The weight of fresh grass eaten by Schistocerca gregaria (Forsk.) on each day of its development from hatching until maturation of the adults was investigated. Hoppers in all instars ate on the average about one gram per gram body-weight per day on the middle days of the instars and the adults ate approximately half-a-gram per gram body-weight each day. The percentage of food assimilated, calculated from the dry faeces and estimated dry weight of food eaten, falls from about 78 in the first to 35 in the fifth instar. There were some indication that the food consumption per hopper increases with the number of hoppers per cage.

The weight of wheat bran eaten by hoppers was also investigated. The amount eaten by hoppers on the middle day of the first instar is approximately the same, in relation to their body weight, as that eaten per day just before and just after moulting by those in the fourth and fifth instars. More is eaten in the middle of these instars.

Measurements of weights and lengths of males and females in the hopper instars and of the weights of adults showed that in the course of their development females become progressively heavier and larger than males. The weight of a newly-hatched hopper is about 10 mg.; a female fledgling weighs about 2 gm. and a male 1·5 gm. Hoppers are approximately 1 cm. long on hatching, and males on reaching maturity are about 4·5 cm. long and females about 5.0 cm.

Laboratory bioassays were carried out with four neonicotinoid insecticides on multiple strains of Bemisia tabaci (Gennadius) to evaluate resistance and cross-resistance patterns. Three imidacloprid-resistant strains and field populations from three different locations in the southwestern USA were compared in systemic uptake bioassays with acetamiprid, dinotefuran, imidacloprid and thiamethoxam. An imidacloprid-resistant strain (IM-R) with 120-fold resistance originally collected from Imperial Valley, California, did not show cross-resistance to acetamiprid, dinotefuran or thiamethoxam. The Guatemala-resistant strain (GU-R) that was also highly resistant to imidacloprid (RR = 109-fold) showed low levels of cross-resistance when bioassayed with acetamiprid and thiamethoxam. However, dinotefuran was more toxic than either imidacloprid or thiamethoxam to both IM-R and GU-R strains as indicated by low LC50s. By contrast, a Q-biotype Spanish-resistant strain (SQ-R) of B. tabaci highly resistant to imidacloprid demonstrated high cross-resistance to the two related neonicotinoids. Field populations from Imperial Valley (California), Maricopa and Yuma (Arizona), showed variable susceptibility to imidacloprid (LC50s ranging from 3.39 to 115 μg ml–1) but did not exhibit cross-resistance to the three neonicotinoids suggesting that all three compounds would be effective in managing whiteflies. Yuma populations were the most susceptible to imidacloprid. Dinotefuran was the most toxic of the four neonicotinoids against field populations. Although differences in binding at the target site and metabolic pathways may influence the variability in cross-resistance patterns among whitefly populations, comparison of whitefly responses from various geographic regions to the four neonicotinoids indicates the importance of ecological and operational factors on development of cross-resistance to the neonicotinoids.

Larval populations of the Anopheles gambiae Giles complex (species A and B) were sampled in Kenya, and from the frequencies of the age distribution of the immature stages time-specific life-tables were constructed. Daily survival rates and instar mortalities showed that there was high mortality amongst fourth-instar larvae, due at least in part to infections of larval nematodes, Coelomomyces and epibionts. Antiserum to the A. gambiae complex was prepared in rabbits so that the precipitin test could be employed to detect the presence of A. gambiae in gut smears taken from possible predators. By this serological technique various spiders, Diptera, Coleoptera and amphibia were identified as predators. These may be very important in regulating populations of A. gambiae even in small collections of water.

A species in the Bactrocera dorsalis (Hendel) complex was detected in Kenya during 2003 and classified as Bactrocera invadens Drew, Tsuruta & White. Having spread rapidly throughout Africa, it threatens agriculture due to crop damage and loss of market access. In a recent revision of the B. dorsalis complex, B. invadens was incorporated into the species B. dorsalis. The potential distribution of B. dorsalis has been previously modelled. However, previous models were based on presence data and did not incorporate information on the seasonal phenology of B. dorsalis, nor on the possible influence that irrigation may have on its distribution. Methyl eugenol-baited traps were used to collect B. dorsalis in Africa. Seasonal phenology data, measured as fly abundance throughout the year, was related to each location's climate to infer climatic growth response parameters. These functions were used along with African distribution records and development studies to fit the niche model for B. dorsalis, using independent global distribution records outside Africa for model validation. Areas at greatest risk of invasion by B. dorsalis are South and Central America, Mexico, southernmost USA, parts of the Mediterranean coast, parts of Southern and Eastern Australia and New Zealand's North Island. Under irrigation, most of Africa and Australia appear climatically suitable.

The effects of nitrogen and silica application to maize plants on various aspects of the bionomics of the maize stem borer, Sesamia calamistis Hampson, were studied in the laboratory. Increasing nitrogen doses significantly increased larval survival (from 18.7% (control) to 37.3% at 2.25 g N/plant), larval weight (from 49.0 mg (control) to 99.5 mg at 2.25 g N/plant), and female fecundity (from 77 eggs per female (control) to 365 eggs per female at 1.69 g N/plant). A significant, positive relationship was found between moth fecundity and female pupal weight. Nitrogen had no effect on larval and pupal developmental time but increased adult longevity. Intrinsic rate of increase, rm, and the net reproductive rate, Ro, were positively related to leaf and stem nitrogen, while generation time, G, was negatively related. Silica had the opposite effect from nitrogen on larval survival. Increasing silica supply reduced ultimate larval survival from 26.0% (control) to 4.0% at 0.56 g Si/plant. Immature developmental time, larval and pupal weight, pupal survival, female fecundity, egg viability and adult longevity of S. calamistis were not affected by silica application.

Temperature and food quality can both influence growth rates, consumption rates, utilization efficiencies and developmental time of herbivorous insects. Gravimetric analyses were conducted during two consecutive years to assess the effects of temperature and food quality on fourth instar larvae of the forest tent caterpillar Malacosoma disstria Hübner. Larvae were reared in the laboratory at three different temperatures (18, 24 and 30°C) and on two types of diet; leaves of sugar maple trees Acer saccharum Marsh. located at the forest edge (sun-exposed leaves) or within the forest interior (shade-exposed leaves). In general, larvae reared at 18°C had lower growth rates and lower consumption rates than larvae reared at the warmer temperatures (24 and 30°C). Moreover, the duration of the instar decreased significantly with increasing temperatures. Type of diet also affected the growth rates and amount of food ingested by larvae but did not affect the duration of the instar. Larvae fed sun-exposed leaves consumed more food and gained higher biomasses. Values of approximate digestibility and efficiency of conversion of ingested food were also higher when larvae were fed sun-exposed leaves. Higher growth rates with increasing temperatures were primarily the result of the shorter stadium duration. The higher growth rates of larvae fed sun-exposed leaves were possibly the result of stimulatory feeding and consequently greater food intake and also a more efficient use of food ingested. This study suggests that the performance of M. disstria caterpillars could be enhanced by warmer temperatures and higher leaf quality.

Genetic (electrophoretic) variation was examined in the two pest species of Heliothis found in Australia, H. armigera (Hübner) and H. punctigera Wallengren. They could be differentiated by seven loci which enabled identification of individual eggs and small larvae not normally distinguishable by their morphology. The traditional criteria for distinguishing the larger larvae of the two species were shown to be unreliable. The genetic distance between the two species was 0·34 ± 0·02. The percentage of loci polymorphic in both species, 32%, and the mean heterozygosities, 11·3% for H. armigera and 10·8% for H. punctigera, are lower than those reported in the American species, H. virescens (F.) and H. zea (Boddie). Populations throughout Australia were differentiated from each other, but there did not appear to be a marked geographic pattern to the variation. Genetic distances between populations of H. armigera were low (<0·01). It was concluded that the effective population size of Heliothis is large and that significant gene flow probably occurs between widely separated regions.

A dual port olfactometer was used to study the response of Anopheles gambiaeGiles sensu stricto to odours of human and animal origin. Human odour consisted of human skin emanations collected on a nylon stocking, which was worn for 24 h. This was tested alone or together with 4.5% carbon dioxide, the concentration in human and cattle breath. Cattle odours consisted of cow skin emanations and/or carbon dioxide. Cow skin emanations were collected by tying a nylon stocking (‘cow sock’) around the hind leg of a cow for 12 h. Anopheles gambiae s.s. was consistently highly attracted by human odour, which is consistent with the high degree of anthropophily in this mosquito. Anopheles gambiae s.s. was not attracted by human or cattle equivalent volumes of carbon dioxide and this gas did not enhance the effect of human skin residues. Furthermore, A. gambiae s.s. showed a high degree of aversion to cow odour. When human odour and cow odour were tested together in the same port, mosquitoes were still highly attracted, indicating that whilst cattle odour may deter A. gambiae s.s., these mosquitoes can detect human odour in the presence of cattle odour. It was concluded that carbon dioxide plays a minor role in the host seeking behaviour of A. gambiae s.s., whilst host specific cues such as human skin residues play a major role and very effectively demonstrated anthropophilic behaviour in the laboratory.

Insects immersed in oils exude minute droplets of water over the surface of the cuticle. These appear more rapidly in light oils than in heavy oils and are more numerous in young insects than in old. Immersed in mixtures of oils and alcohols which separate in contact with water they show a vigorous effervescence at those regions where the mixture penetrates most rapidly to the water in the cuticle. There is a delay of varying duration at different parts of the cuticle when heavy oils are used. This delay disappears if the lipoid layer is first extracted with petroleum ether.

Factors controlling the rate of entry of pyrethrum through the cuticle have been studied by the application of pyrethrum in oil to a restricted area of the abdomen of nymphs and adults of Rhodnius. Entry is more rapid in light than in heavy petroleum oils ; it is much accelerated if the cuticle is first treated with petroleum ether. Entry is very slow in vegetable oils. There are great individual variations, due chiefly to the thickness of the endocuticle, which in turn is determined by the size of meal taken before moulting, the age and the amount of food taken after moulting, and the degree of stretching of the cuticle. There is evidence that the pore canals are important in the passage of pyrethrum through the endocuticle.

Histological examination of the integument shows that in the nymph of Rhodnius oils are taken up first by the epidermal cells in the zone around the bristles ; later by the general epidermis. The uptake of oil is greatly increased if 5 per cent. of oleic acid is added to refined petroleum. In the adult Rhodnius soon after moulting, oils are taken up by the general epidermis. In old insects they are taken up solely by the dermal glands. There is no visible entry through the sockets of the bristles.

The entry of pyrethrum in oils is accelerated by the addition of oleic or other fatty acids.

For insect herbivores, the quality of the larval host plant is a key determinant of fitness. Therefore, insect populations are supposed to be positively correlated with the nutritional quality of their host plant. This study aimed to determine if and how different varieties of grapes (including the wild grape Lambrusque) affect both larval and adult performance of the polyphagous European grapevine moth Lobesia botrana (Denis & Schiffermüller). Significant differences were found in larval development time, but not in pupal mass, adult emergence rate, or sex ratio. Although the fecundity of females is not different among varieties, females fed on some varieties produced eggs of different sizes which are correlated to their fertility. Thus, females adapt resource allocation to eggs depending on their diet as larvae. Using a fitness index, the average reproductive output was found to be highest for females reared on cv. Chardonnay. Females reared on wild grape produced a fitness index identical to the cultivated grapes. However, Lambrusque and Gewurztraminer separate themselves from the cultivated varieties according to our discriminant analyses. It is emphasized, through this study, that cultivars fed on by larvae should be considered in the population dynamics of L. botrana and that egg number is insufficient to determine host plant quality.

Four Old World blow flies (Diptera: Calliphoridae), Chrysomya albiceps (Wiedemann), C. putoria Wiedemann, C. megacephala (Fabricius), and C. rufifacies (Macquart), have recently invaded the New World. The interaction of Chrysomya rufifacies (Macquart) with native carrion flies in Texas, USA, was investigated by reducing oviposition by the invader on rabbit carcasses outdoors. These carcasses produced significantly more Cochliomyia macellaria (Fabricius) adults compared to carcasses on which the invader was not reduced. The results suggest that C. macellaria populations will decline where the two species co-occur. They also support the hypothesis that the carrion community is saturated with species, and provide a mechanism for the possible elimination of Lucilia caesar (Linnaeus) in Madeira and the reduction of C. macellaria in South America by Chrysomya albiceps (Wiedemann).

This paper presents a thorough review and a complete amalgamation, with some modification, of the internal classification of the genus Anopheles. Both formal and informal taxa are included. The classification is intended to aid researchers and students who are interested in analysing species relationships, making group comparisons and testing phylogenetic hypotheses. The genus includes 458 formally named and provisionally designated species divided among six subgenera. The largest subgenera, Anopheles, Cellia and Nyssorhynchus, are further divided into various descending informal groupings down to species level. These groupings include Sections, Series, Groups, Subgroups and Complexes. Other terms applied by some authors to levels of classification represented by these categories have been changed in order to establish a degree of uniformity in the application of informal group names within the genus. A few changes involving groups and species assignments have been made to eliminate duplicity or clarify concepts.

Changes in numbers of adult Coccinellids on nettles infested with Microlophium evansi (Theo.) and on three experimental bean plots infested with Aphis fabae Scop. are described for the spring and summer of 1952 at Rothamsted. Of the three common species, Adalia bipunctata (L.), Coccinella septempunctata L. and Propylea quatuordecimpunctata (L.), the first was always the most abundant.

The overwintered Coccinellids produced two broods: one on nettles in the spring and one on beans in early summer.

On bean plots, changes in numbers of the adult insects occurred in three phases: (1) overwintered ladybirds, dispersing from nettles and other places, accumulated on the beans in the early stages of the infestations by Aphis fabae; (2) ladybirds, which had developed from eggs laid by the overwintered insects on nettles, emigrated from nettles and accumulated on the beans when A. fabae populations were at their height; (3) ladybirds, developed from eggs laid on the beans also by the overwintered insects, reached the adult stage when the bean aphid infestations had finished. The rise and fall of the A. fabae populations are discussed.

One of the bean plots had always a high population of Coceinellids, which was attributed to its situation between two of the nettle sites. The other two bean plots were remote from nettles and one was sheltered by trees and buildings; their coccinellid populations were much lower.

1. Tsetse-flies have been divided into four stages of hunger by their external appearance. These stages have been correlated with the internal appearance determined by dissection. It was found that the fat-bodies became maximally developed shortly after a meal, and thereafter diminished rapidly in bulk. Hunger of flies in samples is expressed as the Mean Hunger Stage (M.H.S.).

2. Flies in different stages of hunger showed different behaviour in the field. Thus hungry flies usually attacked head-upwards, replete flies, if attacking, tending to adopt a head-down attitude. Most replete flies preferred to settle on the ground, or on objects other than the members of the catching party. Hungry flies were the most persistent followers.

3. The true habitats and feeding-grounds of Glossina morsitans, G. swynnertoni, and G. palpalis are described, so far as they are known to the writer.

4. Confirmation of the feeding-ground theory was obtained for G. swynnertoni by an analysis of the to-and-fro movements of marked flies.

5. Considerable independent movement of flies was shown to occur in the absence of carriage on man or game. Some flies under such conditions travelled over 2 miles in 3 hours.

Insect activity was significantly reduced in dung from cattle treated with a recommended topical dose (500 μg/kg) of the endectocide, ivermectin. Reductions affected an ecologically and taxonomically diverse group of insects including coprophagous flies, parasitic wasps, and both predacious and coprophagous beetles. Whereas some species appeared unaffected by faecal residues, the emergence of other species was reduced in dung voided ≤ 12 weeks post-application. The species most affected were the flies, Sepsis sp. and Coproica mitchelli (Malloch). eucoilid wasps. and the beetles, Cercyon quisquilius (Linnaeus) and C. pygmaeus (Illiger). Results were consistent within and between years. Similar reductions have been reported previously, but typically for a much shorter post-application period. The greater duration of effect observed in the current study is attributed to a more accurate bioassay, species-level identifications, and the use of sample sizes generally larger than those used in previous studies. Reduced insect activity was associated with slower dung pat degradation. When ivermectin was added directly to dung, at levels previously reported to occur in dung from treated cattle, the treated dung had not appreciably degraded after 340 days in the field. In contrast, untreated dung pats deposited at the same time and place had largely degraded after 80 days in the field. The effect of ivermectin use on pasture quality is probably ‘moot’ in southern Alberta because of the current pattern of use. Should this pattern change, however, results of the present study validate the need for future studies to assess the off-target effects of ivermectin use on pasture quality.

There are certain smells remarkably attractive to male flies of the genus Dacus and by the employment of these smells the movements of the flies can to a great extent be controlled, in any given direction. The reaction is strictly confined to the male sex, and different species exhibit a variation as regards the smell which is most attractive to them. It is uncertain whether the females emit similar smells; on the whole improbable.

Three of the commoner species of the flies (D. diversus, ferrugineus and zonatus) normally breed respectively in (1) anthers of Cucurbitaceae, (2) fruits of Solanaceae and mango, and (3) peach, guava, mango and other fruits. D. diversus (1) is most strongly attracted by iso-eugenol, zonatus (3) by methyl-eugenol, and ferrugineus (2) by both methyl and iso-eugenol. The smells of these substances have not yet been identified with those of the plants which constitute the normal breeding-places, but male flies have been found attracted to mango, Papaya, a Cycad, and Colocasia. plants with a very characteristic smell similar to that of eugenol-derivatives. Females have never been seen to frequent these plants or to breed in them, but more extended observation is required on this point before it can be definitely said that they never do so. The males give the impression that they might visit these plants for food, for they suck the surface of the flowers, though they do not seem to exhibit any very particular avidity. One species (ferrugineus?) has been bred from a Eugenia at Bangalore.

Three explanations suggest themselves:.—

(a) The smell is a direct sexual guiding smell emitted by the female.

But young crushed females did not attract males.

(b) The smell is not emitted by the female, but may be termed an “indirect” sexual guide to the plants where the females are accustomed to congregate for breeding purposes.

Under these circumstances it is difficult to see why females should not also be attracted by the odoriferous chemicals.

(c) The smell is a food-smell.

It must then be a food attractive only to males.

Two other species of fruit-flies, D. caudatus (breeding in fruits of Cucurbitaceae) and an unidentified species, were captured by means of an amyl-vanillin solution and of eugenol respectively. The rarity of the flies and the small numbers caught (four and one) make it advisable, however, to postpone detailed comment on these two cases, and future work may give more definite information as to their chemical relations. No substance attractive to male cucurbitae has been found.

All the observations described were made at the Pusa Research Institute, India.

Callosobruchus maculatus (Fabricius) develops in the seeds of Vigna unguiculata in the Sahel region of Africa. The reproductive activity of this species and the development of its offspring were studied during the dry season at Ouagadougou and Bobo Dioulasso (Burkina Faso), two zones with different climatic conditions. The length of imaginal life of the females and the developmental time varied with changes in climatic conditions, the lowest values for both parameters being found at the end of the dry season when temperature and humidity increased. In the two zones, variations in the density of the bruchid populations have been studied in experimental stores. The density of the bruchid population was low at the beginning of the study and increased when temperature and humidity rose. The introduction of Dinarmus basalis (Rondani) adults to the stores caused a significant reduction in the numbers of C. maculatus adults. The density of the D. basalis populations was low from December to February and increased when temperatures and then humidity became higher. The increase of the D. basalis population and the rate of parasitism were greater at Bobo Dioulasso than at Ouagadougou. Laboratory studies have demonstrated that fecundity, duration of adult life and developmental times depend on humidity. This study has shown the importance of local climatic conditions on parasitoid reproduction and development and also in the control of bruchid populations in stores.

During investigations on the phytophagous mite fauna of Mauritius, of which up to the present five species only had been recorded, 21 species were collected, of which ten species belong to the TETRANYCHIDAE. In addition, nine species, of which the majority are believed to be predacious, were observed.

An annotated list of the species, together with their food-plants, their geographical distribution and the countries from which they might have been introduced into Mauritius, is given.

Of the species of economic importance the following are worthy of mention: — In the TETRANYCHIDAE: Eutetranychus banksi (McG.) on Citrus, Oligonychus biharensis (Hirst) and Oligonychus mangiferus (Rahm. & Sapra) on various fruit trees, including avocado, mango and litchi, Oligonychus sp. (pratensis (Banks) group) on sugar-cane, Tetranychus marianae McG. and Tetranychus sp. (ludeni Zacher group) on many vegetable crops, including bean, egg-plant, tomato, etc. In the ERIOPHYIDAE: Phyllocoptruta oleivora (Ashm.) and Tegonotus sp., which are potential enemies of Citrus and tomato, respectively. In the TENUIPALPIDAE: Brevipalpus phoenicis (Geijskes) has a very wide range of food-plants and its future importance in the cultivation of tea and Citrus are mentioned, whilst Raoiella indica Hirst is an important pest of coconut plantations. In the TARSONEMIDAE: Hemitarsonemus latus (Banks) ranks as an important pest of a large number of economic plants including bean, tea and water-cress.

Only two predatory mites of great economic importance are recorded, viz.:— Typhlodromus caudatus (Berl.) and T. ovalis Evans. The bionomics of the former species are fully described and its relation to Raoiella indica on coconut is discussed. The life-cycle of T. caudatus from egg to adult varies from 4–8 days, in summer to 13–18 days in winter.

Results are given of a survey of the predatory insects associated with the phytophagous mites. The most important predators on Tetranychus spp. are the Coccinellid, Stethorus vinsoni Kapur, and the Cecidomyiid, Feltiella sp. near tetranychi Rübs. The daily mite consumption of these predators is given. Other predators of secondary importance are Oligota pallidicornis Cam. and Scolothrips sp. near indicus Priesn.

The bionomics of T. marianae on tomato and of Raoiella indica on coconut were studied. Duration of the life-cycle of the former species varies from 4–9 days in summer to 15–22 days in winter. Twenty four to 30 generations of this species are estimated to occur in a year. Raoiella has a development period of 18–26 days from egg to adult in summer and of 30–36 days in winter.

A biological study of the predatory insects, Stethorus vinsoni and Feltiella, sp., is given. Their beneficial effect is discussed. These two predators do not exert a control in time to check the build-up of the mite population. Their abundance is directly proportional to the incidence of the mite and the peak is generally only attained when the depredations of the latter have reached a disastrous level.

The various possible factors which have contributed to the sudden increase in the mite population on tomato during the last 6–8 years are discussed. Amongst the important factors are the following: —

(a) Absence of cyclonic disturbances with their heavy rains and violent gales which generally upset considerably the development of the mite population and reduce it to a minimum each year.

(b) Dressing of tomato and other vegetable crops with nitro-phosphatic fertilisers which were not applied until 10 years ago and which, it is thought, may have favoured an increase in the mite population.

(c) The abundance of natural reservoirs of wild food-plants that harbour Tetranychus spp. from which the subsequent spread results in the building up of mite colonies in cultivated areas.

(d) The ineffectiveness of Stethorus vinsoni as a predator on infested tomato plants.

Between October 1999 and April 2000, nearly 4000 ha of nymphal bands and adult swarms of Chortoicetes terminifera (Walker) were aerially treated using a ULV oil formulation of strain FI-985 of Metarhizium anisopliae var. acridum. During the mild weather (maxima 22–30°C) of spring (October), there was little change in nymphal bands during the first week but at all doses between 25–100 g (1–4 × 1012 conidia) ha−1, the bands rapidly declined 9–12 days after treatment reaching > 90%mortality by 14 days. Metarhizium persisted for some time as there was 50% mortality of locusts fed vegetation collected from the treated blocks seven days after treatment. Persistence was confirmed by the high mortality of bands that invaded from untreated areas and of nymphs that hatched on the plot five to seven days after treatment, though mortality was then delayed until early in the third week. During summer (January), temperatures were high (maxima 36–42°C), and at all doses between 25 and 125 g (1–5 × 1012 conidia) ha−1, there was a rapid decline seven to ten days after treatment. By 12–14 days, there was a > 90% decline in numbers in most blocks which was confirmed by helicopter surveys two weeks after treatment that found very few adults within or near treated areas. Mortality was delayed in the high dose where there were blockages of spray equipment during treatment. The clear demonstration that Metarhiziumcan suppress small local populations of C. terminifera led to the limited operational use of Metarhizium on an organic farm and in a National Park where nearly 2500 ha of bands and swarms were treated. Continued research is needed to develop a commercially viable product so that Metarhizium can form a significant part of a programme of integrated pest management of locusts in Australia.

The susceptibility to several insecticides of 16 and 8 strains of Myzus persicae Sulzer and Aphis gossypii Glover, respectively, received from different European countries in 2001 was investigated. Most of the strains were derived from places known for their aphid resistance problems to conventional insecticides before imidacloprid was introduced. In many regions and agronomic cropping systems imidacloprid has been an essential part of aphid control strategies for a decade, and therefore the susceptibility of aphid populations to imidacloprid using FAO-dip tests and diagnostic concentrations in a leaf-dip bioassay was checked. Additional insecticides tested were cyfluthrin (chemical class: pyrethroid), pirimicarb (carbamate), methamidophos and oxydemeton-methyl (organophosphates). Diagnostic concentrations (LC99-values of reference strains) for each insecticide were established by dose response analysis using a new leaf-disc dip bioassay format in 6-well tissue culture plates. Virtually no resistance to imidacloprid in any of the field-derived populations of M. persicae and A. gossypii was detected. In contrast, strong resistance was found to pirimicarb and oxydemeton-methyl, and to a lesser extent also to cyfluthrin. Two strains of A. gossypii exhibited reduced susceptibility to imidacloprid when tested directly after collection. However, after maintaining them for six weeks in the laboratory, the aphids were as susceptible as the reference strain. The diagnostic concentration of methamidophos did not reveal any resistance in M. persicae, but did so in four strains of A. gossypii.