J.C. van Lenteren

An airflow olfactometer for measuring olfactory responses of hymenopterous parasitoids and other small insects

ABSTRACT. A new type of airflow olfactometer is described, and results given of experiments using it to measure behavioural olfactory responses of hymenopterous parasitoids. Compared with Y‐tube olfactometers it shows several advantages. In its exposure chamber four separate abutting odour fields are presented so that the test insect can readily enter and re‐enter them. More than one odour (or different concentrations of one odour) can be tested at the same time, thereby providing complex preference test situations. The various behavioural measures that can be assessed in the apparatus are examined and discussed.

A greenhouse without pesticides: fact or fantasy?

Abstract Crop protection in European greenhouses became strongly chemically oriented shortly after the Second World War in the 1950s. But an excellent climate for fast reproduction of pests and diseases demanded high spray frequencies and, thus, resulted in quick development of resistance against pesticides. This initiated a search for alternatives of chemical pesticides. The first natural enemies for control of pests in European greenhouse vegetables became available in the 1960s. A change from chemical control to very advanced integrated pest management programs (IPM) took place in a time span of about 20 years. Nowadays, growers worldwide annually introduce millions of natural enemies for pest control. About 100 species of beneficial organisms are commercially available for control of all important insect and mite pests. In the main vegetable crops in northern Europe, most if not all insect problems can now be solved without the use of insecticides. The change to IPM was not based on idealism about a cleaner or healthier environment, but was rooted on clear advantages for the grower. Development of biological control of diseases has started recently, and the first practical successes have been obtained. Integrated management of pests and diseases without conventional chemical pesticides is a goal that will be realised for most of the important vegetables in Europe within a decade. IPM is not limited to greenhouse vegetables, but is increasingly used in ornamentals. New activities in plant breeding research aim at (1) combining aspects of host-plant resistance with biological control, (2) selecting plant cultivars that are able to attract natural enemies after they have been attacked by pest organisms, and (3) selecting plant cultivars that produce a better “working environment” for biological control agents. Because of specific advantages of biological control for the growers, greenhouse crops will be produced without the need to use conventional pesticides in the very near future. At the same time, this results in a cleaner environment, in satisfying the demand of consumers for pesticide-free food, and in sustainable crop protection.

A variable-response model for parasitoid foraging behavior

An important factor inducing variability in foraging behavior in parasitic wasps is experience gained by the insect. Together with the insects genetic constitution and physiological state, experience ultimately defines the behavioral repertoire under specified environmental circumstances. We present a conceptual variable-response model based on several major observations of a foraging parasitoids responses to stimuli involved in the hostfinding process. These major observations are that (1) different stimuli evoke different responses or levels of response, (2) strong responses are less variable than weak ones, (3) learning can change response levels, (4) learning increases originally low responses more than originally high responses, and (5) hostderived stimuli serve as rewards in associative learning of other stimuli. The model specifies how the intrinsic variability of a response will depend on the magnitude of the response and predicts when and how learning will modify the insects behavior. Additional hypotheses related to the model concern how experience with a stimulus modifies behavioral responses to other stimuli, how animals respond in multistimulus situations, which stimuli act to reinforce behavioral responses to other stimuli in the learning process, and finally, how generalist and specialist species differ in their behavioral plasticity. We postulate that insight into behavioral variability in the foraging behavior of natural enemies may be a help, if not a prerequisite, for the efficient application of parasitoids in pest management.

Augmentative biological control of arthropods in Latin America

Augmentative forms of biological control, wherenatural enemies are periodically introduced,are applied over large areas in variouscropping systems in Latin America. About 25%of the world area under augmentative control issituated in this region. Well-known examplesare the use of species of the egg parasitoidTrichogramma for management ofLepidoptera in various crops. In Mexico, forexample, about 1.5 million hectares are treatedwith Trichogramma spp. Application ofTrichogramma also occurs on large areasin Colombia and Cuba, but use is limited inother Latin American countries for economicreasons, the generally low level of educationof farmers, and, more importantly, because ofthe intensive use of pesticides that preventsuse of natural enemies. Of the other eggparasitoids, the main species used incommercial releases are Trissolcusbasalis (Wollaston) against the heteropteranNezara viridula (L.) in soybean inBrazil, and Telenomus remus Nixon againstSpodoptera frugiperda (J.E. Smith) incorn in Venezuela. Natural enemies attackinglarval and pupal stages are not used to a largeextent in augmentative biological control infield crops, with the exception of the use ofCotesia parasitoids against sugarcaneborers in Brazil and several other LatinAmerican countries. In addition to the use ofparasitoids and predators, Latin America isapplying microbial control agents on a largescale, such as viruses for control ofcaterpillars in soybean, fungi for control ofpests in coffee, cotton and sugar cane, andnematodes for control of soil pests. A recentdevelopment in biological control in LatinAmerica is the use of natural enemies andantagonists for disease and pest control inprotected cultivation, for example, inColombia, Brazil and Peru. Up to date, reliablefigures on current use of inundative andseasonal inoculative biological controlappeared hard to obtain, but it is clear thatLatin America currently is a main player in thefield of augmentative releases.

The parasite-host relationship between Encarsia formosa Gahan (Hymenoptera: Aphelinidae) and Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae)

During feeding‐site selection on a tomato plant, adult greenhouse whiteflies, Trialeurodes vaporariorum, do not distinguish between leaves of different age from a distance.

Success in Biological Control of Arthropods by Augmentation of Natural Enemies

Augmentative biological control, where natural enemies are periodically introduced, is commercially applied over large areas in various cropping systems worldwide. It is a popular biological control approach amongst professional and progressive farmers, and has been stimulated by the present international attitudes regarding reduction of pesticide use. Initially augmentative biological control was used to manage pests that had become resistant to pesticides. Now it is applied for reasons of efficacy and costs, which are comparable with conventional chemical control. Farmers are also motivated to use this form of biological control to reduce environmental impacts of pesticide use.

Inferring the origin of populations introduced from a genetically structured native range by approximate Bayesian computation: case study of the invasive ladybird Harmonia axyridis

Correct identification of the source population of an invasive species is a prerequisite for testing hypotheses concerning the factors responsible for biological invasions. The native area of invasive species may be large, poorly known and/or genetically structured. Because the actual source population may not have been sampled, studies based on molecular markers may generate incorrect conclusions about the origin of introduced populations. In this study, we characterized the genetic structure of the invasive ladybird Harmonia axyridis in its native area using various population genetic statistics and methods. We found that native area of H. axyridis most probably consisted of two geographically distinct genetic clusters located in eastern and western Asia. We then performed approximate Bayesian computation (ABC) analyses on controlled simulated microsatellite data sets to evaluate (i) the risk of selecting incorrect introduction scenarios, including admixture between sources, when the populations of the native area are genetically structured and sampling is incomplete and (ii) the ability of ABC analysis to minimize such risks by explicitly including unsampled populations in the scenarios compared. Finally, we performed additional ABC analyses on real microsatellite data sets to retrace the origin of biocontrol and invasive populations of H. axyridis, taking into account the possibility that the structured native area may have been incompletely sampled. We found that the invasive population in eastern North America, which has served as the bridgehead for worldwide invasion by H. axyridis, was probably formed by an admixture between the eastern and western native clusters. This admixture may have facilitated adaptation of the bridgehead population.

Variation in plant volatiles and attraction of the parasitoid Diadegma semiclausum (Hellén)

Differences in allelochemistry of plants may influence their ability to attract parasitoids.We studied responses of Diadegma semiclausum (Hellén), a parasitoid of the diamondback moth (Plutella xylostella L.), to inter- and intraspecific variation in odor blends of crucifers and a non-crucifer species. Uninfested Brussels sprout (Brassica oleracea L. gemmifera), white mustard (Sinapis alba L.), a feral Brassica oleracea, and malting barley (Hordeum vulgare L.) were compared for their attractivity to D. semiclausum in a Y-tube bioassay. Odors from all plants were more attractive to the parasitoid than clean air. However, tested against each other, parasitoids preferred the volatile blend from the three cruciferous species over that of malting barley.Wasps also discriminated between uninfested crucifers: mustard was as attractive as feral B. oleracea, and both were more attractive than Brussels sprout. Attractivity of uninfested plants was compared with that of plants infested by larvae of the host P. xylostella. Host-infested mustard and Brussels sprout were more attractive than uninfested conspecifics. Interestingly, the volatile blends of uninfested white mustard and infested Brussels sprout were equally attractive.We also compared the volatile composition of different plant sources by collecting headspace samples and analysing them with GC-MS. Similarities of volatile profiles were determined by hierarchic clustering and non-metric scaling based on the Horn-index. Due to the absence of several compounds in its blend, the volatile profile of barley showed dissimilarities from blends of crucifers. The odor profile of white mustard was distinctly different from the two Brassicaceae.Feral Brassica oleracea odor profile was different from infested Brussels sprout, but showed overlap with uninfested Brussels sprout. Odor blends from infested and uninfested Brussels sprout were similar, and mainly quantitative differences were found. D. semiclausum appears to discriminate based on subtle differences in volatile composition of odor blends from infested and uninfested plants.

Electrical recording and ultrastructure of stylet penetration by the greenhouse whitefly

Earlier studies have indicated that interior (physical and/or chemical) properties of a plant may be responsible for feeding‐site selection by the greenhouse whitefly, Trialeurodes vaporariorum (Westw.). In order to study the process of feeding‐site selection further, stylet‐penetration activities and the pathway followed by the stylets in host‐plant tissue were investigated using a DC electrical recording method and transmission electron microscopy (TEM). Penetrating whiteflies attached to a gold wire were included in an electrical circuit to record electrical penetration graphs (EPGs). Seven EPG patterns have been distinguished, five of which could be correlated with components of the stylet‐penetration process: 1) one with penetration of the leaf surface, 2) one with intercellular penetration and salivary‐sheath secretion, 3) one with sieve element penetration and ingestion, 4) one with short penetration of a cell, and 5) one with xylem penetration. The stylet pathway is almost completely intercellular before the phloem is reached and in contrast to aphids, brief symplast punctures are very rare. In general, it takes T. vaporariorum more than half an hour from the start of a penetration to reach a sieve element. Rejection of feeding sites occurs within a few minutes of penetration by adult whiteflies, a time span in which stylets are presumed to penetrate just beyond the epidermis. Properties of the apoplast close to the leaf surface seem therefore to play a major role in feeding‐site selection.

Functional Responses in Invertebrates

Predators and parasites may react to density changes of their prey or hosts in many different ways. As regards the numerical aspects of these reactions, two different types are distinguished. When the individual predator attacks more prey when prey density increases, this is called a functional response. When, due to an increase of the density of the prey, the number of predators increases, this is known as a numerical