The potential of pathogens for pest control

A large number of viruses, bacteria, fungi and protozoa can kill or incapacitate insects. Some of these have the potential for a significant role in the management and regulation of pest species of insects, either as naturally occurring entomopathogens or as applied or introduced insecticidal agents. The usefulness of entomopathogens, especially the feasibility of development as microbial insecticides, is examined by consideration of their effectiveness, safety and specificity, production and propagation, and marketability and profitability. Effectiveness, a major consideration in assessing potential, is influenced by several factors including efficacy, dissemination, persistence, compatibility with other regulatory agents, and establishment in the host population, and by factors such as humidity, temperature, chemicals, and formulation, that directly influence the activity of the pathogen.     

Gustatory receptor neuron responds to DEET and other insect repellents in the yellow-fever mosquito, Aedes aegypti

Three gustatory receptor neurons were characterized for contact chemoreceptive sensilla on the labella of female yellow-fever mosquitoes, Aedes aegypti. The neuron with the smallest amplitude spike responded to the feeding deterrent, quinine, as well as N,N-diethyl-3-methylbenzamide and other insect repellents. Two other neurons with differing spikes responded to salt (NaCl) and sucrose. This is the first report of a gustatory receptor neuron specific for insect repellents in mosquitoes and may provide a tool for screening chemicals to discover novel or improved feeding deterrents and repellents for use in the management of arthropod disease vectors.     

Comparative toxicities of some insecticides to Chrysoperla carnea (Chrysopidae: Neuroptera) and trichogramma brasiliensis (Trichogrammatidae: Hymenoptera), two arthropod natural enemies of cotton pests

Relative toxicities of nine insecticides recommended for the control of cotton bollworms were tested against the larvae of Chrysoperla carnea Stephens and adults of Trichogramma brasiliensis Ashmead under laboratory conditions in the Punjab, India. The insecticide concentrations studied were equivalent to those prescribed for field application. When freshly emerged larvae of C. carnea were released on insecticide-treated food (eggs of Corcyra cephalonica Stainton) for 24 h, endosulfan, quinalphos, monocrotophos, phenthoate and fenitrothion caused 74–89% larval mortality over a 72-h period. Phosalone, carbaryl and cypermethrin were moderately toxic (34.1–38.1% mortality), while fenvalerate was the least toxic (19.2% mortality). The surviving larvae in all treatments successfully completed metamorphosis. When T. brasiliensis adults were released on treated host (C. cephalonica eggs), all these insecticides proved highly toxic (84–100% mortality in 24 h). Phosalone, monocrotophos and fenvalerate inhibited parasitism by 26–64%, but did not affect the emergence of parasitoid adults. The remaining six insecticides severely inhibited parasitism and emergence of parasitoid adults. Thus, phosalone and fenvalerate are relatively safe for the above-mentioned natural enemies and can provide selectivity in an integrated control programme for cotton bollworms.     

Species-specific and female host-biased ectophoresy in the roundworm Caenorhabditis japonica

Caenorhabditis japonica is a bacteriophagous nematode species that was discovered on the semi-social burrower bug, Parastrachia japonensis, which demonstrates egg-guarding and provisioning behaviors. To understand the life history of C. japonica in relation to P. japonensis, we demonstrated the specificity of this association and fluctuations in nematode number on the insect throughout the year. C. japonica dauer larvae (DL), larvae in a nonfeeding diapause stage, were predominantly found as clumps on the adult female insects but rarely found on the male insects in all populations examined. This female-biased association was consistent throughout the year, but after the nymphs hatched, nematodes were not detected on the mother insects showing provisioning behavior. DL appeared on the nymphs, and the number of DL on the newly emerged female insects gradually increased thereafter. C. japonica has never been detected on other invertebrates collected from the P. japonensis habitat thus far. Our data suggest that the life cycles of C. japonica and P. japonensis are synchronized.     

The bacterium<Emphasis Type="Italic"> Xenorhabdus nematophila</Emphasis> inhibits phospholipases A<Subscript>2</Subscript> from insect,prokaryote, and vertebrate sources

The bacterium, Xenorhabdus nematophila, is a virulent insect pathogen. Part of its pathogenicity is due to impairing cellular immunity by blocking biosynthesis of eicosanoids, the major recognized signal transduction system in insect cellular immunity. X. nematophila inhibits the first step in eicosanoid biosynthesis, phospholipase A₂ (PLA₂). Here we report that the bacterium inhibits PLA₂ from two insect immune tissues, hemocytes and fat body, as well as PLA₂s selected to represent a wide range of organisms, including prokaryotes, insects, reptiles, and mammals. Our finding on a bacterial inhibitor of PLA₂ activity contributes new insight into the chemical ecology of microbe–host interactions, which usually involve actions rather than inhibitors of PLA₂s.     

Occurrence of phytophagous insects on wild Vigna sp. and cultivated cowpea: comparing the relative importance of host-plant resistance and millet intercropping

The effects of host-plant resistance on cowpea phytophagous insects and their natural enemies under pure and mixed crop conditions was evaluated at Minjibir, Kano State, Nigeria, in 1992–1994 crop seasons. Cowpea Vigna unguiculata cv ‘IT86D-715' (susceptible to insect pests) and a wild Vigna line Vigna vexillata ‘TVnu 72' (resistant to most insect species) were planted alone and in mixtures with millet (Pennisetum glaucum) in plots of 25×25 m.Mixed cropping had limited effect on major insects and natural enemies. Colonies of Aphis craccivora were significantly smaller and there were more adults of Maruca vitrata in crop mixtures than in monocultures. However, flower and raceme infestation by larval M. vitrata, Megalurothrips sjostedti, and Sericothrips sp. were similar in crop mixtures and monocultures. Empoasca sp. populations and seedling infestation by beanfly Ophiomyia phaseoli were also similar in mixtures and monocultures as well as pod damage by M. vitrata and populations of Clavigralla tomentosicollis. Parasitization rates of M. vitrata, C. tomentosicollis and O. phaseoli and predator–prey ratios of spiders and Orius sp. were similar across cropping systems. Host-plant resistance in TVnu 72 drastically reduced insect populations and damage. Grain yield per hill was high in cowpea IT86D-715 and was not affected by intercropping with millet. Grain yield of TVnu 72 was poor and reflected the low yield potential of this accession.Host-plant resistance is an effective means of controlling insect pest damage in cowpea and there is no evidence that high levels of resistance reduced natural biological control.     

Altruism during predation in an assassin bug

Zelus annulosus is an assassin bug species mostly noted on Hirtella physophora, a myrmecophyte specifically associated with the ant Allomerus decemarticulatus known to build traps on host tree twigs to ambush insect preys. The Z. annulosus females lay egg clutches protected by a sticky substance. To avoid being trapped, the first three instars of nymphs remain grouped in a clutch beneath the leaves on which they hatched, yet from time to time, they climb onto the upper side to group ambush preys. Long-distance prey detection permits these bugs to capture flying or jumping insects that alight on their leaves. Like some other Zelus species, the sticky substance of the sundew setae on their forelegs aids in prey capture. Group ambushing permits early instars to capture insects that they then share or not depending on prey size and the hunger of the successful nymphs. Fourth and fifth instars, with greater needs, rather ambush solitarily on different host tree leaves, but attract siblings to share large preys. Communal feeding permits faster prey consumption, enabling small nymphs to return sooner to the shelter of their leaves. By improving the regularity of feeding for each nymph, it likely regulates nymphal development, synchronizing molting and subsequently limiting cannibalism.     

Recilia banda Kramer (Hemiptera: Cicadellidae), a vector of Napier stunt phytoplasma in Kenya

Napier grass (Pennisetum purpureum) is the most important fodder crop in smallholder dairy production systems in East Africa, characterized by small zero-grazing units. It is also an important trap crop used in the management of cereal stemborers in maize in the region. However, production of Napier grass in the region is severely constrained by Napier stunt disease. The etiology of the disease is known to be a phytoplasma, 16SrXI strain. However, the putative insect vector was yet unknown. We sampled and identified five leafhopper and three planthopper species associated with Napier grass and used them as candidates in pathogen transmission experiments. Polymerase chain reaction (PCR), based on the highly conserved 16S gene, primed by P1/P6-R16F2n/R16R2 nested primer sets was used to diagnose phytoplasma on test plants and insects, before and after transmission experiments. Healthy plants were exposed for 60 days to insects that had fed on diseased plants and acquired phytoplasma. The plants were then incubated for another 30 days. Nested PCR analyses showed that 58.3% of plants exposed to Recilia banda Kramer (Hemiptera: Cicadellidae) were positive for phytoplasma and developed characteristic stunt disease symptoms while 60% of R. banda insect samples were similarly phytoplasma positive. We compared the nucleotide sequences of the phytoplasma isolated from R. banda, Napier grass on which these insects were fed, and Napier grass infected by R. banda, and found them to be virtually identical. The results confirm that R. banda transmits Napier stunt phytoplasma in western Kenya, and may be the key vector of Napier stunt disease in this region.     

A versatile and effective trap for detecting and monitoring stored-product coleoptera

An efficient multi-species trap of corrugated paper and utilizing plastic pitfall devices containing a vegetable-oil based food attractant, was developed for the purpose of detecting and monitoring stored-product insects. Adults of Oryzaephilus surinamensis (L.), Tribolium confusum Jacquelin duVal and larvae of Trogoderma variabile Ballion were trapped successfully in laboratory assays.Using a multiple-choice olfactometer, Tribolium confusum and Tribolium castaneum (Herbst) adults were shown to respond to 4,R-8,R-dimethyldecanal, their natural aggregation pheromone, and to distinguish between this and other diastereomers when tested simultaneously. The S-R and S-S diastereomers may be slightly inhibitory to T. castaneum response. Tribolium confusum was attracted to racemic 4, 8-dimethyldecanal applied to rubber septa lures. High doses of the pheromone reduced attraction. The pheromone is currently being used with food attractants in the trap for monitoring field populations of Tribolium and other stored-product insects.     

Consequences of electrical conductivity in an orb spider's capture web

The glue-coated and wet capture spiral of the orb web of the garden cross spider Araneus diadematus is suspended between the dry silk radial and web frame threads. Here, we experimentally demonstrate that the capture spiral is electrically conductive because of necks of liquid connecting the droplets even if the thread is stretched. We examine how this conductivity of the capture spiral may lead to entrapment of charged airborne particles such as pollen, spray droplets and even insects. We further describe and model how the conducting spiral will also locally distort the Earth's ambient electric field. Finally, we examine the hypothesis that such distortion could be used by potential prey to detect the presence of a web but conclude that any effect would probably be too small to allow an insect to take evasive action.     

Field vole (Microtus agrestis) seasonal spacing behavior: the effect of predation risk by mustelids

There are numerous studies showing that predation risk may change different aspects of the behavior of prey, such as habitat use, activity pattern, and foraging. Prey should exhibit the strongest antipredatory response against their most deadly predator. Small mustelids are considered the most important mammalian predators of voles. Nevertheless, there is no general agreement as to whether strong antipredatory reactions exist in natural free-living populations of voles. Here, we studied the field vole Microtus agrestis spatial reaction to high predation risk from small mustelids in the breeding (August) and nonbreeding (October) seasons under natural conditions. Voles were exposed to a caged weasel (Mustela nivalis) and a stoat (Mustela erminea), as well as to the odors of these predators. The reactions of 30 field voles were monitored with radiotelemetry. The field voles were found to display antipredator reactions that varied with season. In the breeding period, in response to predation risk, voles reduced locomotory activity and daily-range size, whereas in the nonbreeding period they did not. Changes in home range position were similar for control and treatment voles, in both the breeding and nonbreeding periods. The results indicate that mustelid predators modify the spatial behavior of small rodents in natural conditions depending on season. This might be a reflection of differences in state-dependent responses to predation from sexually active or inactive individuals. This suggests that the basic antipredatory reaction of voles under high predation risk from small mustelids limits their locomotory activity.     

“You are what you eat”: Diet modifies cuticular hydrocarbons and nestmate recognition in the Argentine ant, Linepithema humile

 Nestmate recognition plays a key role in the behavior and evolution of social insects. We demonstrated that hydrocarbons are the chemical cues used in Argentine ant, Linepithema humile, nestmate recognition, and that these hydrocarbons can be acquired from insect prey. Consequently, Argentine ant cuticular hydrocarbon patterns reveal the same hydrocarbons present in their diet. Diet alters both the recognition cues present on the cuticular surface and the response of nestmates to this new colony odor, resulting in aggression between former nestmates reared on different insect prey. Received: 16 June 2000 / Accepted: 14 August 2000     

Ubiquity of parasporin-1 producers in <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> natural populations of Japan

Parasporin, a Bacillus thuringiensis parasporal protein, is unique in having a strong cytocidal activity preferential for human cancer cells. In this study, we characterized parasporin activities associated with three novel geographical isolates of B. thuringiensis. Parasporal inclusion proteins of the three isolates were highly toxic to human uterus cervix cancer cells (HeLa), but not to non-cancer uterine smooth muscle cells (UtSMC). Inclusions of the isolates lacked insect toxicity and hemolytic activity against sheep erythrocytes. Ouchterlony immunodiffusion tests revealed that the proteins of the three isolates are immunologically closely related to parasporin-1 (Cry31A), but dissimilar to the three other existing parasporin groups. Our results provide evidence that the parasporin-1-producing organism is a common member in B. thuringiensis populations occurring in natural environments of Japan.     

Reward and non-reward learning of flower colours in the butterfly Byasa alcinous (Lepidoptera: Papilionidae)

Learning plays an important role in food acquisition for a wide range of insects. To increase their foraging efficiency, flower-visiting insects may learn to associate floral cues with the presence (so-called reward learning) or the absence (so-called non-reward learning) of a reward. Reward learning whilst foraging for flowers has been demonstrated in many insect taxa, whilst non-reward learning in flower-visiting insects has been demonstrated only in honeybees, bumblebees and hawkmoths. This study examined both reward and non-reward learning abilities in the butterfly Byasa alcinous whilst foraging among artificial flowers of different colours. This butterfly showed both types of learning, although butterflies of both sexes learned faster via reward learning. In addition, females learned via reward learning faster than males. To the best of our knowledge, these are the first empirical data on the learning speed of both reward and non-reward learning in insects. We discuss the adaptive significance of a lower learning speed for non-reward learning when foraging on flowers.     

When attempts at robbing prey turn fatal

Because group-hunting arboreal ants spread-eagle insect prey for a long time before retrieving them, these prey can be coveted by predatory flying insects. Yet, attempting to rob these prey is risky if the ant species is also an effective predator. Here, we show that trying to rob prey from Azteca andreae workers is a fatal error as 268 out of 276 potential cleptobionts (97.1?%) were captured in turn. The ant workers hunt in a group and use the “Velcro?” principle to cling firmly to the leaves of their host tree, permitting them to capture very large prey. Exceptions were one social wasp, plus some Trigona spp. workers and flies that landed directly on the prey and were able to take off immediately when attacked. We conclude that in this situation, previously captured prey attract potential cleptobionts that are captured in turn in most of the cases.     

Engineered nanoparticles: safer substitutes for toxic materials, or a new hazard?

Nanotechnology has the potential for the development of new materials and processes that can substitute for toxic materials now used in industry. Excitement over this possibility is tempered, however, by the potential adverse environmental health and safety aspects of the new nanomaterials. Although a few examples from the literature are encouraging, e.g., wire and cable insulation, great care must be taken to perform complete alternatives assessment evaluations of any new nanotechnology-enabled product before its adoption.     

Colour mimicry and sexual deception by Tongue orchids (Cryptostylis)

Typically, floral colour attracts pollinators by advertising rewards such as nectar, but how does colour function when pollinators are deceived, unrewarded, and may even suffer fitness costs? Sexually deceptive orchids are pollinated only by male insects fooled into mating with orchid flowers and inadvertently transferring orchid pollinia. Over long distances, sexually deceptive orchids lure pollinators with counterfeit insect sex pheromones, but close-range deception with colour mimicry is a tantalising possibility. Here, for the first time, we analyse the colours of four sexually deceptive Cryptostylis orchid species and the female wasp they mimic (Lissopimpla excelsa, Ichneumonidae), from the perspective of the orchids’ single, shared pollinator, male Lissopimpla excelsa. Despite appearing different to humans, the colours of the orchids and female wasps were effectively identical when mapped into a hymenopteran hexagonal colour space. The orchids and wasps reflected predominantly red-orange wavelengths, but UV was also reflected by raised bumps on two orchid species and by female wasp wings. The orchids’ bright yellow pollinia contrasted significantly with their overall red colour. Orchid deception may therefore involve accurate and species-specific mimicry of wavelengths reflected by female wasps, and potentially, exploitation of insects’ innate attraction to UV and yellow wavelengths. In general, mimicry may be facilitated by exploiting visual vulnerabilities and evolve more readily at the peripheries of sensory perception. Many sexually deceptive orchids are predominantly red, green or white: colours that are all potentially difficult for hymenoptera to detect or distinguish from the background.     

Injury to young cashew plants, Anacardium occidentale L., by the red-banded thrips Selenothrips rubrocinctus Giard (Thysanoptera: Thripidae)

The damage caused to young cashew plants, — Anacardium occidentale L. — by the red-banded thrips, Selenothrips rubrocinctus Giard was studied with the objective of establishing the economic injury threshold of this insect on the crop to effect a timely control.Six infestation levels (0, 40, 80, 160, 240 and 320) of adult thrips were used on 6-week-old cashew seedlings in a randomized complete block design of five replicates. The results showed that the mean leaf number and shoot weight bore significant linear relationships (P<0.001) (without detectable higher power components) to increased number of thrips. Damage by up to 160 adult thrips on 6-week-old cashew plants did not differ significantly from the control at P<0.05. On the other hand, infestation rates of 240 thrips and above caused significant defoliation and shoot weight loss when compared with the control. The need to spray chemical insecticides to control thrips on the basis of pest numbers as against spraying to a schedule is discussed.     

Avoiding incidental predation by mammalian herbivores: accurate detection and efficient response in aphids

Mammalian herbivores eat plants that may also provide food and shelter for insects. The direct trophic effect of the browsing and grazing of mammalian herbivory on insects, which is probably prevalent in terrestrial ecosystems, has been mostly neglected by ecologists. We examined how the aphid Uroleucon sonchi L. deals with the danger of incidental predation by mammalian herbivores. We found that most (76%) of the aphids in a colony survive the ingestion of the plant by a feeding herbivore. They do so by sensing the combination of heat and humidity in the herbivore’s breath and immediately dropping off the plant in large numbers. Their ability to sense the herbivore’s breath or their tendency to drop off the plant weakens as ambient temperature rises. This could indicate a limitation of the aphids’ sensory system or an adaptation that enables them to avoid the hostile conditions on a hot ground. Once on the ground, U. sonchi is highly mobile and capable of locating a new host plant by advancing in a pattern that differs significantly from random movement. The accurate and efficient defense mechanism of U. sonchi emphasizes the significance of incidental predation as a danger to plant-dwelling invertebrates.     

Chronic toxicity and cytotoxicity of synthetic pyrethroid insecticide cis-bifenthrin

With the increasing use of synthetic pyrethroids (SPs), the significance of ecological safety and health risk is an emerging concern. In this study, we evaluated the chronic aquatic toxicity of cis-bifenthrin (cis-BF) in Daphnia magna and its cytotoxicity in Chinese hamster ovary (CHO) cells as well as human cervical carcinoma (Hela) cells. Chronic aquatic toxicity tests showed that cis-BF could significantly affect the reproduction of D. magna. The lowest observed effective concentration and the non-observed effective concentration of cis-BF to D. magna were 0.02 and 0.01 μg/L, respectively, and the chronic value was 0.014 μg/L. The intrinsic rate of natural increase was significantly decreased (p < 0.05) to 0.02 μg/L. The cytotoxicity assay demonstrated that cis-BF decreased cell viability in CHO and Hela cells in a concentration-and time-dependent manner. The IC₅₀ values for Hela and CHO cells were 4.0 × 10⁻⁵ and 3.2 × 10⁻⁵ mol/L, respectively. Together, these results indicated that cis-BF induced chronic toxicity in both aquatic invertebrate animals and mammalian cells. These findings assist in understanding the impact of SPs on health and environmental safety. Considering the wide spectrum of SPs, a more comprehensive understanding of the negative effects is indispensible for planning future application and regulation of these pesticides.