gewächshauskulturen eingesetzt. Sowohl der Ernährungszustand der räuberischen Milben als auch die Grösse der Thripslarven haben Einfluss auf das Ausmass der Beutenahme. Die Prädatoren sind erfolgreicher, wenn sie eine Zeitlang ohne Nahrung gehalten wurden. Beim Zusammentreffen mit einer ausgehungerten Raubmilbe besteht für Thripslarven des zweiten Stadiums ein geringeres Risiko erbeutet und gefressen zu werden als für Larven des ersten Stadiums. T. tabaci Larven mindern den Angriffserfolg der Prädatoren durch kräftiges Hin- und Herschlagen des Abdomens und durch Abgabe eines Tropfens Rektalflüssigkeit. Wird dieses Abwehrverhalten der Larven durch Anaästhesie mit CO₂ verhindert, erhöht sich der Angriffserfolg der Prädatoren. Anästhesie nivelliert jedoch nicht das für beide Larvenstadien unterschiedlich hohe Risiko erbeutet zu werden. Mögliche Ursachen für diesen Unterschied werden diskutiert.Die Verfügbarkeit geeigneter Beutetiere hängt ab von der zeitlichen Entwicklung der Altersstruktur ihrer Population. Das Angebot an wirklich geeigneten Beutetieren kan also unter Umständen geringer sein, als dies die Gesamtthripsdichte zunächst vermuten lässt. Ist das der Fall, dürften alternative Nahrungsquellen für die Ernährung der Prädatorenpopulation wichtig sein.

     

Impact of native predators and parasites on Spodoptera exigua, an introduced pest of alfalfa hay in northern California

A long-term study of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) revealed that a complex of native predators and parasites exploit this introduced pest in alfalfa-hay fields in northern California. In field trials, predators typically fed on >50% of sentinel egg masses during a given sample interval; mean predation rate ranged from 40 to 100% and percentage predation per individual egg mass was density-independent (spatial context) for 16 of 17 sample intervals. The egg-predator guild consisted primarily of adults and nymphs of Lygus hesperus Knight & L. elisus Van Duzee (Miridae), Nabis americoferus Carayon (Nabidae), and Orius tristicolor (White) (Anthocoridae); and adults of Collops vittatus (Say) (Melyridae) and Hippodamia convergens Guerin-Meneville (Coccinellidae). In laboratory feeding trials, these species readily fed on eggs, as well as neonate larvae of S. exigua. The parasite guild associated with small- to medium-sized larvae consisted of Hyposoter exiguae (Viereck) and Pristomerus spinator (F.) (Ichneumonidae), both larval endoparasites; and Chelonus insularis Cresson (Braconidae), an egg-larval endoparasite. Total parasitization by these species in field samples ranged from 5 to 75%, and was density-independent (spatial context) for each of four sample dates. A few larvae were infected with a nuclear polyhedrosis virus but no epizootics were observed. These results are consistent with the hypothesis that native natural enemies, especially generalist predators, can maintain populations of S. exigua at levels well below the treatment threshold for this exotic pest of alfalfa grown for hay.     

Intraguild predation on Eretmocerus sp. nr. emiratus,a parasitoid of Bemisia tabaci,by three generalist predators with implications for estimating the level and impact of parasitism

Intraguild predation (IGP) is pervasive in many managed and unmanaged ecosystems and may have negative, neutral or positive effects on the biological control of pest insects. Both generalist predators and aphelinid parasitoids attack Bemisia tabaci (Gennadius) Biotype B (=B. argentifolii Perring & Bellows) on cotton in the southwestern USA. Free-choice and no-choice laboratory assays were conducted to quantify prey consumption patterns and preference by three representative generalist predators, Geocoris punctipes (Say), Orius insidiosus (Say), and Hippodamia convergens Guérin-Méneville, offered fourth instar B. tabaci nymphs and nymphs parasitized by Eretmocerus sp. nr. emiratus. All three predators showed a significant preference for larval and pupal stage parasitoids over early fourth instar nymphs, but G. punctipes and O. insidiosus were non-discriminating when offered a choice of larval parasitoids and late fourth instar nymphs. The potential implications of these observed patterns for the field were examined through sensitivity analyses of existing field life table data. First, preference for parasitized hosts alters the methods required for calculating marginal rates of parasitism. Incorporating a preference variable in the estimation procedure had a very small positive effect (0.02–1.13% change) on total generational mortalities observed in previous life table studies. However, further hypothetical analyses suggested that under circumstances of lower generational mortality and higher levels of either apparent parasitism or predation, high levels of predator preference for parasitized prey could alter estimates of total mortality as much as 14%. Second, although intraguild predation was demonstrated, the implications for biological control are unclear. Based on field life table data the rate of IGP ranged from 0.019 to 0.126 depending on predator species and prey comparison, but accounting for these levels of IGP had only small negative effects on total generational mortalities (0.193–1.278% change).     

Control ofThrips tabaci [Thysanoptera: Thripidae] on glasshouse cucumber using large introductions of predatory mitesAmblyseius barkeri [Acarina: Phytoseiidae]

The phytoseiid miteAmblyseius barkeri (Hughes) (=Amblyseius mckenziei Sch. & Pr.) was used for biological control ofThrips tabaci Lind. in 7 commercial glasshouses with cucumber (a total of 5780 m²). Predatory mites were introduced 3–4 times in densities ranging from 40 to 300/m² at each release. In 6 of the 7 glasshouses, control of thrips was satisfactory throughout the growing season. Thrips densities were kept below 15 individuals per leaf. In 1 glasshouse, thrips damage was seen on the fruits at densities of 25 thrips per leaf, but the thrips population was quickly reduced and remained at low densities for the next 3 months.      

The influence of landscape diversity and heterogeneity on spatial dynamics of agrobiont linyphiid spiders: An individual-based model

Spiders are important generalist predators in natural pest control. However, agricultural fields are highly disturbed and ephemeral habitats, which present a number of challenges to the organisms living there; likewise landscape diversity and heterogeneity are also thought to be important factors in determining spider spatial dynamics. To investigate the interactions between these factors, we present an individual-based simulation model, which integrates life history characteristics of a typical agrobiont linyphiid spider with a dynamic spatially explicit landscape representation. The landscape contains several habitat types of varying quality and varies in time and space. Simulations showed that spatial landscape diversity (number of habitat types available for the spiders) is crucial for the persistence of spiders, but that spatial heterogeneity (spatial arrangement of patches) only had little impact on spider abundance. The necessary landscape diversity could either be provided by a diverse crop rotation or by including refuges in the form of less frequently managed habitats in the landscape. The presence of refuges greatly boosted numbers of spiders in the landscape as a whole. The most important characteristics of refuge were sanctuary from pesticides and extra prey availability, whereas tillage frequency mattered less. The simulations indicated that agrobiont linyphiids combination of high dispersal abilities and high reproductive rate enables it to exploit the transient resources of the different habitats in the agricultural landscape.     

Pearl bodies of Cayratia japonica (Thunb.) Gagnep. (Vitaceae) as alternative food for a predatory mite Euseius sojaensis (Ehara) (Acari: Phytoseiidae)

On the young leaves, shoots, and buds of Cayratia japonica (Thunb.) Gagnep. (Vitaceae), we observed nutritious bodies called pearl bodies and hypothesized that they are utilized by generalist predators as alternative foods. Some ambulate organisms consume pearl bodies in the wild and the predatory mite Euseius sojaensis (Ehara) (Acari: Phytoseiidae) was considered as a primary candidate. Pearl bodies promoted E. sojaensis settlement on C. japonica leaves and E. sojaensis could prey on the phytophagous mite Tetranychus kanzawai Kishida (Acari: Tetranychidae) when the predators settle on a leaf before the prey. In addition, the presence of pearl bodies did not reduce predation of E. sojaensis on T. kanzawai. This was seemingly because food quality of T. kanzawai was higher than pearl bodies. These results implied that pearl bodies on C. japonica leaves are utilized by E. sojaensis as alternative foods.     

Host plant influences pathogenicity of Beauveria bassiana to Bemisia tabaci and its sporulation on cadavers

Laboratory bioassays were conducted to determine host plant effect on pathogenicity of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuill. (Ascomycota: Hypocreales) to the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae). Fourth instar B. tabaci reared on cucumber, tomato, melon, green pepper, potato, eggplant, marrow, cabbage, bean or cotton, were treated with 1 × 10⁷ conidia/ml B. bassiana EABb 93/14-Tp isolate. Mortality caused by B. bassiana significantly increased with time and it was significantly affected by the host plant on which the nymphs were reared. Mean mortality of nymphs 8 days after inoculation ranged between 52.3±7.3 for nymphs reared on cotton and 91.8±5.8 for nymphs reared on cucumber. Average survival times of nymphs treated with the fungal suspensions were also significantly influenced by the host plant, with a mean of 4.7±0.1 days for nymphs reared on cucumber, 6.6±0.2 days for cotton and 6.9±0.1 days for green pepper. The production of newly formed conidia was also affected by host plant and varied from 111000±8600 conidia/cadaver for nymphs reared on cotton to 597000±28000 conidia/cadaver for those reared on melon.