Impact of Recombinant Baculovirus Applications on Target Heliothines and Nontarget Predators in Cotton

Recombinant baculoviruses have been genetically engineered to reduce the time to kill infected pests, thus reducing crop damage. In this study, wild-type viruses and recombinant viruses expressing a scorpion toxin were applied to cotton in response to larval infestations of Helicoverpa zea and Heliothis virescens in 1997 and 1998. A chemical standard and an untreated control acted as comparison treatments. The goals of this field study were to (1) assess the efficacy of recombinant baculoviruses in protecting cotton from larval feeding damage; (2) assess the impact of recombinant virus introductions on predator densities and diversity; and (3) determine if cotton predators acquire baculovirus by consuming infected heliothines. When applications were timed at larval emergence, certain recombinant virus treatments protected cotton from damage better than wild-type virus treatments and as well as the chemical standard. Differences in efficacy between recombinant and wild-type baculoviruses were not apparent if treatments were applied 3 to 4 days after peak larval emergence. Predator densities and diversity were similar among recombinant and wild-type baculovirus treatments, whereas plots treated with the chemical standard had consistently smaller predator populations. From polymerase chain reaction analyses of predators in 1997 and 1998, 1.7 and 0.2%, respectively, of predators had consumed a virus-infected heliothine. Nine of the 26 predators carrying viral DNA were positive for recombinant virus. Additionally, 13 of the 26 predators were found to disperse 13.5 to 105 m 2 to 5 days after initial virus applications. Five of these dispersing predators (0.2% of all predators evaluated) carried recombinant viral DNA. These results suggest that the potential for the inadvertent spread of recombinant viral DNA via dispersing predators is low.     

Populations of Predaceous Natural Enemies Developing on Insect-Resistant and Susceptible Tomato in North Carolina

Naturally occurring populations of immature and adultGeocoris punctipes,adultColeomegilla maculataand immature coccinellids were monitored on field-grown tomato lines susceptible and resistant toManduca sextaandHelicoverpa zea. Helicoverpa zeaandHeliothis virescenseggs and small larvae that serve as prey for these predators also were monitored. MoreH. zeaandH. virescenseggs and small larvae were found on resistant than on susceptible plant lines. However, similar populations of largeH. zeaandH. virescenslarvae were found on resistant and susceptible plants. The number of adultGeocoris punctipes,adultColeomegilla maculataand immature coccinellids on resistant plants was always as high or higher than the number on susceptible plants. The data demonstrate no incompatibility of host-plant resistance with biological control provided by these predaceous insects, but indicate that the number ofG. punctipesand coccinellids required to provide effective biological control may develop too late in the season to be of practical value. Large populations of stilt bugs (Jalysus wickhami,Hemiptera: Berytidae) and spiders were observed to occur earlier in the growing season than eitherG. punctipesor coccinellids and may be a significant source of mortality forH. zeaeggs and small larvae.     

Predators as potential dispersal agents of the nuclear polyhedrosis virus ofAnticarsia gemmatalis [Lep.: Noctuidae] in soybean

The relationships were surveyed among an introduced nuclear polyhedrosis virus (AgNPV), populations of its larval hostAnticarsia gemmatalis, and other invertebrates associated with soybean. Early applications of AgNPV to small plots effectively suppressedA. gemmatalis populations without causing detectable perturbations in populations of other phytophagous insects or invertebrate predators. Bioassays of predators collected within and outside treated areas demonstrated that many invertebrates readily fed upon AgNPV-infectedA. gemmatalis under field conditions. These results suggest that the predator complex enhances the epidemic potential of this baculovirus. The predators, by feeding upon virus infected larvae, are believed to play an important role in maintaining and disseminating virus inoculum in soybean. Florida Agricultural Experiment Station Journal Series No. 6272.     

Short-term changes in consumption and oviposition rates of Neoseiulus californicus strains (Acari: Phytoseiidae) after a diet shift

Short-term effects on consumption and oviposition rates of four strainsof Neoseiulus californicus (McGregor) after a diet shiftwere evaluated. The new feeding conditions experienced by the predators weresixfixed densities of eggs or protonymphs of Tetranychusurticae Koch placed on excised strawberry leaflet discs andmaintained under laboratory conditions (25±1°C,75–85% RH, 16L: 8D). The observations were made on the first and thefifthday of the experiment. The phytoseiids came from three long-term mass-rearedstrains fed on T. urticae, Dermatophagoides farinaeHughes,or Quercus spp. pollen, respectively. The fourth strainwascollected directly in a strawberry field. Time since diet transfer can be addedto the factors (i.e. feeding history and prey density) already known to affectthe functional and numerical responses of N. californicus,both when it feeds on prey eggs and protonymphs. If consumption rates wereaveraged over all strains and densities, 9.04 and 11.41 eggs, and 6.97 and 6.48protonymphs were consumed on the first and the fifth day, respectively. If thesame was done for oviposition rates, predators feeding on eggs produced 1.46and2.36 eggs/female/day, whereas predators feeding on protonymphs produced 1.35and2.29 eggs/female/day. Time had the greatest impact on the functional responseofthe strain that had previously fed on tetranychids, while an effect of time onthe numerical response was detectable in all strains.     

Comparison of wild type and genetically engineered nuclear polyhedrosis viruses of Autographa californica for mortality, virus replication and polyhedra production in Trichoplusia ni larvae

Virus replication and polyhedra production of two polyhedron-positive recombinant nuclear polyhedrosis viruses of Autographa californica, AcJHE.KK and AcAaIT which encode juvenile hormone esterase and scorpion toxin, respectively, were compared with those of a plaque purified wild-type nuclear polyhedrosis virus, AcMNPV-C6, in Trichoplusia ni larvae. Though average times required to kill the T. ni larvae increased with the age of the larvae, killing time by either recombinant virus was significantly shorter than that by wild-type virus. Killing time was reduced ca. 30% for AcAaIT-infected larvae and 5 to 8% for AcJHE.KK-infected larvae as compared to that for AcMNPV-C6-infected larvae. The average weight of larvae infected with AcAaIT was significantly lower than that of larvae infected with AcJHE.KK and AcMNPV-C6. The mean numbers of polyhedra produced in each larva inoculated with AcAaIT and AcJHE.KK were ca. 20% and 60%, respectively, of those for AcMNPV-C6. Total virus titers in AcMNPV-C6-infected larvae were significantly higher than those in AcJHE.KK- and AcAaIT-infected larvae until 2 days post infection.     

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.     

Resource-Dependent Giving-Up Time of the Predatory Mite, Phytoseiulus persimilis

We examined the effect of prey (Tetranychus urticae) egg density on leaving rate of the predatory mite, Phytoseiulus persimilis, from leaf disks using predators with different feeding experiences and levels of external volatile cues related to their prey. Predators stayed longer on disks with prey eggs than on those without prey eggs. However, at each prey egg density predators stayed longer in the absence of prey-related volatiles from an external source. Starved predators stayed longer in a prey patch than those that had not experienced starvation. At each prey density, starved P. persimilis consumed a greater proportion of prey eggs than satiated predators. The total prey consumption of starved predators appears to be related to their longer residence time on source disks compared to satiated predators and also the per capita consumption rate was greater for starved predators compared to satiated predators.     

Effects of chlorogenic acid-and tomatine-fed caterpillars on the behavior of an insect predator

If generalist insect predators are a selective force contributing to patterns of feeding specialization by insect herbivores, then predators should be deterred from eating allelochemical-fed prey. The attack and feeding behaviors of naive predators (Podisus maculiventris stinkbugs) reared on control caterpillars (Manduca sexta) fed plain diet were compared to experienced predators reared on caterpillars fed tomato allelochemicals. Tomatine-fed prey were found more quickly by both naive and tomatine-experienced predators, and chlorogenic acid-experienced predators were more stimulated to begin searching for prey. However, experienced predators were less likely to attack both chlorogenic acidfed and tomatine-fed caterpillars than were naive predators. These results indicate that allelochemical-fed prey were easier for predators to locate, but allelochemical-containing prey often deterred predation by experienced predtors.     

An evaluation of some natural enemies of Spodoptera exigua on sugarbeet in northern California

Eggs and small to medium-sizedlarvae of Spodoptera exigua (Hübner)(Lepidoptera: Noctuidae) are exploited by acomplex of natural enemies in spring-plantedsugarbeet fields in northern California. Fieldstudies revealed that predation on sentinel eggmasses ranged from 20 to 100%egg mass;predation rate was lowest in fields previouslytreated with methomyl and highest innon-treated fields. Predators typicallydestroyed all of the eggs in a given egg mass;percentage predation per egg mass was densityindependent (spatial context). Survival of eggs(to neonate larvae) in cages that excludedpredators ranged from 80 to $>$90%. Theegg-predator guild consisted of adults andnymphs of Orius tristicolor(White)(Anthocoridae), Nabis americoferusCarayon (Nabidae), Lygus hesperus Knight(Miridae), and Geocoris punctipes (Say)(Geocoridae); larvae of Chrysoperlacarnea (Stephens) (Chrysopidae); and adults ofCollops vittatus (Say) (Melyridae). Laboratory evaluation revealed that largelarvae of C. carnea and the adults of theother species (except for O. tristicolor)could consume 100 eggs of S. exigua in a48 h period. The parasite guild associatedwith small and medium-sized larvae consisted ofthree species: Hyposoter exiguae(Viereck) and Pristomerus spinator (F.)(Ichneumonidae), both larval endoparasites; andChelonus insularis Cresson (Braconidae),an egg-larval endoparasite. Parasitization in field samples ranged from 30 to 65%. Smalland medium-sized larvae were also infected witha nuclear polyhedrosis virus (NPV); rates ofNPV infection ranged from 0 to 35% in fieldsamples. These results are consistent withanecdotal evidence that natural enemies,primarily generalist predators, are largelyresponsible for maintaining populations ofS. exigua at relatively low levels innontreated sugarbeet fields.     

Influence of feeding interval on reproduction and longevity ofPodisus sagitta (Het.: Pentatomidae)

The effect of various feeding intervals on reproduction and longevity of the predatory bugPodisus sagitta (Fabricius) (Heteroptera: Pentatomidae) was studied in the laboratory. Isolated pairs of adult bugs were given moisture in excess and were fed 1 seventh-instar larva of the greater wax moth,Galleria mellonella (L.), with a mean weight of 90 mg every 1, 3, 7, 14 and 21 days. Reproduction ofP. sagitta reflected well the feeding intervals. Females fed at increasing time intervals reduced reproductive outputs by decreasing oviposition frequency and size of egg batches. Predators fed every 21 days produced a mean of only 1.0 egg per day in comparison with 9.9 eggs per day for females fed daily. Longevity of females fed daily was 110.7 days; females fed less frequently tended to live longer, with a maximum mean longevity of 217.5 days for females fed every 14 days. No cannibalism among adults was observed, but females were occasionally seen to feed on their own eggs. Availability of moisture was found to be essential for the predators to survive during periods of low food supplies. These findings are discussed in relation to the predator's adaptation strategies in conditions of prey scarcity.      

Use of Proteases to Improve the Insecticidal Activity of Baculoviruses

Basement membranes that surround the tissues of lepidopterous larvae act as potential barriers to baculovirus movement and establishment of systemic infection. Hence, one potential approach to improving the insecticidal activity of baculoviruses is to perforate or eliminate the basement membranes of their hosts, thereby facilitating the process of infection. Toward this end, we constructed six recombinant clones of Autographa californica nucleopolyhedrovirus (AcMNPV) that express three proteases that digest basement membrane proteins: rat stromelysin-1, human gelatinase A, and flesh fly (Sarcophaga peregrina) cathepsin L. Expression of these proteases was directed from either the ie-1 promoter (in AcIE1TV3.STR1, AcIE1TV3.GEL, and AcIE1TV3.ScathL) or the p6.9 promoter (in AcMLF9.STR1, AcMLF9.GEL, and AcMLF9.ScathL). Recombinant proteases were detected in the culture medium of cells infected with recombinant viruses by either zymography or azocoll assay. AcMLF9.STR1 and AcMLF9.ScathL caused premature cuticular melanization of 5th instar Heliothis virescens. Melanization and fragmentation of internal tissues were observed in half of the larvae infected with AcMLF9.ScathL and not at all in larvae infected with AcMLF9.STR1 or wild-type AcMNPV. Lethal-concentration bioassays revealed no significant differences in virulence toward H. virescens among the protease-expressing recombinants and wild-type AcMNPV. However, in survival-time bioassays, AcMLF9.ScathL killed H. virescens approximately 30% faster than AcMLF9.LqhIT2, a virus expressing an insect-selective scorpion neurotoxin from the p6.9 promoter. Larvae infected with AcMLF9.ScathL consumed approximately 26-fold less lettuce than wild-type virus-infected larvae. These results highlight the potential of improving baculovirus efficacy through the expression of proteases.     

Effect of cassava exudate and prey densities on the survival and reproduction of Typhlodromalus limonicus (Garman & McGregor) s.l. (Acari: Phytoseiidae), a predator of the cassava green mite,Mononychellus tanajoa (Bondar) (Acari: Tetranychidae)

The effects of cassava exudate and prey densities on reproduction and survival of the predatory mite, Typhlodromalus limonicus (Garman & McGregor) (Acari: Phytoseiidae), were investigated in the laboratory. Females were provided either cassava exudate ad lib. daily, low or high numbers of the cassava green mite prey, Mononychellus tanajoa (Bondar) (Acari: Tetranychidae) daily, or exudate for 5 or 10 days before switching to a low or high prey diet. Females fed only exudate laid no eggs. Females fed exudate before prey experienced a significant decrease (30%) in the number of eggs laid compared to females fed high numbers of prey daily. The reduction in fecundity was the result of prolonged preoviposition periods (2.0 days on prey daily vs 4.0 days on exudate before prey) and reduced number of eggs laid per female per day (1.7 eggs per female per day on prey daily vs 0.4 eggs per female per day on exudate before prey). Females fed only exudate had a greater survival rate and longevity than females fed prey daily or females fed exudate before a diet of prey. These results suggest that T. limonicus can survice for a limited period on cassava exudate during periods of low prey availability, but requires prey to complete oögenesis and propagate the population.     

Predators of Mesoplatys ochroptera in sesbania planted-fallows in eastern Zambia

Mesoplatys ochroptera Stål (Coleoptera: Chrysomelidae) is a serious pest of the tropical legume sesbania (Sesbania sesban (L.) Merrill) widely used for soil fertility improvement in southern Africa. Surveys were conducted between October 1997 and June 1999 in order to identify the predators of M. ochroptera in sesbania fallows in eastern Zambia. The Heteroptera Afrius yolofa (Guérin-Méneville), Glypsus conspicuus (Westwood), Macrorhaphis acuta Dallas, Mecosoma mensor (Germar), Rhinocoris segmentarius (Germar), and Deraeocoris ostentans (Stål), the carabid beetle Cyaneodinodes fasciger (Chaudoir), the ants Tetramorium sericeiventre Emery and Pheidole sp., and the lacewing Mallada sp. were recorded as predators of M. ochroptera for the first time. The pentatomid bugs G. conspicuus, M. acuta and M. mensor were the most common predators in sesbania fallows. Adults and the different nymphal stages of the three species preyed on larvae, adults and occasionally on eggs of the beetle. In the insectary, the adults of G. conspicuus, M. acuta and M. mensor consumed a significantly higher number of larvae (p < 0.001) compared to adults of M. ochroptera under both free-choice and no-choice conditions. The voracity of the three species showed a positive response to prey density. The fifth instar nymphs showed variation in daily consumption of larvae, the highest number of prey being consumed two to three days after the fourth molt. These predators also attacked other pests associated with sesbania and crop plants. The potential role of the predators in the natural control of pests in agroforestry systems is discussed, with a review of their prey species.     

The brine shrimp Artemia sp. as alternative prey for rearing the predatory bug Macrolophus caliginosus

The predatory bug Macrolophus caliginosus, which is widely used in greenhouse crops, is limited in its application by its high price. An important factor in the cost is the high price of Ephestia kuehniella eggs, the prey used in their mass rearing. In order to reduce their price, alternatives to moth eggs are currently being investigated. The brine shrimp Artemia sp. is produced in large quantities in saline lakes and is fed as live food source to the larvae of a variety of marine and freshwater organisms. In this study, we tested Artemia sp. as prey for rearing M. caliginosus from two strains. We evaluated developmental and reproduction parameters of the predator when fed nauplii, enriched nauplii with a fatty acid, dry cysts and hydrated cysts, and were compared with those obtained when the predator was fed with E. kuehniella eggs. Nauplii had a significant reduction in survivorship, a delay in development of nymphs and a low reproduction of adults. Nauplii enriched with docosahexaenoic acid (DHA, 22:6n − 3), a common practice for larviculture of some marine fish species, resulted toxic to M. caliginosus nymphs and survival was quite low. On the contrary, either dry or hydrated cysts from the two strains tested of the brine shrimp produced the same nymphal survivorship, nymphal development time and weight and fecundity of adults as those obtained with E. kuehniella eggs. Demographic parameters of the eighth generation of the predator reared with cysts of the two strains, either dry or hydrated, were as good as those of moth eggs. We concluded that Artemia sp. cysts were a good substitution prey for the mass rearing of M. caliginosus.     

Comparative studies of predation among feral,commercially-purchased,and laboratory-reared predators

The predatory activities of commercially-purchased adult Hippodamia convergens Guérin-Méneville and two laboratory-reared strains of adult Geocoris punctipes (Say) were compared with their feral counterparts. In single prey choice feeding tests, commercially-purchased and feral H. convergens were provided copious amounts of silverleaf whitefly, Bemisia argentifolii Bellows & Perring adults or eggs of pink bollworm, Pectinophora gossypiella (Saunders). Commercially-purchased H. convergens devoured more pink bollworm eggs and at a faster rate than the feral H. convergens. In contrast, feral H. convergens consumed more adult whiteflies and at a faster rate than commercial H. convergens. In multiple feeding choice tests, two distinctly different laboratory-reared strains and feral G. punctipes were provided a cotton leaf disk containing copious amounts of silverleaf whitefly eggs, nymphs, and adults. Virtually no predation was observed on whitefly eggs, but both laboratory strains of G. punctipes fed on more whitefly nymphs and adults than the feral G. punctipes. Moreover, both of the laboratory strains had a significantly faster feeding rate on adult whiteflies and spent significantly less time feeding on plant tissue than the feral strain. These results suggest that the augmentative biological control candidates retained their ability to prey on these selected prey items, even after being displaced from their overwintering site (H. convergens) or being reared in captivity (G. punctipes) for over 40 generations. Handling editor: Patrick De Clercq. This article reports the results of research only. Mention of a proprietary product does not constitute an endorsement or a recommendation for its use by the USDA.     

Effect of Coleus Size and Variegation on Attack Rates, Searching Strategy, and Selected Life History Characteristics of Cryptolaemus montrouzieri (Coleoptera: Coccinellidae)

Twenty-four-hour attack rates and the search strategy of third instar Cryptolaemus montrouzieri Mulsant (Coleoptera: Coccinellidae) attacking 1 to 16 third instar Planococcus citri Risso (Homoptera: Pseudococcidae) were measured on green and yellow-variegated Solenostemon scutellarioides (L.) Codd (= Coleus blumei (Bentham)) (Labiatae) plants of different sizes. Selected life history characteristics of C. montrouzieri fed different amounts of P. citri as prey from third instar to adults were also examined. On average, predators attacked 1 to 4 mealybugs, depending on the number of mealybugs and plant size. There was no effect of plant color on attack rates. Attack rates were positively related to prey density, whereas the estimated area searched by predators was inversely related to prey density. Analyses suggest that leaf area was the plant characteristic that most affected attack rates. Predators fed few prey had a decrease in body weight and survival. The implications for the use of C. montrouzieri in biological control are discussed.     

Interaction between Podisus maculiventris and Harmonia axyridis two predators used in augmentative biological control in greenhouse crops

Intraguild predation (IGP) betweenthe pentatomid Podisus maculiventris(Say) and the coccinellid Harmoniaaxyridis (Pallas) in the absence or presenceof the extraguild prey Spodopteralittoralis (Boisduval) and Myzuspersicae (Sulzer) was studied in thelaboratory. Interactions were asymmetric infavor of the pentatomid. Podisusmaculiventris readily fed upon eggs and larvaeof H. axyridis, but adult beetles wererarely attacked. Success of attacks by P.maculiventris was stage dependent, fourthinstars and adults being more successful inkilling ladybeetle larvae than second instars.Attacks by H. axyridis on the pentatomidwere rare and none of them were successful. Theeffect of introducing extraguild prey on thelevel of IGP was tested both in petri dishesand on caged sweet pepper plants. Whensufficient numbers of S. littoralislarvae were present to satiate the pentatomid,predation on H. axyridis larvae decreasedsignificantly, indicating that the coccinellidis a less preferred or less vulnerable prey.When the aphid M. persicae was presentedas extraguild prey, levels of IGP were notaltered. Nymphs of P. maculiventrissuccessfully completed development whenexclusively fed on larvae of H. axyridis,but developmental time was longer than onlepidopteran prey. No pentatomid nymphs reachedadulthood on aphids alone. IGP by P.maculiventris on H. axyridis may be ofsome importance in greenhouse crops, where bothpredators are being used increasingly inaugmentative biological control programs.Nonetheless, it is expected that in practicelarger larvae and adults of H. axyridiswill escape most attacks by the pentatomid.     

Life history characteristics of Orius insidiosus (Say) fed Aphis glycines Matsumura

The soybean aphid, Aphis glycines Matsumura, is a new invasive pest of soybeans throughout most of the soybean production areas of North America. Field studies have demonstrated that the indigenous predator, Orius insidiosus (Say), is an important natural enemy of the soybean aphid early the soybean crop season. Because soybean aphid is newly introduced into North America, the life history characteristics of predators fed this aphid are not known. In laboratory assays, we measured the survival, development, longevity and reproduction of O. insidiosus fed 1, 3, 6 or 12 seconds to third instars of soybean aphid. O. insidiosus nymphal development decreased from 34.0 to 21.4 days as the number of soybean aphid nymphs provided increased from 1 to 6 aphid nymphs daily. Stage-specific mortality was highest at 68% for first instar O. insidiosus nymphs fed 1 soybean aphid nymph per day. Adult longevity (43.9 days) and fecundity (49.7 eggs per female) was highest for O. insidiosus fed 6 soybean aphid nymphs daily, but longevity (23.5 days) and fecundity (10.1 eggs per female) declined for adults fed 1 soybean aphid nymph daily. The intrinsic rate of increase of O. insidiosus ranged from 0.048 to 0.133. Compared to other prey species, soybean aphid is an adequate prey item for O. insidiosus. Our results suggest that O. insidiosus will be most effective in suppressing soybean aphid population growth in the initial phase of the aphid’s colonization of soybeans.     

Spatial distribution of interacting insect predators: Possible roles of intraguild predation and the surrounding habitat

Predator foraging behaviour affects the outcome of enemy–enemy interactions. Using a combination of fieldwork and laboratory experiments, we show that intraguild predation may be important in the field distribution of generalist predators that share a common prey: the eggs (and larvae) of the leaf beetle Phratora vulgatissima, a major insect pest in coppicing willow plantations. We focused on a species from the hoverfly genus Parasyrphus (Syrphidae), which may exhibit large temporal and spatial variation in density. Predator and prey densities were quantified in 40 field plots in willow plantations. The likelihood of finding hoverfly eggs declined with increasing densities of two predatory mirids, Orthotylus marginalis and Closterotomus fulvomaculatus, which exhibit less mobile behaviour similar to that of hoverfly larvae. The density of a more mobile predatory bug species, the anthocorid Anthocoris nemorum, was not associated with hoverfly occurrence. These results corroborate the hypothesis that less mobile predators should be stronger intraguild predators than mobilepredators. Further partial support for this hypothesis was obtained in the laboratory study where individual predators were presented with clutches of P. vulgatissima eggs containing one hoverfly egg: the less mobile C. fulvomaculatus and O. marginalis tended to consume the hoverfly egg more readily than the more mobile A. nemorum. However, most individuals of all three bug species consumed the egg of the potential competitor – the syrphid – within 24 h. The field study also showed that hoverfly occurrence was positively associated with the density of their prey and with the presence of nearby forests. We conclude that intraguild predation, abundance of prey and the surrounding habitat affect the distribution of hoverflies in this system and should be considered when developing biological control methods.     

Patch-Leaving Decision of the Predatory Mite Amblyseius womersleyi (Acari: Phytoseiidae) Based on Multiple Signals from Both Inside and Outside a Prey Patch

We compared the emigration rates of Amblyseius womersleyi from prey patches (leaf disks) of different conditions in airflow containing either infested plant volatiles (volatiles airflow) or uninfested plant volatiles (control airflow). Both airflow and prey patch conditions significantly affected the emigration rates. Emigration rates from patches carrying prey products (feces, exuviae, webs, etc.) and prey eggs were significantly lower in control airflow than in volatiles airflow. Under other patch conditions, the rates were lower in control airflow than in volatiles airflow, although the difference was not significant. In both airflows, the lowest emigration rates were observed when predators were in a heavily infested patch. Patches carrying prey products and prey eggs resulted in lower emigration rates than patches carrying eggs alone and patches emitting prey-infested plant volatiles but carrying no prey. Thus, A. womersleyi appears to decide the timing of emigration based on two criteria: prey products in the patch and prey-infested plant volatiles from outside.