Influence of foliar and systemically applied azadirachtin on host‐plant evaluation behaviour of the sweetpotato whitefly,Bemisia tabaci

Abstract The behaviour of female adult Bemisia tabaci is observed for a period of 20 min after initial contact with untreated cucumber leaves, or leaves either foliar or systemically treated with azadirachtin, to determine whether application of azadirachtin affects the host‐evaluation behaviour and whether the behaviour on treated leaves differs between application methods. Application of azadirachtin deters settling of the whiteflies on host plants. The whiteflies probe for shorter duration and less frequently but spend longer and engage more frequently in labial grooming on treated leaves than on untreated leaves. Behavioural transition between probing and other behavioural elements is less common, and that between labial grooming and other behavioural elements more common, on both treated leaves than on untreated leaves. No difference is detected in the host‐evaluation behaviour of B. tabaci between leaves foliar treated and systemically treated with azadirachtin.     

Blue–green opponency and trichromatic vision in the greenhouse whitefly (Trialeurodes vaporariorum) explored using light emitting diodes

Visual orientation in the greenhouse whitefly (Trialeurodes vaporariorum Westwood, Hemiptera: Aleyrodidae) is the result of “wavelength‐specific behaviours.” Green–yellow elicits “settling behaviour” while ultraviolet (UV) radiation initiates “migratory behaviour.” The only available physiological study of the photoreceptors' spectral efficiency showed peaks in the green and the UV range and whitefly vision was said to be dichromatic so far. In order to study the visual behaviour of T. vaporariorum, 19 narrow‐bandwidth light emitting diodes (LEDs) covering the UV‐A and visible range were used in combination with light scattering acrylic glass screens in a small‐scale choice arena under greenhouse conditions. Multiple‐choice and dual‐choice assays were performed, resulting in LED‐based behavioural action spectra of settling (green) and migratory behaviour (UV). A potential inhibitory blue–green chromatic mechanism was studied by combining yellow with different bluish LEDs. Intensity dependencies were illustrated by changing LED intensities. Regarding the “settling response,” highest attraction was achieved by a green LED with a centroid wavelength of 550 nm, while a blue LED with 469 nm proved to be most inhibitory. Besides this inhibitory interaction, an intensity dependence was observed within the action spectrum in the green–yellow range. “Migratory behaviour” was elicited the most by the UV LED with the shortest available wavelength of 373 nm. The results provide compelling behavioural evidence for the presence of a green and a yet undescribed blue sensitive photoreceptor and a blue–green opponent mechanism. Furthermore, empirical colour choice models were built and receptor peaks were estimated around 510–520 nm (green), 480–490 nm (blue) and 340–370 nm (UV). Consequently, a trichromatic receptor setup is suggested for T. vaporariorum.     

Female false black widow spiders,Steatoda grossa,recognize webs based on physical and chemical cues

Females of the false black widow, Steatoda grossa CL Koch (Araneae: Theridiidae), invest significant energy and time weaving cobwebs. We tested the hypothesis that S. grossa females select sites for their webs based, in part, on the presence of con‐ or heterospecific webs, sensing both physical and chemical web cues. In bioassays, we offered female S. grossa a choice between an empty control frame and a frame bearing the web of a conspecific female or that of a female common house spider, Parasteatoda tepidarium CL Koch (Araneae: Theridiidae), recording (1) the time she spent, and (2) the time she spent inactive (a proxy for settling behaviour) on each frame. We also tested the effect of (1) silk micro‐ and macrostructure (wrapped‐up silk or intact web, each semiochemical‐deprived), (2) plastic webs, and (3) silk semiochemical extract on the responses of S. grossa females. Females settled on both con‐ and heterospecific webs and chose test stimuli based on their chemical and physical characteristics. Even plastic webs in cobweb‐like arrangement readily prompted settling behaviour by females. Our results suggest that web architecture, rather than web silk, mediates settling responses by female S. grossa on pre‐existing webs which may provide structural support for a new web and indicate habitat suitability.     

Mealybug avoidance responses to ladybird semiochemicals

The predatory ladybird Cryptolaemus montrouzieri Mulsant is a very effective natural enemy of the citrus mealybug, Planococcus citri (Risso), and has a worldwide distribution. This study investigated how the citrus mealybug responded to semiochemicals from the ladybird. In laboratory experiments, mealybug response to semiochemicals left by ladybirds on leaf surfaces was measured. The results indicated that the presence of ladybirds can change the settling behaviour of P. citri. The exposure of plant material to C. montrouzieri had a significant influence on the settling of mealybugs added to the same plant. The distribution of citrus mealybugs in the Petri dishes was significantly affected by the previous presence of ladybirds. The avoidance response may aid in the biological control of mealybugs by coccinellids released onto crops infested with mealybugs.     

Cucurbit aphid‐borne yellows virus (CABYV) modifies the alighting,settling and probing behaviour of its vector Aphis gossypii favouring its own spread

Plant pathogens are able to influence the behaviour and fitness of their vectors in such a way that changes in plant–pathogen–vector interactions can affect their transmission. Such influence can be direct or indirect, depending on whether it is mediated by the presence of the pathogen in the vector's body or by host changes as a consequence of pathogen infection. We report the effect that the persistently aphid‐transmitted Cucurbit aphid‐borne yellows virus (CABYV, Polerovirus) can induce on the alighting, settling and probing behaviour activities of its vector, the cotton aphid Aphis gossypii. Only minor direct changes on aphid feeding behaviour were observed when viruliferous aphids fed on non‐infected plants. However, the feeding behaviour of non‐viruliferous aphids was very different on CABYV‐infected than on non‐infected plants. Non‐viruliferous aphids spent longer time feeding from the phloem in CABYV‐infected plants compared to non‐infected plants, suggesting that CABYV indirectly manipulates aphid feeding behaviour through its shared host plant in order to favour viral acquisition. Viruliferous aphids showed a clear preference for non‐infected over CABYV‐infected plants at short and long time, while such behaviour was not observed for non‐viruliferous aphids. Overall, our results indicate that CABYV induces changes in its host plant that modifies aphid feeding behaviour in a way that virus acquisition from infected plants is enhanced. Once the aphids become viruliferous they prefer to settle on healthy plants, leading to optimise the transmission and spread of this phloem‐limited virus.     

Dispersal and settling of translocated populations: a general study and a New Zealand amphibian case study

Translocations are widely used to reintroduce threatened species to areas where they have disappeared. A continuum multi-species model framework describing dispersal and settling of translocated animals is developed. A variety of different dispersal and settling mechanisms, which may depend on local population density and/or a pheromone produced by the population, are considered. Steady state solutions are obtained using numerical techniques for each combination of dispersal and settling mechanism and for both single and double translocations at the same location. Each combination results in a different steady state population distribution and the distinguishing features are identified. In addition, for the case of a single translocation, a relationship between the radius of the settled region and the population size is determined, in some cases analytically. Finally, the model is applied to a case study of a double translocation of the Maud Island frog, Leiopelma pakeka. The models suggest that settling occurs at a constant rate, with repulsion evidently playing a significant role. Mathematical modelling of translocations is useful in suggesting design and monitoring strategies for future translocations, and as an aid in understanding observed behaviour.      

Induced resistance by Myzus persicae in the peach cultivar `Rubira'

The effect of a previous infestation by the green peach aphid Myzus persicae (Sulzer) on the settling behaviour and reproduction of the same aphid species was investigated in the resistant peach cultivar Rubira, and compared with that observed in the susceptible control cultivar GF305. A previous infestation of 48 h triggered induced resistance in Rubira. There were significantly fewer aphids settling on preinfested than on uninfested plants, indicating an increased rejection of Rubira as a host plant. The level of induced resistance in preinfested plants was positively related to the duration of the first infestation. In GF305, previous infestation had no detrimental effect on aphid settlement and even slightly enhanced larviposition by adult females. The aphid probing behaviour after a 48-h preinfestation was also monitored for 8 h with the electrical peneration graph (EPG) technique. On preinfested GF305, most EPG parameters indicated an enhanced host plant acceptance. On preinfested GF305, aphids produced less sieve element salivation and more continuous sap ingestion than on uninfested GF305, indicating that the previous aphids provoked changes in plant properties beneficial to the test aphids. In Rubira, a major induced factor of resistance was thought to be expressed in the sieve element as phloem sap ingestion was 4-fold shorter on preinfested than on uninfested plants. The time taken by the aphid stylets to reach a sieve element was also significantly increased on preinfested Rubira, suggesting the induction of resistance factors outside the phloem. The originality of the Rubira/M. persicae interaction is discussed in the perspective of a better understanding of plant induced responses to aphids.     

Role of leaf glandular trichomes of melon plants in deterrence of Aphis gossypii Glover

External characteristics of the leaf epidermis and their effects on behaviour of Aphis gossypii Glover were evaluated in two Cucumis melo L. genotypes, ‘Bola de Oro’ (aphid susceptible) and TGR‐1551 (aphid resistant) in order to explore their role in the early rejection of TGR‐1551 by this aphid. No differential effects of epicuticular waxes on aphid behaviour were observed. The type, distribution and number of trichomes on melon leaves were also studied. Pubescence in melon, measured as the number of non‐glandular trichomes per cm², was not sufficient to prevent aphid settling. However, there was a high density of type I glandular trichomes on leaves of the aphid‐resistant genotype. According to microscopic observations and stain testing, these trichomes store and secrete phenols and flavonoids. Free‐choice tests were conducted to determine the effect of these glandular trichomes on A. gossypii preference, revealing that aphids reject leaf disks of TGR‐1551 from the onset of the experiment. Additional experiments after removal of leaf type I glandular trichome exudates showed that A. gossypii preferred washed TGR‐1551 leaf disks over unwashed disks, while this effect was not observed in experiments using washed and unwashed ‘Bola de Oro’ leaf disks. These results suggest that a high density of glandular trichomes and chemicals secreted by them deter A. gossypii and disturb aphid settling on TGR‐1551.     

Settlement behaviour of larvae of the Stripey Snapper, <Emphasis Type="Italic">Lutjanus carponotatus</Emphasis> (Teleostei: Lutjanidae)

Larval behaviour is important to dispersal and settlement, but is seldom quantified. Behavioural capabilities of larval Lutjanus carponotatus in both offshore pelagic and reef environments at Lizard Island, Great Barrier Reef were observed in situ to determine if they were sufficient to influence dispersal. Offshore, larvae swam with higher directional precision and faster on the windward side of the island (28 cm.s⁻¹) than on the leeward side (16 cm s⁻¹). Most larvae swam directionally. Mean swimming directions were southerly in the windward area and northerly in the leeward area. Larvae avoided the surface and remained mostly between 3–15 m. Larvae released near reefs were 2–3 times faster swimming away from reefs (19 cm s⁻¹) than swimming toward or over them (6–8 cm s⁻¹). Speed swimming away was similar to that offshore. Of 41 larvae released near reefs, 73% reached the reef, 59% settled, and 13% of those reaching the reef were eaten. Larvae settled onto hard and soft coral (58%), topographic reef features (29%) and sand and rubble (13%). Settlement depth averaged 5.5 m (2–8 m). Before settling larvae spent up to 800 s over the reef (mean 231 s) and swam up to 53 m (mean 14 m). About half of the larvae interacted with reef residents including predatory attacks and aggressive approaches by residents and aggressive approaches by settling larvae. Settlement behaviour of L. carponotatus was more similar to a serranid than to pomacentrids. Settlement-stage larvae of L. carponotatus are behaviourally capable, and have a complex settlement behaviour.     

Observations on the settling of Simulium damnosum larvae on artificial substrates,in the Ivory Coast

The settling rates of the larvae of the blackfly Simulium damnosum Theobald, were monitored at a Simulium breeding site on the Bandama River in the Ivory Coast. Significantly greater numbers of larvae were found to settle during the night than during the day, and there was a significant increase in the settling rate immediately following the onset of darkness.The drifting larval population contained significantly greater numbers of smaller larvae than the general population.     

Characterization of the physical properties of yeast flocs

Summary Physical characteristics, namely floc density function, floc size distribution, and relative floc strength, of a number of flocculent yeast types were measured. A straight-line relationship was found to exist between log values of size and density for the yeasts examined. Each yeast type had coefficients from this relationship which could be used to interpret settling behaviour. Indices of relative floc strength were also obtained and together with the floc density function allowed fuller interpretation of yeast settling than with simpler theories.Symbols a constant (g·cm^-3) - B ₂/B ₁ floc binding strength of floc₂ relative to floc₁ - d _f floc diameter (cm) - d _i image diameter on print (cm) - d _max maximum floc diameter (cm) - f _d Ploc effective density (g·cm^-3) - g gravitational constant (981 cm·s^-1) - K _p constant (-) - R _l rate of enlargement on film - R ₂ rate of enlargement on print - S _s density of suspending liquid phase (g·cm^-3) - S _f density of solid (floc) phase (g·cm^-3) - U _t terminal settling velocity (cm·s^-1) - u liquid viscosity (g·cm^-3·^-1)     

Rapid change in the settling behavior of the arrowhead scale Unaspis yanonensis as an avoidance mechanism against introduced parasitoids, Aphytis yanonensis and Coccobius fulvus

Abstract Parasitoids are thought to exert immense selection pressures that shape the traits of herbivores. We examined whether two species of parasitoid wasps, Aphytis yanonensis DeBach et Rosen and Coccobius fulvus Compere et Annecke (Hymenoptera: Aphelinidae), affect the settling behavior of the arrowhead scale Unaspis yanonensis Kuwana (Hemiptera: Diaspididae), in order to demonstrate the evolution of antiparasitism behavior of herbivores using parasitoids in the field. We used the following five methods: a comparison of parasitism risk in different settling behaviors‐parasitoid introduction into a parasitoid‐free population; a comparison of the settling behavior between parasitoid‐present and parasitoid‐free populations; a common garden experiment, in which scales were transferred from parasitoid‐present and parasitoid‐free populations into the same garden; and a laboratory observation of the settling behavior of the first instars derived from the two population categories. Both parasitoids were introduced into a parasitoid‐free population in Wakayama in 1987, and the settling mode of the scales was examined in 1987, 1994, and 1995. The introduction of parasitoids modified the scale‐settling mode so that more crawlers settled under another scale (called burrowing), which was consistent with the results observed in parasitoid‐present (including South China) and parasitoid‐free populations. Moreover, only the burrowing scale exhibited a lower parasitism rate compared to scales settling singly and being burrowed. The common garden experiment demonstrated that scales introduced from the parasitoid‐present population had a greater proportion of burrowers than the parasitoid‐free population, even in the same field cage under parasitoid‐free conditions. Laboratory observations demonstrated that the population difference in parasitism rate was principally due to intrinsic differences in the settling behavior of nymphs; some first‐instar nymphs derived from the parasitoid‐present population burrowed under another scale settled. These results strongly suggest that the natural selection pressure imposed by the parasitoids modified the settling behavior of the arrowhead scale.     

On the settling of Bacillus sphaericus spores by pH adjustment

The influence of pH and hydrophobicity on the degree of flocculation and settling velocity of Bacillus sphaericus spores were investigated. The highest value of settling velocity (97.1 cm h^–1) with a cell recovery of around 90% occurred at pH 3. This also gave the maximum degree of hydrophobicity. Adjustment of pH to 3 does not influence the biological activity of the product.     

Honesty of agonistic signalling and effects of size and motivation asymmetry in contests

Game theoretical models predict that the main function of fighting behaviour is to assess the relative fighting ability of opponents. The sequential assessment game has often been used to investigate contests, while honest signalling theory has received much less attention. With the wolf spider Hygrolycosa rubrofasciata we investigated whether male agonistic signalling can reveal honest information about fighting ability, and how size and motivation asymmetries affect male fighting behaviour. We also determined whether male–male competition affects the courtship behaviour of the males. We found that agonistic drumming activity is an honest indicator of male fighting ability, and that relative size asymmetry and motivation to fight both contribute to the fighting ability. We also found that male–male competition decreases the courtship drumming rate of subdominant males, suggesting that male–male competition limits the opportunities for female choice. We conclude that sequential assessment and honest signalling may both be used in settling contests, and that they may be used simultaneously. Received: 10 December 1998 / Received in revised form: 23 June 1999 / Accepted: 5 July 1999     

Settling-site selection and survival of two scale insects,Ceroplastes rubens andC. ceriferus,on citrus trees

We studied settling-site selection and the resulting survival of two sessile scale insects, Ceroplastes rubens and C. ceriferus, in the citrus tree, Citrus unshiu, in central Japan. C. rubens preferred 0-year-old twigs most as a settling-site; the density of nymphs settling on 0-year-old twigs was significantly higher than those on ≥1-year-old twigs, and few nymphs settled on ≥3-year-old twigs. The mean survival rates from settling until reproduction in the next year were significantly higher on more preferred twigs than on less preferred ones. In C. ceriferus, nymphs significantly preferred 1- and 2-year-old twigs to 0- and ≥3-year-old ones, and the mean survival rates on the more preferred 1- and 2-year-old twigs were significantly higher than those on less preferred ≥3-year-old twigs. However, the survival rate on less preferred 0-year-old twigs was slightly higher than those on 1- and 2-year-old ones. Thus, in both species of scale, it was the preferred twigs which were more profitable sites for survival after settling, except for less preferred 0-year-old twigs for C. ceriferus. In both scale species, most mortality was due to growth cessation, which is believed to be related to the twig quality as a food source. Predators and parasitoids were minor mortality factors. Both species showed constant survival rates until the density of settled nymphs exceeded double the “upper-limit” density, whereupon they decreased drastically. Nymphs of C. rubens settling on twigs of high scale density showed a spacing-out distribution, those of C. ceriferus did not. In C. rubens, an increase in preference for originally less profitable twigs at the later stage of the settling season was observed, but not in C. ceriferus. Accordingly, individuals of C. rubens showed a stronger tendency to avoid conspecifics than did C. ceriferus. Although nymphs of the two scales clearly preferred more profitable sites, their settling-site selection did not agree with the predictions from the ideal free distribution theory (Fretwell and Lucas, 1970). The discrepancies were (1) frequent settling on less profitable sites at the early stage of the settling season, (2) insufficient utilization of the most profitable twigs, and (3) virtually 100% mortality on overcrowded twigs under conditions where unoccupied profitable twigs still remained. These discrepancies are thought due to the limited dispersal time of nymphs.     

Variation in the Settling Velocity of Suspended Particulate Matter in Shallow Lakes,with Special Implications for Mass Balance Modelling

Measurements of sedimentation were combined with water samples to calculate settling velocity of suspended particulate matter (SPM) in lakes. The study sites were open water stations and enclosures in Lake Erken (Sweden) and Lake Balaton (Hungary). Settling velocities were found to vary considerably both inside and outside the enclosures. Within sites, the differences between 25th and 75th percentiles of measured settling velocities of SPM were two‐ or three‐fold. Median settling velocities of SPM ranged from around 0.5 m/d in the enclosures of Lake Erken to more than 8 m/d in the open water of Lake Balaton. Special relevance was attributed to flocculation, which is known to be affected by, e.g., SPM concentration and turbulence. Even though not directly measured, the less turbulent environment inside the enclosures was suggested to explain the low settling velocity compared to the open water environment. Settling velocity apparently correlated with water current speed (r² = 0.66; n = 12). Stepwise multiple linear regressions were used to relate the variability in settling velocity of SPM to the variability of possible controlling factors in a number of data subsets. In most cases, one variable describing the total amount of settling material (e.g., SPM) and one variable reflecting the composition of settling material (e.g., total phosphorus) were chosen. The use of suspended solids concentration to predict settling velocity in mass balance models was discussed. It was found that the mean slope between SPM and v_SPM was close to 0.1 (m⁴d^–1 g^–1). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microplanktonic distributional patterns west of the Antarctic Peninsula,with special emphasis on the Tintinnids

Summary Microplankton was sampled with a centrifugal suction pump in the surface layer (approx. 9 m) of the Bellingshausen Sea and the Bransfield Strait in March 1987, and concentrated with a 26m-mesh net. Bulk microplanktonic settling volumes were assessed, silicoflagellates and large thecate dinoflagellates were counted, and tintinnids were counted and identified to species. Average (and maximum) values for the entire area surveyed were as follows, settling volume: 6.7 (43.3) ml/m³; silicoflagellates: 674 (7777) ind./l, 0.57 (6.54) mg C/m³; dinoflagellates: 109 (1321) ind./l, 1.40 (16.98) mg C/m³; tintinnids: 52 (589) ind./l, 1.15 (9.87) mg C/m³. The three geographic zones defined objectively on the basis of tintinnid specific assemblages also differed sharply in their surface salinity, overall microplanktonic abundance and bulk settling volume. The Bransfield Strait, with lowest settling volume values (2.1 ml/m³) and cell concentrations, was characterized by the dominance of Cymatocylis affinis/convallaria. In waters around the tip of the Antarctic Peninsula microplanktonic settling volumes averaged 4.6 ml/m³, cell concentrations were intermediate, and 79% of the tintinnids were represented by Codonellopsis balechi. The Bellingshausen Sea was characterized by the lowest salinities and the highest settling volumes (8.7 ml/m³) and cell counts; Laackmanniella spp. and Cymatocylis drygalskii, f. typica dominated this area. Almost all biological variables were significantly intercorrelated, and showed strong and mostly significant negative correlations with surface salinity, yet relationships between enhanced standing stock and ice meltwater were not obvious; rather, highest microplanktonic concentrations seemed to be due to ice-associated growth. Extremely high spatial correlations were found between the tintinnids and the dinoflagellates (r²: 0.941), suggesting the existence of close links between these two groups. Tintinnid species-specific assemblages show a coherent distributional pattern and well defined environment-related trends; most clearly differentiated preferences are exhibited by Laackmanniella prolongata (closely associated with ice-covered areas), Cymatocylis affinis/convallaria (oligotrophic open-ocean waters), and Codonellopsis balechi (coastal regions).     

Impact of temperature on the breeding performance and selection patterns in lesser kestrels Falco naumanni

Adjusting breeding phenology to climate fluctuations can be problematic for migratory birds as they have to account for local environmental conditions on the breeding grounds while migrating from remote wintering areas. Predicting general responses to climate change is not straightforward, because these responses vary between migrant species due to the species‐specific ecological drivers of breeding behaviour. Therefore more information is needed on species with different ecological requirements, including data on heritability of migration, factors driving phenological changes and how climate affects selection pressures. Here, we measure heritability in settlement dates and the effect of local climate at the breeding grounds on settlement dates, reproductive success and selection patterns in a French population of a trans‐Saharan migratory insectivorous raptor, the lesser kestrel Falco naumanni, monitored and ringed since 1996. Heritability of settlement dates was low (0.07 ± 0.03), indicating a weak evolutionary potential. Nevertheless, plasticity in settlement dates in response to temperatures allowed earlier settlement when early spring was warmer than average. Reproductive success and selection patterns were strongly affected by temperature during settlement and chick rearing respectively. Warmer spring decreased selection for earlier settling and warmer early summer increased reproductive success. Interestingly, selection for earlier settling was more intense in cooler springs, contrasting with patterns from passerines lagging behind food peaks. Altogether, these results suggest a positive effect of warmer temperatures on breeding performances of lesser kestrels most likely because the French population is at the coolest boundary of the species European breeding range.     

Genotypic variation in induced resistance and induced susceptibility in the peach-Myzus persicae aphid system

In response to herbivore damage or stress, plants may express physiological or morphological changes known as induced responses. We tested whether previous herbivory by the aphid Myzus persicae differentially altered the expression of resistance and susceptibility among five genotypes of peach that differ in their resistance phenotype (avoidance resistance, antibiosis resistance or susceptibility). We measured behavioural and performance parameters of aphid success on plants previously infested by conspecifics as compared to uninfested controls. Significant variation was found both among genotypes and among resistance phenotype, including between genotypes showing a same resistance phenotype. Genotypes with avoidance resistance showed either induced resistance to aphid settling or no response. Genotypes with antibiosis resistance showed induced susceptibility to aphid settling, but the effects of previous herbivory on aphid development were either positive or negative depending on the genotype. In the susceptible genotype, most parameters of aphid settlement and performance, including reproduction, were positively influenced by previous herbivory. Using electronic recording, the aphid probing behaviour was examined to tentatively identify host plant tissues most likely to play a role in induced defenses. Probing behaviour was significantly affected by plant genotype, previous herbivory, and their interaction, indicating complex relations between the two factors. In the genotypes with avoidance resistance, aphids were deterred before they reach the phloem. In the genotypes expressing susceptibility or antibiosis resistance, previous herbivory triggered instead the induction of a phloem‐mediated response, that however diverged depending on the resistance status (facilitation or reduction of phloem sap uptake respectively). Genotypic variation in induction found in the peach‐M. persicae system establishes a useful framework to improve our knowledge of the ecological role of induced plant responses to aphids.     

Interaction between individuals during the colonisation of Douglas fir needles by sistentes of Adelges cooleyi

Summary A laboratory study of Adelges cooleyi sistentes emerging from diapause shows that the sistentes redistribute themselves over the needles in the form of aggregations. At low densities needle bases are preferred to the tips, this preference becoming less marked as sistentes density increases. Within each needle aggregation a certain amount of spacing out occurs which minimises the number of sistentes settling in close proximity. Over a given period of time adult sistentes produce more eggs at the base of the needles than at the tips. The settling behaviour ensures that at low densities the most favourable parts of the needle at the base are utilised first while the spacing out factor provides sufficient space for the development of the egg masses and minimises the possibility of over-exploitation of one part of the needle in relation to the remainder.