Microhabitat selection and audible sexual signalling in the wolf spider Hygrolycosa rubrofasciata (Araneae, Lycosidae)

We studied the microhabitat selection and male sexual signalling behaviour in the wolf spider Hygrolycosa rubrofasciata (Ohlert). Males strike dry leaves with their abdomen, producing an audible sexual drumming signal, and females use this signal to choose mating partners. In the field we followed male drumming rate and microhabitat selection using both the mark–recapture method and direct observations. In the laboratory we conducted an experiment on male microhabitat and drumming substrate selection. We found that in the field males were not distributed randomly among the habitat; fewer males were found in areas that had high sedge cover, low elevation, and low dry leaf cover. In the laboratory experiment, males spent more time on dry leaf substrate. Drumming rate in the field was positively correlated with dry leaf cover and in the laboratory males clearly preferred dry leaves as drumming substrate. Temperature was positively correlated with male drumming rate, and with male and female mobility. We conclude that in H. rubrofasciata male distribution and sexual signalling rate, and thus mating success, are greatly affected by environmental factors. Therefore, males may be sexually selected to make effective use of their signalling habitat by active microhabitat choice. Received: 27 December 1999 / Received in revised form: 14 February 2000 / Accepted: 14 February 2000     

Competition, resource partitioning and coexistence of an endoparasitoid Encarsia perniciosi and an ectoparasitoid Aphytis melinus of the California red scale

Abstract. 1. Laboratory experiments and field studies were conducted to explain the coexistence of an endoparasitoid, Encarsia perniciosi Tower, and an ectoparasitoid, Aphytis melinus DeBach, both of which were introduced into California to control the California red scale, Aonidiella aurantii (Mask.).
2. Encarsia parasitized all scale stages but it preferred first and second instar scales. This is in contrast to Aphytis melinus , in which previous studies have shown that it parasitizes second and third instar females and second instar males but prefers third instar female scales. Encarsia developed most rapidly when it parasitized an early second instar and slowest when it parasitized the mature female scale. However, on early second instar scales it was about 80% as fecund as a wasp that emerged from a mature female scale.
3. Second instar scales parasitized by Encarsia were accepted by Aphytis as readily as unparasitized scales.
4. Encarsia did not distinguish between unparasitized hosts and those previously parasitized by Aphytis.
5. Encarsia always outcompeted by Aphytis when both species parasitize the same host.
6. Encarsia prefers scale on stems whereas Aphytis prefers those on leaves and fruits. This, too, may be a result of interspecific competition with Aphytis.
7. The partitioning of the scale resource by the two species explains why they coexist in coastal southern California but it does not explain why Encarsia disappeared from citrus groves in the inland valleys coincident with the introduction of Aphytis melinus into southern California.     

Competition between two parasites of the Florida red scale in Israel

ABSTRACT. 1. During the last decade, the endoparasitic wasp Pteroptrix smithi (Compere) has gradually replaced the ectoparasitic wasp Aphytis holoxanthus DeBach on the Florida red scale, Chrysomphalus aonidum (L.), on citrus in Israel. The studies reported herein were aimed at elucidating some of the mechanisms of this process.
2. Aphytis adults detected, and were attracted to hosts containing eggs of Pteroptrix , whereas adults of Pteroptrix did not detect hosts containing eggs of Aphytis. In this extrinsic aspect of host discrimination, Aphytis is superior to its competitor. Neither species responded to hosts containing heterospecific first-instar larvae, and both tended to avoid hosts containing more advanced developmental stages of the other species.
3. Aphytis outcompeted Pteroptrix in most cases of multiple parasitism, as observed under both controlled and field conditions.
4. Aphytis was superior in mixed-species cultures; however, its dominance was affected by the availability of adult food.
5. Aphytis adults were found to be more sensitive (×4.11) to the insecticide malathion as compared to adults of Pteroptrix. This finding provides a possible explanation to the displacement of Aphytis by Pteroptrix in the citrus groves along the coastal plain of Israel.     

Dispersal and foraging behaviour of Platygaster californica: hosts can't run, but they can hide

Abstract.  1. Host–parasitoid models often identify foraging behaviour and dispersal distance as important for system persistence.
2. Laboratory observations and field trials were used to characterise foraging behaviour and dispersal capability of Platygaster californica Ashmead (Platygasteridae), a parasitoid of the gall midge Rhopalomyia californica Felt (Cecidomyiidae).
3. Although foraging parasitoids meticulously searched plants in laboratory observations, none of the laboratory trials resulted in 100% parasitism, and the proportion of parasitism declined as midge egg density increased.
4. The field trials showed that the distribution of parasitism over distance from a central release point was hump-shaped, as predicted by a simple diffusion model. Mean parasitoid dispersal distance was 4.5 m, considerably farther than the 1.7 m mean midge dispersal found in previous work.
5. Although the parasitoid appears to search thoroughly for midge eggs and to disperse farther than the midge, the results of this study show how this host–parasitoid system may persist due to spatially variable incomplete parasitism.     

Searching and oviposition behaviour of Amitus fuscipennis, a parasitoid of the greenhouse whitefly

Abstract: Amitus fuscipennis MacGown & Nebeker (Hym., Platygasteridae) is a parasitoid of the greenhouse whitefly, Trialeurodes vaporariorum (Westwood) (Hom., Aleyrodidae) on some crops as bean and snap bean (both Phaseolus vulgaris L.) in Colombia. The searching and oviposition behaviour of A. fuscipennis was studied in the laboratory, using T. vaporariorum as a host on bean leaves. The parasitoid's basic search pathway consisted of walking, encountering the host, and drumming and probing it. While walking, the parasitoid stopped for short periods of time, partly to preen itself. Walking while searching comprised 61% of the adults' time budget and probing hosts represented 16%. After a host nymph was parasitized by A. fuscipennis , the parasitoid remained close by and continued searching for new hosts in the immediate vicinity. Such behaviour suggests area-restricted searching. The parasitoid preferred first instars of T. vaporariorum for oviposition, resulting in long developmental times. Amitus fuscipennis had a high percentage of host acceptance resulting in a high percentage parasitism (60%) of all encountered hosts. Amitus fuscipennis , on average, walked faster before an oviposition (1.4 mm/s) than other whitefly parasitoids. The implications of these findings for the control of T. vaporariorum are discussed.     

Host selection and egg allocation behaviour by Aphytis melinus and A.lingnanensis: comparison of two facultatively gregarious parasitoids

Abstract. 1. The ovipositional and egg allocation behaviour of individual females of Aphytis melinus DeBach and A.lingnunensis Compere were compared.
2. Both Aphytis species exhibit the same behavioural sequence during oviposition.
3. Aphytis melinus laid most of its female eggs on the dorsum of a scale-insect beneath its cover, and most of its male eggs under the scale-insect's body. Aphytis lingnanensis also oviposited both dorsally and ventrally on scale-insect hosts, but female and male progeny arose with equal frequency from eggs laid in both locations.
4. Both A.melinus and A. lingnanensis are facultatively gregarious parasitoids. The degree of gregariousness depends on host size, i.e. the larger the host, the more the Iikelihood that several eggs will be deposited at each visit by the parasitoid.
5. When two eggs were laid during the same host visit, both A.melinus and A.lingnanensis laid one female and one male egg more often than would be expected under an assumption of random allocation of sexes.
6. Because A.melinus successfulIy utilize smaller hosts than A.lingnanensis to produce progeny, these parasitoids should not be considered ecological homo-logues, as suggested by DeBach & Sundby (1963).     

A test of the hypothesis that a pathway of intraguild predation limits densities of a wolf spider

Abstract.  1. The high number of potential predatory interactions among the many arthropod generalist predators in terrestrial food webs makes exhaustive testing of interaction strengths by field experiments unfeasible. Thus, correlative patterns and laboratory observations of behaviour often form the basis of inferences about the strength of interaction pathways involving generalist predators (intraguild predation).
2. Previous research has revealed a negative correlation between survival of juvenile wolf spiders of the genus Schizocosa (Lycosidae) and densities of another abundant spider family, the Gnaphosidae.
3. Feeding trials in laboratory microcosms with a leaf-litter substrate revealed that gnaphosids prey on juvenile Schizocosa in a structurally complex habitat.
4. Gnaphosid densities were manipulated in two different field experiments, each conducted in a different year, in order to test directly the hypothesis that intraguild predation by gnaphosids limits densities of juvenile Schizocosa .
5. Reducing numbers of gnaphosids, and doubling their numbers to two times the mean natural density, had no impact on the survival of juvenile Schizocosa in either field experiment. This finding suggests that correlative patterns in nature and feeding trials in the laboratory may at times provide deceptively simple and potentially misleading generalisations about the strengths of interaction pathways in complex networks of generalist predators.     

Activity patterns and oviposition rates of Aphytis lingnanensis females,a parasitoid of California red scale Aonidiella aurantii: implications for successful biological control

1. Activity patterns of Aphytis lingnanensis Compere (Hymenoptera: Aphelinidae) females were determined. Females have a preovipositional period of 2–6.5 h and oviposit daily until they die. The average lifetime fecundity was 191 eggs (n = 10 females), with a maximum of 13 eggs per day, but usually far fewer. Females do not lay eggs at night as low light levels reduce the propensity to oviposit. 2. Aphytis lingnanensis females showed a repeatable pattern of activity during the course of the day, ovipositing actively during the first 1.5 h of their 7 h exposure to hosts. Thereafter, activity levels decreased sharply and females spent relatively long intervals resting. Activity occurred in fairly discrete bouts, with long periods of inactivity, ranging from 12 to 231 min between bouts. 3. Each day, females laid most eggs during their first bout of oviposition, with availability of mature eggs in the ovaries dictating the duration and intensity of ovipositional bouts. Once females completed a bout of oviposition in the laboratory, they moved away from the hosts even though many suitable hosts remained. They stood immobile, sheltering, until they matured more eggs. 4. These results suggest how general biological control models and dynamic state variable models of behaviour can be made more realistic. In particular, the behaviour of females does not remain constant over extended periods, particularly because of egg depletion, which suggests that the interaction between the ovipositional history of the individual and its physiological capabilities dictates the response of females to particular circumstances, and thus contributes to patterns of parasitism in the field. 5. It is predicted that A. lingnanensis females in the field are inactive at night and begin ovipositing when it becomes light enough in the morning. Egg depletion is likely if enough hosts are available, but should occur later in the day than was recorded in the laboratory. Egg-depleted females are likely to shelter while they mature more eggs, but may undertake interpatch movement.     

Male mating success and risk of predation in a wolf spider: a balance between sexual and natural selection?

1. Traits that benefit males through sexual selection are simultaneously expected to impair males by provoking costs through natural selection. If we consider the two male fitness components, mating success and viability, then we may expect that the increase in male mating success resulting from a larger trait size will be counterbalanced by an increase in viability costs.
2. We studied the benefits and costs of male mate searching and sexual signalling activity in the wolf spider Hygrolycosa rubrofasciata . In the field, males search females actively and court them by drumming dry leaves with their abdomen. Females have been shown to prefer males with high drumming rate. Male moving and especially drumming is energetically highly demanding and drumming results in significant mortality costs.
3. Our objective in this study was to determine whether male mate-searching activity or drumming activity affect male mating success and the risk of males being predated.
4. It was evident that both higher mate-searching activity and higher drumming activity benefited males by increasing their mating success. Higher mate-searching activity clearly impaired males by causing direct increase in predation risk. There was also a slight tendency that more actively drumming males had higher risk of predation and from all of the predated males 13.3% were caught directly after they had drummed. Furthermore, male drumming activity decreased drastically in the presence of the predator.
5. We conclude that in H. rubrofasciata both increased mate-searching activity and drumming activity benefit males through sexual selection, but at the same time natural selection provokes direct balancing costs on the same traits.     

Oppositional responses to host density in Aphytis holoxanthus (Hymenoptera: Aphefinidae), an efficient gregarious parasite

Abstract. 1. Ovipositional responses of an efficient gregarious ectoparasite ( Aphytis holoxanthus ) to increasing densities of its host (the Florida red scale, Chrysomphalus aonidum , Homoptera: Diaspididae) were studied in a laboratory system.
2. The female parasite may lay one to several eggs at each encounter with a host.
3. As host density increased, the number of eggs laid by the female parasite during the experimental period increased at a decreasing rate. This type of functional response resulted in less parasitism at higher host densities.
4. As host density increased, the number of eggs laid per encounter decreased. This behavioural response partially compensated for the decrease in parasitism.     

Behaviour of a specialist parasitoid, Cotesia melitaearum: from individual behaviour to metapopulation processes

Abstract. 1. Foraging behaviour and movement within and among host patches of the specialist parasitoid wasp Cotesia melitaearum (Braconidae) attacking the larvae of Melitaea cinxia (Nymphalidae) were studied in the field and in the laboratory.
2. In the field, female wasps aggregated in large host groups in the autumn and caused positive spatial density-dependent parasitism in the field. Wasps stayed longer with groups of pre-diapause caterpillars than with post-diapause caterpillars, but attacked them less frequently.
3. In the laboratory, wasps attacked larger larvae more readily than smaller larvae. Also in the laboratory, wasps exposed to larvae outside their protective webs showed differences in the rates at which they attacked larvae fed different diets, implicating host plant-derived chemicals as proximate cues for foraging wasps.
4. Mark–recapture studies indicated that there was a low rate of successful movement of wasps among groups of young larvae within a habitat patch in the autumn and no successful movement of wasps across non-habitat. In contrast, wasps moved frequently among groups of late-instar caterpillars in the spring.
5. Host caterpillars of different ages responded very differently to wasp attacks. Pre-diapause larvae remained in groups and used collective head-jerking behaviour to defend themselves, whereas post-diapause larvae dispersed away from the group immediately after being attacked.
6. Population and metapopulation level dynamics of the host–parasitoid interaction are discussed in light of these observations of the behaviour of individual wasps.     

Host handling behavior of the egg parasitoid Telenomus triptus to the egg mass of the stink bug Piezodorus hybneri

Host handling behavior of Telenomus triptus Nixon (Hymenoptera: Scelionidae) to an egg mass of Piezodorus hybneri Gmelin (Heteroptera: Pentatomidae) was studied in the laboratory. Five distinct behavioral events could be distinguished in the host handling behavior: drumming, ovipositor-insertion, marking, walking, and resting. Female wasps showed two types of behavioral sequence in an ovipositional bout. One consisted of drumming, ovipositor-insertion, and marking, and the other drumming, ovipositor-insertion, walking, and resting. Females did not seem to lay an egg when ovipositor-insertion was not followed by marking. This was observed frequently in the early oviposition bout, on average 2.9 times per female. Females finally succeeded in parasitizing all the eggs in a host egg mass in most cases. The durations of drumming, ovipositor-insertion, and marking on an egg mass were nearly constant, while the total time spent by a female on and beside a host egg mass varied considerably as a result of variable durations of walking and resting. Self-superparasitism occurred when almost all the eggs in a host egg mass were parasitized. Females laid the first male egg within the first four eggs; this could be an adaptation to small egg masses or single egg.     

Effects of time limitation and egg limitation on lifetime reproductive success of a parasitoid in the field

We used field observations of freely foraging Aphytis aonidiae parasitoids in conjunction with results of laboratory studies of A. aonidiae and other Aphytis species to simulate lifetime patterns of behavior and reproduction. Field observations provided estimates of encounter rates with three classes of hosts, the mortality rate from predation on adult parasitoids, and host-handling times for oviposition and host feeding by adult wasps. A series of physiological parameters, including the egg maturation rate and the value of host-feeding meals, were estimated from previously published studies. Plasticity in parasitoid behavior was incorporated in two ways. For one set of simulations we used a behavioral rule derived empirically from observations of parasitoids made in the field, and for another we used a dynamic state-variable model to generate a set of behavioral rules that maximize lifetime reproductive success. As was expected, the empirically derived rule led to better matches with field observations than did simulations using the output of the dynamic model. Projections of lifetime reproductive success in the field ranged between three and 37 eggs within the 95% confidence intervals of the mortality rate and host encounter rate and depending on which behavioral rule was used. Lifetime reproductive success from the simulation with central estimates of the mortality and host encounter rates that incorporated the empirical rule was 6.25 eggs. Using the empirical versus the theoretical rule in the simulations led to a 10%-30% decline in projections of lifetime reproductive success, depending on mortality and host encounter rates. Regardless of the behavioral rule, the simulations underscored the observation that the host encounter rate was greater than the egg maturation rate. The overall oviposition rate was sufficiently high to lead to daily episodes of temporary egg limitation during which parasitoids must mature an egg before being able to oviposit.     

Calling behaviour of the potato aphid Macrosiphum euphorbiae oviparae under laboratory and field conditions

Abstract.  1. The calling behaviour of virgin oviparae of the potato aphid Macrosiphum euphorbiae (Thomas) (Homoptera: Aphididae) was studied at three different constant temperatures under laboratory conditions. The mean age of calling for the first time decreased with a decrease in temperature from 2.9 days at 20 °C to 2.1 days at 10 °C. At all temperature regimes, the mean onset time of calling advanced from about 5 to 3 h after the onset of the photophase, and the mean time spent calling increased by > 4 h over the 8 days.
2. Cohorts of oviparae were also observed at two different periods in late summer–early autumn in the field, to examine the effects of fluctuating abiotic conditions (temperatures, wind velocity, rain) and age on calling behaviour. As under constant laboratory conditions, the mean age of calling for the first time declined with declining temperature, from 3.7 days in early September to 1.6 days at the end of September. Age-related changes in the mean onset time of calling and the mean time spent calling were much less evident under field conditions, due to the inhibitory effects of low temperatures, high winds, and rain on female calling activity.
3. The results are discussed within the context of reproductive success and address a previously proposed hypothesis suggesting that species-specific calling windows may serve as a reproductive isolating mechanism for sympatric aphid species.     

A regional analysis of weather mediated competition between a parasitoid and a coccinellid predator of oleander scale

The regulation of an asexual population of the oleander scale [Aspidiotus nerii Bouchè (Hemiptera: Diaspididae)] on California bay tree [Umbellularia californica (Hopk. & Arn.) Nut.] by two natural enemies; an idiobiont, ectoparasitoid Aphytis chilensis Howard (Hymenoptera: Aphelinidae) and a coccinellid predator (Rhysobius lophanthae (Blaisd.) (Coleoptera: Coccinellidae), was examined using a general weather-driven, tri-trophic, physiologically based age-mass structured demographic model. The model is of intermediate complexity and was parameterized using extensive laboratory data and field observations from Albany, CA. Temperature-dependent physiological indices were estimated from the laboratory data and used to scale per capita growth, fecundity and survivorship rates from maximal values in a time varying environment. The tri-trophic model was integrated in a GIS (geographic information system) and the species dynamics examined across years and across the ecological zones of California. Field data and simulation results suggested the coccinellid predator was the most important regulating agent of oleander scale in the mild climate of Albany. However, multiple linear regression analysis of simulation data across all ecological zones of California shows that the parasitoid A. chilensis is the most important agent in suppressing oleander scale densities in warmer climates, while the predator R. lophanthae increases scale density an average of 9.7% across all regions.     

Pollen-feeding and the host specificity and fecundity of flower thrips (Thysanoptera)

Abstract. 1. Flower thrips could feed on pollen of a wide range of sizes and were unaffected by exine spines. One species of thrips was unable to feed on very adhesive grains.
2. The proportion of time spent feeding on each species of pollen (in seconds per minute of exposure) reflected the known host specificities. Host-specific thrips could distinguish their host pollen, apparently without probing.
3. Kakothrips pisivorus (Westwood) appeared to have a higher rate of ingestion of a host pollen than of four non-host pollens.
4. In cages in the laboratory, Thrips fuscipennis Haliday laid significantly more eggs over 48h when given pollen than when given other floral tissues or no food. K. pisivonis and Ceratothrips ericae (Haliday) also laid significantly more eggs over 48h in the presence of pollen than in its absence.
5. In comparisons between pollen diets, T. fuscipennis, a generalist, laid similar numbers of eggs with each species of pollen, but K. pisivorus and C. ericae laid significantly more eggs with their respective host pollens. The differences in oviposition rate between pollen diets reflected the differences in the proportion of time spent feeding.     

Behavioral Attraction of Two Parasitoids,Venturia canescens and Bracon hebetor,to Silk Extracts of a Host Plodia interpunctella

Kairomonal activities of silk extracts of host Plodia interpunctella were determined by measuring the rates of behavioral responses of two parasitic wasps, Venturia canescens and Bracon hebetor. Silk of P. interpunctella larvae attracted both parasitic wasps but the cocoon silk of silkworm, Bombyx mori and the web silk of two-spotted spider mite, Tetranychus urticae did not. Silk components of P. interpunctella were extracted by using either hexane or methanol, and tested the rates of three serial responses of wasps; host location, antennal drumming and ovipositor probing behaviors. The patterns of each behavioral response were similar in two wasps. The rates of each response were increased at the higher concentrations of both extracts. Antennal drumming behavior was much more responsive to lower concentrations of both extracts than ovipositor probing behavior was. Furthermore, the rate of antennal drumming response was higher in hexane-extracts rather than methanol-extract in both wasps; V. canescens and B. hebetor for 20 and 17 times, respectively. However, ovipositor probing response was similar in two different extracts. Both extracts elicited 100% of antennal drumming response but ovipositor probing response was only 60 to 80% of all tested individuals. Our results were shown that silk extracts of host larvae elicited strong behavioral responses of two parasitic wasps and could be applied for practical application of parasitoids attraction in the biological control of agricultural pests.     

Behavioural responses of male ruffed grouse (Bonasa umbellus,L.) to playbacks of drumming displays

Birds use a variety of sounds in their courtship displays, but the majority of behavioural studies have focused on vocalizations. In contrast, little is known about how non‐vocal sounds, or sonations, are used, even though many avian species produce them. The ruffed grouse (Bonasa umbellus) is a useful species to examine non‐vocal sounds because they lack vocal components in their courtship and rely on a non‐vocal sound to attract mates and defend their territory. Their courtship display, known as “drumming,” is created by the wings, and the number of pulses and speed (pulse rate) varies significantly among males. Anecdotal evidence suggested that males can affect the drumming behaviour of neighbouring males in drumming “duels” in an analogous way to song contests. Here, we test whether males do respond to the playback of drumming sounds of an unfamiliar male. Using a portable speaker system, we played recordings of drumming displays to males that were actively drumming themselves. Throughout each playback, we recorded the drumming behaviour of target males so that we could assess whether drumming activity changes following a playback as well as whether males change the speed of their display. Overall, male grouse were equally likely to approach the speaker or continue drumming following a playback. For those males that continued drumming, their drumming pulse rate was significantly faster following playbacks, but they drummed less often. These results indicate that male ruffed grouse do respond to drumming sounds, but the specific response differs among males. Because the differential response was not related to colour phase or whether a male was drumming in proximity to other males, we suggest that the response of individuals likely varies with other traits, such as hormone levels or behavioural syndrome.     

Oviposition Behavior of the Eastern Spruce Budworm Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae)

We documented 13 behaviors associated with oviposition in unfertilized and fertilized spruce budworm moths, Choristoneura fumiferana, by videotaping active moths with a macro lens. Apart from resting, the most pronounced behaviors were probing, drumming + probing, and egg laying. Probing was a bending of the abdomen and extension of the ovipositor to touch the substrate in a rocking, back and forth motion. Drumming + probing involved tapping of the substrate with the pro- and mesothocacic legs with concurrent bending of the abdomen as above. Egg laying was the actual process of egg deposition onto the needle surface of a balsam fir twig. Both the frequency and the duration of these behavioral elements varied depending on the mating status of the female. Unfertilized females exhibited a higher frequency of probing and egg laying. The duration of probing was longer in unfertilized females, while drumming + probing and egg laying were longer for fertilized females. This study is the basis for future work on the chemosensilla associated with the perception of host-plant surface chemicals by ovipositing females.     

Understanding gregariousness in a larval Lepidopteran: the roles of host plant, predation, and microclimate

Abstract.  1. Many moth and butterfly larvae are gregarious early in development, but become solitary in late instars. This ontogenetic variation in behaviour is probably the result of temporal changes in the costs and benefits associated with gregariousness. This study provides observational and experimental evidence that, in one particular moth species, a series of different ecological factors influence larval behaviour at different times during development.
2. Field observations show that young caterpillars of the limocodid Doratifera casta form large aggregations while foraging, but that mature larvae are largely solitary.
3. A field experiment revealed that individual first to third instar larvae in larger groups develop more rapidly, but that group size had no detectable influence on survival. The developmental advantage associated with gregariousness is affected by host plant species, but not by predator exclusion, suggesting that group living in these cryptic early instar larvae promotes feeding facilitation, but does not provide individuals with protection from natural enemies.
4. Laboratory experiments revealed that aposematic fourth instar caterpillars in large groups were less likely to be attacked by a generalist insect predator than those in small groups.
5. Field observations provided no evidence that group living affects body temperature, suggesting that microclimatic factors do not favour gregariousness in this species.
6. It is concluded that gregariousness in D. casta confers at least two different advantages on larvae at different stages early in development, but that these advantages disappear, or are outweighed by costs associated with intraspecific competition, in final instars.