Linking habitat complexity with predation of pests through molecular gut-content analyses

Improving the diversity of farm systems or landscapes can lead to more effective biological control by providing refuge and alternative resources for colonising natural enemies. Within an experimental cabbage agroecosystem, we examined the effects of habitat management (i.e. herbicide use and cover crops) on pest populations and predator community structure, and report one of the first studies on the trophic links in this system using molecular gut-content analysis. In response to herbicide and cover crop management treatments designed to create different levels of habitat diversity, we quantified the abundance of two pests, Plutella xylostella Linnaeus (Lepidoptera: Plutellidae) and Pieris rapae Linnaeus (Lepidoptera: Pieridae), and predators. We designed species-specific primers to detect prey DNA in predators' guts. Pieris rapae were significantly more abundant in plots where cover crops were killed early in the season, and habitat management generated unique predator communities in response to weed management treatments. Thirty-five per cent of predators tested positive for prey DNA, and habitat management had interactive effects on predation of P. xylostella. Combined we found that habitat management has variable effects on natural enemy–pest interactions.     

Comparing the predatory performance of green lacewing on cotton bollworm on conventional and Bt cotton

We compared the survival of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) eggs and larvae on Bt and conventional cotton, in the presence or absence of the generalist predator, green lacewing larvae, Mallada signatus, (Schneider) (Neuroptera: Chrysopidae). In small arenas, green lacewings consumed a similar number of H. armigera eggs (ave. 15.8 ± 1.3 on conventional, 12.6 ± 1.4 on Bt cotton per predator over 24 h) and larvae (ave. 6.8 ± 0.7 conventional, 6.5 ± 0.8 Bt per predator over 24 h) whether on Bt or conventional cotton leaves. Likewise, similar numbers of eggs were consumed by each lacewing larva searching whole plants of either Bt (ave. 15.5 ± 0.6 of 49 over 24 h) or conventional (ave. 13.6 ± 1.1 of 49 over 24 h). On conventional plants over 72 h, survival of H. armigera larvae was 72.8% and decreased to 37.7% when lacewings were present, giving a net consumption rate of 35.1% (8.6 prey per predator over 72 h). On Bt cotton plants, 13.6% of the H. armigera larvae survived after 72 h and this decreased to 1.7% when lacewings were present. This combination of mortality factors operated synergistically. Helicoverpa armigera larvae moved to fruiting structures on conventional or Bt cotton but failed to survive in the squares (young flower buds) when the impacts of Bt and lacewings were combined. The removal of first to second instar H. armigera larvae from squares of Bt cotton by predators has the potential to reduce immediate pest damage and, perhaps more importantly, remove potentially Bt‐resistant genotypes.     

Functional response and predation of Nabis kinbergii （Hemiptera. Nabidae） to Plutella xylostella （Lepidoptera. Plutellidae）

The functional response of adult Nabis kinbergii （Hemiptera： Nabidae） to density of diamondback moth Plutella xylostella （Lepidoptera： Plutellidae） was investigated under laboratory conditions. Holling＇ s （1959） type Ⅱ model was found to be a good fit for the observed functional response of this predator. The numbers of P. xylostella consumed increased with temperature from 15℃ to 35℃. The maximum number of prey killed was observed at 35℃, with average of 10.3 and 8.3 forth instar larvae consumed by adult females and males of N. kinbergii, respectively. The predation of N. kinbergii on P. xylostella increased with successive immature stages. The number of prey consumed by predators decreased as the body size of prey increased. An average of 131 eggs or 95 larvae of P. xylostella were killed by a single of female adult in 24 hours at 24＂C. The pupae of P. xylostella were observed to be eaten by fifth instar nymphs and adults N. kinbergiiin numbers of less than an average of 0.7 pupae per predator in 24 hours at 24＂C. Predation preference by N. kinbergii was also investigated. The number of P. xylostella and Myzus persicae killed by female N. kinbergii was not significantly different, but males killed significantly more P. xylostella than M. persicae. Both eggs and larvae of P. xylosteUa were killed in significantly greater number than those of Pieris rapae in the same feeding arena.     

Life table of Xylocoris afer (Hemiptera: Anthocoridae) feeding on eggs of Corcyra cephalonica (Lepidoptera: Pyralidae) and Plutella xylostella (Lepidoptera: Plutellidae)

The aim was to evaluate the biological aspects of Xylocoris afer fed on Corcyra cephalonica and Plutella xylostella eggs. The experiment was conducted with X. afer nymphs aged 12–24?h, one per Petri dish, resulting in a total of 60 nymphs (60 repetitions). On each dish, we placed P. xylostella and C. cephalonica eggs daily, together with a cotton roll moistened with distilled water and conducted daily evaluations. We evaluated the duration, viability, and consumption of nymphal stages, the longevity of males and females, the number of eggs per female, and egg fertility. In addition, the biological data were used to determine the parameters of the fertility life table. The consumption by adults (male and female) of X. afer differed between the preys, the highest consumption was C. cephalonica eggs. Females that consumed eggs of C. cephalonica produced about 90% more eggs and more offspring than those that fed on P. xylostella eggs. Based on our results, it can be inferred that the predator completed its development feeding both on natural prey and the alternative, evidencing the possible potential of X. afer to be used in the biological control of P. xylostella as well as the use of C. cephalonica in mass rearing of this predator under laboratory conditions, aiming its use in applied biological control. However, before being accepted for biological control agent against agricultural pests, various factors should be examined in ecosystem level, as the behavior of predator in different densities of prey.     

Resistant cabbage cultivars change the susceptibility of Plutella xylostella to Bacillus thuringiensis

1 Laboratory studies demonstrated that the susceptibility of larvae of the lepidopteran crucifer pest Plutella xylostella to the insect pathogen Bacillus thuringiensis (Bt) was influenced by the host plant. 2 Larvae reared on the resistant cabbage cultivars Minicole F₁ and Red Drumhead were significantly more susceptible to Bt (the LC₅₀ fell to one half) than larvae fed leaves of susceptible cultivars. 3 However, a third resistant cultivar, Aquarius F₁, had no synergistic effect on Bt‐related mortality. 4 Actual uptake of Bt was monitored in the bioassays, as a preliminary experiment showed that the plant resistance reduced consumption of Bt‐treated leaf discs. However, differences in feeding rate did not explain the observed differences in mortality.     

Life stage and population density of Plutella xylostella affect the predation behavior of Euborellia annulipes

The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), is the main pest of brassica crops worldwide. The ringlegged earwig, Euborellia annulipes (Lucas) (Dermaptera: Anisolabididae), has been reported as a potential predator of lepidopteran larvae, including this pest, and may therefore be used for biological control. Knowledge about predator–prey interactions is important to establish pest management strategies. Therefore, the objective of this work was to evaluate the influence of the developmental stage (larva and pupa) and density of P. xylostella on the preference and functional response of E. annulipes adult females. We used choice and no‐choice tests to evaluate the foraging behavior and preference of E. annulipes on DBM life stages and varied prey density to assess the type of functional response of the ringlegged earwig. Larvae were preferred over pupae, and the predator’s functional response was type II for both prey stages. Our results report the potential of E. annulipes as a biocontrol agent of P. xylostella. Understanding their interactions may help in decision‐making and optimization of integrated management strategies.     

Behavioral and physiological responses of susceptible and resistant diamondback moth larvae to Bacillus thuringiensis

To determine whether field-selected resistance of diamondback moth (Plutella xylostella L.) (Lepidoptera: Plutellidae) to Bacillus thuringiensis is based on behavioral or physiological adaptation, we measured mortality, consumption, and movement of larvae from a susceptible and a resistant colony when placed on untreated and B. thuringiensis treated cabbage. Colonies did not differ in mortality, consumption, or movement on untreated cabbage. However, for a given amount of consumption of treated cabbage, resistant larvae had lower mortality than susceptible larvae, demonstrating that resistance had a physiological basis. The movement patterns could not account for the differences between colonies in survival. Resistant larvae did not avoid B. thuringiensis more than did susceptible larvae. Thus, we found no evidence for behavioral resistance.     

Comparative bioassay of different isolates of Bacillus thuringiensis subsp. kurstaki on the third larval instars of diamondback moth,Plutella xylostella (L.) (Lep.: Plutellidae)

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae) is one of the most important pests of cruciferous plants throughout the world. In recent years, this insect has been a serious pest for cabbage fields in Tehran province. Resistance of P. xylostella to all main groups of insecticides has been recorded and it is ranked in the 20 most resistant pest species reported up to now. According to many researchers, to eliminate the problem of pest resistance to chemical pesticides, an integrated pest management programme should be used. In line with this, the uses of microbial control agents (MCAs) are discussed. The bacterium, Bacillus thuringiensis (Bt) is one of microbial control agents of pests. It is characterised by its ability to produce proteic crystalline inclusions during sporulation. Cry1 protein has insecticidal activity and is highly specific to certain insects and not toxic to unrelated insects, plants or vertebrates. In this work, the pathogenicity of some Bt isolates, including Dipel, 20, 29, 79 and 87, was tested against P. xylostella and the lethal concentrations (LC₅₀) of their crystal proteins to P. xylostella third larval instar was determined. The experiment was designed in factorial in randomised complete design with 5 treatments (different concentrations including 10⁴, 10⁵, 10⁶, 10⁷, 10⁸ CFU/ml and 5 replications and with 10 third larval instars. Spore–crystal complex was applied to the surface of natural diets (cabbage leaves) and the mortality of P. xylostella larvae was assessed 120?h after exposure of Bt toxin in each treatment. Results showed that percentage of survival was significantly higher for control treatment. Results also showed that after 5?days, LC₅₀ for isolates of Dipel, 20, 29, 79 and 87 were equal to 1?×?10⁶, 1?×?10⁵, 5?×?10⁵, 4?×?10⁵ and 1?×?10⁴ CFU/ml, respectively. LT₅₀ were equal to 93.71, 48.04, 71, 40.49 and 75.28?h. Of and most the percentage larval mortality relate to attendance 87 and also at least percentage mortality is related to the groom Dipel.     

Does Bacillus thuringiensis have adverse effects on the host egg location by parasitoid wasps?

This study investigated the interaction between two pest biological control agents, the parasitoid wasp Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) and the entomopathogen Bacillus thuringiensis (Bacillales: Bacillacea) (Bt). The aim of this study was to evaluate if the presence of Bt (formulated products Agree^®, Dipel^® and HD1 and HD11 strains) interferes in the oviposition preference of T. pretiosum to eggs of Helicoverpa zea (Lepidoptera: Noctuidae). Using an olfactometry test, the eggs of H. zea were bathed with the commercial formulations, with the Bt suspensions or distilled water, and offered to the parasitoid wasps in order to evaluate parasitism. The results showed that H. zea eggs sprayed with commercial formulations and Bt strains did not interfere in the choice made by the parasitoid. The parasitoid wasp is not able to distinguish between eggs with or without B. thuringiensis treatment, independently of strains suspension or commercial formulations. Therefore, these two control agents may be used together without negative interaction.     

Relationship between predation by Podisus nigrispinus and developmental phase and density of its prey,Plutella xylostella

This study evaluated the predation by Podisus nigrispinus (Dallas) (Hemiptera: Pentatomidae) at various densities of larvae and pupae of the pest Plutella xylostella (L.) (Lepidoptera: Plutellidae). We tested predator behavior of female P. nigrispinus at six experimental densities (1, 5, 10, 15, 20, or 25 prey items in a 1‐l transparent plastic container, replicated 15 times for each density) of both the fourth instar and pupae of P. xylostella. The number of prey consumed was monitored every 15 min for 12 h and was subsequently monitored at 24 h. Podisus nigrispinus females were weighed before and after the experiments to determine the effect of different densities of prey on their weight gain. Female predators had a Type‐II functional response, with attack rate estimated at 1.387 and 0.260 and a handling time of 0.091 and 0.183 h^?1 for larvae and pupae, respectively. Podisus nigrispinus consumed on average 10.9 larvae or 5.5 pupae in 24 h. Despite the similarity of the response type, P. nigrispinus preferred to feed on larvae, rather than on pupae.     

Host Preferences of Trichogramma pretiosum and the Influence of Prior Ovipositional Experience on the Parasitism of Plutella xylostella and Pseudoplusia includens Eggs

Trichogramma wasps are generalist egg parasitoids used in biological control efforts. In a multi host situation they may preferentially parasitize a non-target host species to the detriment of the control program. Plutella xylostella Linnaeus (Lepidoptera: Plutellidae) is a very serious pest of cabbage, but is only one in a number of species in the ‘cabbageworm’ complex. We investigated the host preferences of Trichogramma pretiosum Riley (Hymenoptera: Chalcidoidea) when offered the eggs of Plutella xylostella and the eggs of Pseudoplusia includens Hübner (Lepidoptera: Noctuidae), another species in the ‘cabbageworm’ complex. Trichogramma pretiosum reared on the eggs of the factitious host Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) parasitized both Plutella xylostella and Pseudoplusia includens eggs under laboratory conditions. For both choice and no-choice experiments, T. pretiosum parasitized significantly more P. xylostella eggs than P. includens eggs. Prior ovipositional experience with one or other of the two host species had no effect on the subsequent parasitism levels of the two host species. The preference for P. xylostella eggs was also not affected by this prior ovipositional experience.     

Spider mite infestations reduce Bacillus thuringiensis toxin concentration in corn leaves and predators avoid spider mites that have fed on Bacillus thuringiensis corn

Perceived benefits of insecticidal transgenic crops include reduced usage of broad‐based insecticides, and therefore lower risk to non‐target organisms. Numerous studies have documented low or no direct toxicity of Bacillus thuringiensis (Bt)‐derived toxins against non‐target organisms, but there has been less research on (a) effects of secondary pest infestations on Bt expressing in crops and (b) behavioural responses by predators feeding on host arthropods from Bt crops – both topics are investigated in this study. We quantified predation by the obligate spider mite predator Phytoseiulus persimilis of carmine spider mites (Tetranychus cinnabarinus), reared on Bt or non‐Bt corn (Zea mays). Both no‐choice and two‐choice studies were conducted. In addition, we quantified toxin levels in corn leaves with/without spider mite infestation. Under no‐choice conditions, P. persimilis consumed non‐Bt spider mites at a faster rate than Bt spider mites. Under two‐choice conditions, P. persimilis spent more time in the vicinity of non‐Bt spider mites than near Bt spider mites. Corn infested with spider mites exhibited lower toxin levels than non‐infested plants. These results suggest potentially complex interactions among non‐target herbivores, their natural enemies and Bt crops.     

The combined use of <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> and <Emphasis Type="Italic">Nesidiocoris tenuis</Emphasis> against the tomato borer <Emphasis Type="Italic">Tuta absoluta</Emphasis>

Since Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) was first detected at the end of 2006 in the Mediterranean Basin, several endemic natural enemies have been reported to prey on this exotic pest. The predator Nesidiocoris tenuis Reuter (Hemiptera: Miridae) can regulate T. absoluta populations, because it is able to prey efficiently on T. absoluta eggs. Furthermore, previous studies have demonstrated that first-instar larvae of T. absoluta are highly susceptible to Bacillus thuringiensis (Bt) treatments. In this work, we tested the combination of both approaches under greenhouse conditions. B. thuringiensis formulations were sprayed weekly for two months, three months or throughout the growing cycle, and in all cases, one N. tenuis per plant was also released. Control plants were completely destroyed by the infestation levels reached by T. absoluta. In contrast, all treatments based on B. thuringiensis treatments and releases of N. tenuis reduced leaf damage by more than 97% when compared to the untreated control, with no significant differences among them. Furthermore, yield in the control plants was significantly reduced when compared with all Bt–N. tenuis treatments. Our results demonstrate that when B. thuringiensis treatments are applied immediately after the initial detection of T. absoluta on plants, they do not interfere with N. tenuis establishment in the crop because T. absoluta eggs are available. According to our data, treatments with B. thuringiensis later in the growing season would no longer be necessary because mirids alone would control the pest.     

Effect of treating eggs of cotton bollworm with Bacillus thuringiensis Berliner on functional response of Trichogramma brassicae Bezdenko

Cotton bollworm, Helicoverpa armigera (Hübner) is a cosmopolite insect pest of a wide spectrum of crops such as cotton, maize, tomato, soybean, etc. Egg parasitoids mainly Trichogramma brassicae Bezdenko and Bacillus thuringiensis Berliner (Bt) are biological control agents, that are used as components of sustainable and environmentally compatible IPM systems. Although Bt does not come in direct contact with egg parasitoids, it may persist within the host’s body and affect the quality of the host’s eggs via biochemical changes in their mother and possibly behaviour and potency of the parasitoids. In this study, the functional response of T. brassicae to different densities of H. armigera eggs was investigated in two sets of experiments at 26?±?1?°C, 65?±?5% RH, and 16: 8?h photoperiod. The first group was a control and the second one were eggs laid by hosts treated as 3rd instar larvae with LC₂₀ of Bt (determined as 9.8?×?10^5?IU/l of artificial diet based on a preliminary bioassay). A type III functional response was observed in both treatments with a direct density dependent mortality up to eight host eggs and an inverse one upward. Both handling time and searching efficiency were affected by Bt treatment as the handling time was increased by a factor of 1.5 and the searching efficiency was decreased by a factor of 0.6. The searching efficiencies were 0.0310?±?0.003 and 0.0182?±?0.005?h^?1, and handling times were 1.134?±?0.042 and 1.672?±?0.082?h in control and Bt treatment respectively.     

Insecticidal effects of selected biological control agents on the larvae of Plutella xylostella (Lepidoptera: Plutellidae)

To identify a more effective and safe biological control agent against a common cabbage pest, Plutella xylostella (L.) (Lepidoptera: Plutellidae), the insecticidal effects of selected biological agents were evaluated. The highest insecticidal effects determined were 100, 73.5, 45.5, 47 and 55.3% using toxin HD‐1 (isolated from the Harry Dumagae strain of Bacillus thuringiensis), toxin BTS‐1 (isolated from the tenebrionis strain of B. thuringiensis), B. thuringiensis Berliner, B. thuringiensis israelensis and B. thuringiensis kurstaki, respectively.     

Functional response of Macrolophus pygmaeus (Rambur) and Nesidiocoris tenuis (Reuter) feeding on two different prey species

Functional response type and predatory parameters of Macrolophus pygmaeus and Nesidiocoris tenuis (Hemiptera: Miridae), the two important predators widely used in IPM programmes at tomato greenhouses, were investigated. The predators fed on Tuta absoluta (Lepidoptera: Gelechiidae) or Ephestia kuehniella (Lepidoptera: Pyralidae) eggs. Different densities of prey eggs including 1, 2, 3, 4, 5, 8, 10, 15, 20, 30 and 40 (latest only for E. kuehniella) were used at laboratory conditions. The results showed that both predatory bugs had type II functional response. Also, predation indices were significantly different between the predators. Although N. tenuis was more efficient against T. absoluta eggs than M. pygmaeus, M. pygmaeus predatory indices showed that this predator was more efficient on E. kuehniella eggs. Thus, minimum and maximum attack rate were observed in N. tenuis fed on E. kuehniella (0.0871 h^?1) and T. absoluta (0.2514 h^?1) eggs, respectively. Whilst, the minimum and maximum handling time were observed in M. pygmaeus fed on E. kuehniella (1.8695 h) and T. absoluta (2.7415 h) eggs, respectively.     

Bt rice could provide ecological resistance against nontarget planthoppers

Genetically engineered (GE) rice lines expressing Lepidoptera‐active insecticidal cry genes from the bacterium Bacillus thuringiensis (Bt) have been developed in China. Field surveys indicated that Bt rice harbours fewer rice planthoppers than non‐Bt rice although planthoppers are not sensitive to the produced Bt Cry proteins. The mechanisms underlying this phenomenon remain unknown. Here, we show that the low numbers of planthoppers on Bt rice are associated with reduced caterpillar damage. In laboratory and field‐cage experiments, the rice planthopper Nilapavata lugens had no feeding preference for undamaged Bt or non‐Bt plants but exhibited a strong preference for caterpillar‐damaged plants whether Bt or non‐Bt. Under open‐field conditions, rice planthoppers were more abundant on caterpillar‐damaged non‐Bt rice than on neighbouring healthy Bt rice. GC–MS analyses showed that caterpillar damage induced the release of rice plant volatiles known to be attractive to planthoppers, and metabolome analyses revealed increased amino acid contents and reduced sterol contents known to benefit planthopper development. That Lepidoptera‐resistant Bt rice is less attractive to this important nontarget pest in the field is therefore a first example of ecological resistance of Bt plants to nontarget pests. Our findings suggest that non‐Bt rice refuges established for delaying the development of Bt resistance may also act as a trap crop for N. lugens and possibly other planthoppers.     

Determining the frequency of heteropteran predation on sweetpotato whitefly and pink bollworm using multiple ELISAs

The gut contents of field-collected, predaceous Heteroptera were assayed for the presence of eggs of the sweetpotato whitefly,Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and the pink bollworm,Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae) using multiple enzyme-linked immunosorbent assays (ELISAs). Of seven species examined,Geocoris species andOrius tristicolor (Say) were the most frequent predators of sweetpotato whitefly with 32–39% of the individuals tested over the whole season scoring positive for whitefly antigens. With the exception ofLygus hesperus Knight, a major insect pest as well as a predator, the frequency of predation on pink bollworm eggs was much lower (0.7–14.3% positive over the season). Relatively few predators tested positive for both antigens (0.3–12.5%).     

Plant-mediated vulnerability of an insect herbivore to Bacillus thuringiensis in a plant-herbivore-pathogen system

Laboratory studies were performed to explore the effects of host-plant quality on the vulnerability of Plutella xylostella to Bacillus thuringiensis. P. xylostella were kept on different host plants, including Brassica pekinensis (Chinese cabbage) cv. Hero, Brassica oleracea var. botrytis (cauliflower) cv. Royal, and B. oleracea var. capitata (common cabbage) cv. Globe Master (white cabbage) and cv. Red Dynasty (red cabbage) for at least two generations. These host plants are considered as the high (Chinese cabbage), intermediate (cauliflower and white cabbage) and low-quality (red cabbage) hosts for P. xylostella. The vulnerability of the pest larvae was then tested using two formulation of B. thuringiensis var. kurstaki, including Biolarv^® and Biolep^®. The results demonstrated that the susceptibility of P. xylostella to B. thuringiensis was influenced by host-plant quality. Indeed, B. thuringiensis acted better on the pest fed on the low-quality host plant compared with that on the high-quality host plant. The interaction between the pathogen and plant quality/resistance resulted in more mortality of the pest larvae, implying a synergistic effect. From a pest management viewpoint, these findings may be promising for the integration of the pathogen and the low-quality/partially resistant host plants against P. xylostella in field studies.     

Interactions of Bacillus thuringiensis bioinsecticides and the predatory stink bug Podisus nigrispinus to control Plutella xylostella

The diamondback moth (DBM), Plutella xylostella (L.), is a major pest of brassica crops worldwide. Control of this pest is difficult because it rapidly develops resistance to synthetic and biological insecticides and because of the effects of insecticides on its natural enemies. Podisus nigrispinus (Dallas) is a predator that feeds on its prey, as well as on the host plants of its prey, and is an important biological control agent of DBMs. The aim of this study was to determine the susceptibility of P. xylostella larvae to two bioinsecticides: the HD1 strain of Bacillus thuringiensis (B. thuringiensis var. kurstaki) and the commercial product Agree^® (B. thuringiensis var. aizawai CG 91). In addition, the impact of these bioinsecticides on the P. nigrispinus consumption of DBM larvae and phytophagy was evaluated. Both the HD1 strain and Agree^® caused 100% mortality in P. xylostella larvae. P. nigrispinus nymphs fed only with kale leaves (Brassica oleracea var. acephala) sprayed with water, the HD1 strain, or Agree^® did not complete their nymphal development. When prey was also available, P. nigiripinus fed on kale leaves to obtain water. Both nymphs and adults of P. nigrispinus consumed greater numbers of DBM larvae, and fed less on kale leaves, when sprayed with the HD1 strain or Agree^®. These results suggest a positive interaction of B. thuringiensis‐based products and the predator P. nigrispinus in the control of P. xylostella larvae.