Ontogenetic change in effectiveness of chemical defence against different predators in Oxycarenus true bugs

Many prey species change their antipredator defence during ontogeny, which may be connected to different potential predators over the life cycle of the prey. To test this hypothesis, we compared reactions of two predator taxa – spiders and birds – to larvae and adults of two invasive true bug species, Oxycarenus hyalinipennis and Oxycarenus lavaterae (Heteroptera: Oxycarenidae) with life-stage-specific chemical defence mechanisms. The reactions to larvae and adults of both true bug species strikingly differed between the two predator taxa. The spiders were deterred by the defences of adult bugs, but the larval defences were ineffective against them. By contrast, birds attacked the larvae considerably less often than the adult bugs. The results indicate a predator-specific ontogenetic change in defence effectiveness of both Oxycarenus species. The change in defence is likely linked to the life-stage-specific composition of secretions in both species: whereas secretions of larvae are dominated by unsaturated aldehydes, secretions of adults are rich in terpenoids, which probably serve dual function of defensive chemicals and pheromones. Our results highlight the variation in defence between different life stages and the importance of testing responses of different types of predators.     

Beobachtungen zur Lebensweise von Pycnogonum litorale (Ström) (Pantopoda)

Zusammenfassung Am Leitdamm des Jadebusens lebt Pycnogonum litorale im Lückensystem des Miesmuschelbesatzes. Dieser bietet mit hartem Untergrund, hoher Feuchtigkeit bei Niedrigwasser, genügend Actinien als Nahrung und guter Durchströmung bei gleichzeitigem Schutz vor Vertragung—offensichtlich günstige Lebensbedingungen für Pycnogonum litorale.Der Eiablage im Februar geht eine Reiterstellung des Männchens auf dem Weibchen von durchschnittlich 24 Tagen voraus. Unter künstlichen Kurztagbedingungen kann diese Reiterstellung auch außerhalb der Fortpflanzungsperiode eingenommen werden. Die Eier werden durch Rumpfbewegungen beider Partner zu den Ovigeren des Männchens bewegt. Bei 12°C schlüpfen die Larven etwa 41 bis 46 Tage nach der Eiablage aus, bei 19°C, im Sommer, schlüpften keine Larven.Im Jadebusen leben die Larven etwa 1/2 Jahn endoparasitisch in Hydrozoen. Die an die Metamorphose anschließende juvenile Phase, in der die Tiere frei leben, dauert ein knappes Jahr, die Reifehäutung erfolgt normalerweise im Sommer des zweiten Jahres, die Fortpflanzungsperiode etwa 6 Monate später, im Winter.

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Observations on the life biology of Pycnogonum litorale (Ström) (Pantopoda)

Summary Pycnogonum litorale lives in an interstitial system, of the mussel zone on the embankment of the Jadebusen. Hard substrate, high humidity at low tide, sufficient Metridium senile as food, and active currents together with protection from drifting, constitute favourable conditions for this pycnogonid.Prior to laying egg in February, the male remains in a riding position upon the female for approximately 24 days. Under artificial short-day conditions the riding position may also be assumed outside of the reproductive period. The eggs are transported to the ovigers of the male by trunk movements of both partners. At 12°C the larvae hatch about 41–46 days after egg-laying. No larvae hatched from eggs laid during summer at 19°C.The larvae live endoparasitically in Hydrozoa for about 1/2 year. Following metamorphosis, the freeliving juvenile phase lasts barely a year. The maturation moult normally takes place in the summer of the second year, the reproductive period beginning about 6 months later, in winter.

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Mit Unterstützung der Deutschen Forschungsgemeinschaft.     

Potential predation of non-webbuilding spider assemblage on cotton pests Helicoverpa armigera and Spodoptera littoralis (Lepidoptera: Noctuidae)

The relative feeding rates and preferences of a hunting-spider assemblage inhabiting southern Spanish cotton fields for two major cotton pests, Helicoverpa armigera (Hübner) and Spodoptera littoralis (Boisdubal) (Lepidoptera: Noctuidae), were analyzed under laboratory conditions. First, a no-choice feeding test was used to determine relative feeding rates for hunting-spider families and species, offering a fixed number of 10 neonate larvae of H. armigera or S. littoralis and observing predation after 2 h, 4 h, 8 h, and 24 h. In a second test, Drosophila melanogaster, a very palatable alternative prey, was used to determine the degree of preference for cotton pest larvae. The mean number of first-instar lepidoptera larvae consumed by hunting spiders after 24 h was 8.57±0.25. As expected, spiders showed no preference for either of the two cotton pest species H. armigera and S. littoralis over the other. Results also showed that cursorial spiders of the families Miturigidae (represented here by Cheiracanthium pelasgicum) and Philodromidae consumed significantly higher percentages of larvae than crab spiders belonging to the Thomisidae family after 2 h and 24 h, respectively. In the prey choice test, Cheiracantium pelasgicum displayed a strong preference for cotton pest larvae while Thomisidae and Oxyopidae showed no significant preference. In addition, as the attack sequence progressed, Ch. pelasgicum showed a clear tendency towards the alternation of prey while Thomisidae, and more irregularly Oxyopidae, maintained their preference for D. melanogaster. These findings confirmed both the considerable potential value of some cursorial spiders (e.g. Ch. pelasgicum) in the biological control of lepidopteran cotton pests and the relatively low impact of other hunting spiders, e.g. Thomisidae, on pests of this kind.     

Resident spiders as predators of the recently introduced light brown apple moth,Epiphyas postvittana

The invasive light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), has not reached damaging levels on crops in California (USA), suggesting that its populations and impact are limited by generalist natural enemies. In a series of laboratory experiments, we examined resident spiders as predators of E. postvittana larvae on two host plants, the ornamental Australian tea tree, Leptospermum laevigatum (Gaertn.) F. Muell (Myrtaceae), and the weed French broom, Genista monspessulana (L.) L.A.S. Johnson (Fabaceae). Of three abundant spider species in Australian tea tree, two hunting spiders measurably reduced the numbers of E. postvittana larvae and plant damage, whereas a web‐weaving spider had no detectable impact. The adult stage of the dominant hunting spider Anyphaena aperta Banks (Anyphaenidae) consumed E. postvittana larvae, but neither large nor small juveniles had statistically detectable effects on numbers of larvae. However, plant damage was reduced in the presence of large juvenile A. aperta, suggesting that A. aperta may also have non‐consumptive effects on the feeding behavior of E. postvittana larvae. Anyphaena aperta consumed larvae only when larval densities exceeded a low threshold in a functional response experiment, leading to a type III functional response. Adult A. aperta showed no preference for different E. postvittana instars, whereas Cheiracanthium mildei L. Koch (Miturgidae), an abundant hunting spider on French broom, showed a partial preference for late‐instar larvae. The generalist feeding habits of the spiders may have precluded strong prey preferences. Results show that hunting spiders may help limit E. postvittana populations in California, and that they may in turn reduce the impact of E. postvittana on its host plants.     

A scanning electron microscopy study of the embryonic development of Pycnogonum litorale (Arthropoda,Pycnogonida)

The phylogenetic position of the enigmatic Pycnogonida (sea spiders) is still controversial. This is in part due to a lack of detailed data about the morphology and ontogenesis of this, in many aspects, aberrant group. In particular, studies on the embryonic development of pycnogonids are rare and in part contradictory. Here, we present the first embryological study of a pycnogonid species using scanning electron microscopy (SEM). We describe the late embryogenesis of Pycnogonum litorale from the first visible appendage anlagen to the hatchling in 11 embryonic stages. The three pairs of appendage anlagen gain in length by growth, as well as by extension of furrows into the embryo. The opening of the stomodaeum is located far in front of the anlagen of the chelifores and has a Y‐shaped lumen from the onset. During further embryogenesis, the position of the mouth shifts ventrally, until it is located between the chelifores. The proboscis anlage grows out as a circumoral wall‐like structure, which is initially more pronounced ventrally. Hypotheses about the evolution of the proboscis by fusion of originally separated components are critically discussed, because the proboscis anlage of P. litorale shows no indications of a composite nature. In particular, a participation of post‐cheliforal elements in proboscis formation is rejected by our data. Further, no preoral structure and no stage in proboscis formation was found, which could plausibly be homologized with the labrum of othereuarthropods. Thus, our study supports the assumption of a complete lack of a labrum in Pycnogonida. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Differential efficacy of toxic pederin in deterring potential arthropod predators of Paederus (Coleoptera: Staphylinidae) offspring

This study investigates the effects of pederin, a hemolymph toxin that is accumulated in the eggs of most Paederus females, on potential arthropod predators of the offspring of P. fuscipes and P. riparius. Insects generally do not respond to pederin present in the prey. Paederus larvae are sufficiently agile to escape from these predators by running away, and the eggs are hidden by the females. Unlike insects, (wolf) spiders are deterred by prey with pederin. They turn away from larvae they have already captured and exhibit cleansing behavior. Larvae containing pederin survive the attacks of spiders without damage, whereas larvae descended from females that do not transfer pederin into their eggs are often killed and eaten. In the case of sudden attacks by spiders, the larvae have no chance of escape. Their survival thus depends on chemical defense. These investigations show for the first time why pederin might be of considerable importance for Paederus in the field.     

Consumptive and non‐consumptive effects of wolf spiders on cotton bollworms

Larvae of the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) that survive on genetically modified Bt cotton (Gossypium hirsutum L., Malvaceae) contribute to the risk of widespread resistance to Bt toxins. Current resistance management techniques include pupae busting, which involves deep tilling of the soil to kill overwintering pupae. Unfortunately, pupae busting runs counter to soil and water conserving techniques, such as minimum tillage. This problem could be relieved with biological control methods, whereby predators attack either larvae going to ground to pupate or moths emerging from the ground. We found that the wolf spider Tasmanicosa leuckartii (Thorell) (Araneae: Lycosidae), a common inhabitant of Australian cotton agroecosystems, is an effective predator of H. armigera, attacking and killing most larvae (66%) and emerging moths (77%) in simple laboratory arenas. Tasmanicosa leuckartii also reduced the number of emerging moths by 66% on average in more structurally complex glasshouse arenas. Males, females, and late‐instar juveniles of T. leuckartii were similarly effective. Tasmanicosa leuckartii also imposed non‐consumptive effects on H. armigera, as when a spider was present larvae in the laboratory areas spent less time on the cotton boll and more time on the soil and more mass was lost from the cotton boll. Increased loss of boll mass likely reflects changes in H. armigera foraging behavior induced by the presence of spiders (indirect non‐consumptive effects). Helicoverpa armigera spent more time as pupae when the spider was present in simple laboratory arenas, but not in more complex glasshouse enclosures. Overall, results indicate that T. leuckartii spiders can be effective predators of H. armigera late instars and moths but also suggest that, under some conditions, the presence of spiders could increase the damage to individual cotton bolls.     

Sensilla on maxillary and labial palps in a helicophagous ground beetle larva (Coleoptera,Carabidae)

Sensilla on the labial and maxillary palp of Carabus lefebvrei Dejean, 1826 larvae were investigated using scanning and transmission electron microscopy. Two identical sets of sensilla were present on the tips of both palp pairs, and four morphological types of sensilla were identified: sensilla basiconica types 1 and 2, sensilla coeloconica and sensilla digitiformia. Ultrastructure indicates that the sensilla basiconica type 1 and coeloconica have a chemical role as gustatory and olfactory receptors, respectively, while sensilla basiconica type 2 are mechanoreceptors and the sensilla digitiformia are probably thermo‐ and hygroreceptors. Their function is discussed in relation to specialized prey detection and habitat adaptations.     

Central projections of cheliceral mechanoreceptors in the spider Cupiennius salei (Arachnida,Araneae)

Central projections of lyriform organs and tactile hairs on the chelicerae of the wandering spider Cupiennius salei were traced using anterograde cobalt fills. Different fibers arising from both mechanoreceptor types arborize in the cheliceral ganglia, which are part of the tritocerebrum, and in sensory longitudinal tracts in the center of the suboesophageal nerve mass together with afferent fibers arising from mechanoreceptors on the walking legs and the pedipalps. This convergence of sensory projections in the sensory longitudinal tracts might provide the anatomical basis for the coordination of the movements of different extremities during prey capture and feeding. The findings also support the hypothesis that the tritocerebrum originally was a preoral ganglion in spiders. © 1993 Wiley-Liss, Inc.     

Low metabolic rates in primitive hunters and weaver spiders

The rates of oxygen consumption and carbon dioxide release of primitive hunters and weaver spiders, the Chilean Recluse spider Loxosceles laeta Nicolet (Araneae: Sicariidae) and the Chilean Tiger spider Scytodes globula Nicolet (Araneae: Scytodidae), are analyzed, and their relationship with body mass is studied. The results are compared with the metabolic data available for other spiders. A low metabolic rate is found both for these two species and other primitive hunters and weavers, such as spiders of the families Dysderidae and Plectreuridae. The metabolic rate of this group is lower than in nonprimitive spiders, such as the orb weavers (Araneae: Araneidae). The results reject the proposition of a general relationship for metabolic rate for all land arthropods (related to body mass) and agree with the hypothesis that metabolic rates are affected not only by sex, reproductive and developmental status, but also by ecology and life style, recognizing here, at least in the araneomorph spiders, a group having low metabolism, comprising the primitive hunters and weaver spiders, and another group comprising the higher metabolic rate web building spiders (e.g. orb weavers).     

Sensilla on the antennae,mouthparts, and body of the larva of the alfalfa weevil,hypera postica (gyllenhal) (Coleoptera : Curculionidae)

The morphology of the antennal complex, major mouthpart sensilla and body sensilla of alfalfa weevil larvae, Hypera postica (Gyllenhal) (Coleoptera : Curculionidae), is described. All instars have a short 1-segmented antenna with a large, multiporous sensillum basiconicum (s.b.) on its apex. Surrounding the s.b. are 2 unusual sensilla auricularia, and 3 small s.b., 2 of which are uniporous. The galea of the maxilla contains 11 s.b. and the apex of the palpus bears 12 thick-walled, uniporous s.b. The labial palpus has 10 similar s.b. apically.First-instar larvae have long, rodlike sensilla with a large, thick-walled bulbous apex; they are probably mechanoreceptors. The other 3 instars bear unusual nodulated and ridged sensilla with thin walls and a fragment-filled lumen. Sensilla on the apex of the thoracic legs and prolegs are also illustrated.     

Comparative biology of Portia africana,P. albimana,P. fimbriata,P. labiata,and P. shultzi,araneophagic, web-building jumping spiders (Araneae: Salticidae): Utilisation of webs,predatory versatility,and intraspecific interactions

Abstract

Portia is a behaviourally complex and aberrant salticid genus. The genus is of unusual importance because it is morphologically primitive. Five species were studied in nature (Australia, Kenya, Malaysia, Sri Lanka) and in the laboratory in an effort to clarify the origins of the salticids and of their unique, complex eyes. All the species of Portia studied were both web builders and cursorial. Portia was also an araneophagic web invader, and it was a highly effective predator on diverse types of alien webs. Portia was an aggressive mimic, using a complex repertoire of vibratory behaviour to deceive the host spiders on which it fed. The venom of Portia was unusually potent to other spiders; its easily autotomised legs may have helped Portia escape if attacked by its frequently dangerous prey. Portia was also kleptoparasitic and oophagic when occupying alien webs. P. fimbriata from Queensland, where cursorial salticids were superabundant, used a unique manner of stalking and capturing other salticids. The display repertoires used during intraspecific interactions were complex and varied between species. Both visual (typical of other salticids) and vibratory (typical of other web spiders) displays were used. Portia copulated both on and away from webs and frequently with the female hanging from a dragline. Males cohabited with subadult females on webs, mating after the female matured. Adult and subadult females sometimes used specialised predatory attacks against courting or mating males. Sperm induction in Portia was similar to that in other cursorial spiders. Portia mimicked detritus in shape and colour, and its slow, mechanical locomotion preserved concealment. Portia occasionally used a special defensive behaviour (wild leaping) if disturbed by a potential predator. Two types of webs were spun by all species (Type 1, small resting platforms; Type 2, large prey-capture webs). Two types of egg sacs were made, both of which were highly aberrant for a salticid. Responses of different species and both sexes of Portia were quantitatively compared for different types of prey. Many of the trends in behaviour within the genus, including quantitative differences in predatory behaviour, seemed to be related to differences in the effectiveness of the cryptic morphology of Portia in concealing the spider in its natural habitat (‘effective crypsis’). The results of the study supported, in general, Jackson & Blest’s (1982a) hypothesis of salticid evolution which, in part, proposes that salticid ancestors were web builders with poorly developed vision and that acute vision evolved in conjunction with the ancestral spiders becoming proficient as araneophagic invaders of diverse types of webs.     

Two closely linked genes encoding thioredoxin and thioredoxin reductase in Clostridium litorale

The genes encoding thioredoxin and thioredoxin reductase of Clostridium litorale were cloned and sequenced. The thioredoxin reductase gene (trxB) encoded a protein of 33.9 kDa, and the deduced amino acid sequence showed 44% identity to the corresponding protein from Escherichia coli. The gene encoding thioredoxin (trxA) was located immediately downstream of trxB. TrxA and TrxB were each encoded by two gene copies, both copies presumably located on the chromosome. Like other thioredoxins from anaerobic, amino-acid-degrading bacteria investigated to date by N-terminal amino acid sequencing, thioredoxin from C. litorale exhibited characteristic deviations from the consensus sequence, e.g., GCVPC instead of WCGPC at the redox-active center. Using heterologous enzyme assays, neither thioredoxin nor thioredoxin reductase were interchangeable with the corresponding proteins of the thioredoxin system from E. coli. To elucidate the molecular basis of that incompatibility, Gly-31 in C. litorale thioredoxin was substituted with Trp (the W in the consensus sequence) by site-directed mutagenesis. The mutant protein was expressed in E. coli and was purified to homogeneity. Enzyme assays using the G31W thioredoxin revealed that Gly-31 was not responsible for the observed incompatibility with the E. coli thioredoxin reductase, but it was essential for activity of the thioredoxin system in C. litorale. Received: 19 September 1996 / Accepted: 21 May 1997     

Microcosm studies on intraguild predation between female erigonid spiders and lacewing larvae and influence of single versus multiple predators on cereal aphids

Abstract: Intraguild predation between female erigonid spiders [Erigone atra (Blackwall) and Oedothorax apicatus (Blackwall), Araneae, Erigonidae] and lacewing larvae (second instar larvae of Chrysoperla carnea (Stephens), Neuropt., Chrysopidae) and interaction effects of predator combinations on cereal aphids were investigated in a microcosm system under laboratory conditions. The microcosm experiments were run for 7 days and consisted of 15wheat seedlings, 15 Sitobion avenae (F) (Hom., Aphididae) as start population, plus a female spider or a lacewing larva or a combination of a spider plus a lacewing larva. The mortality rate of lacewing larvae was significantly increased by 44 and 31% due to intraguild predation by female spiders of E. atra and O. apicatus in comparison with lacewing larvae that were kept alone. The final aphid numbers in the microcosms were significantly reduced by all single predator treatments (spiders, lacewing larvae) and the predator combinations in comparison with controls without predators. The predation effect on aphid populations due to both spider species was similar and not statistically different. An additive effect of the predator combinations ‘spider plus surviving lacewing larva’ was found for both spider species resulting in reduced aphid numbers compared with the single predator treatments. When the lacewing larva was killed by an E. atra female the effects on aphids were non‐additive, but aphid numbers were not statistically increased compared with the lacewing larva treatment. When the lacewing larva was killed by an O. apicatus female, the effects of spider and C. carnea larva were additive on aphid numbers. In the presence of additional prey (fruit flies and Collembola) intraguild predation was not found and E. atra females had no significant effect on the survival of lacewing larvae. In addition, E. atra females had no significant effect on aphid numbers in the presence of fruit flies and Collembola, but in combination with a lacewing larva that survived, a significantly greater reduction of the aphid population was observed compared with the lacewing larva treatment. The body mass of lacewing larvae at the end of the experiment was not statistically influenced by the presence or absence of an E. atra female.     

Fossil cocoons associated with a dinosaur egg from Patagonia,Argentina

Abstract: Eight fossil (Cretaceous) insect cocoons were discovered within the infillings of a broken dinosaur egg of a clutch from a Patagonian locality. Cocoons are considered to be in situ based on detailed preservation of thin, delicate walls with surface texture, infillings that are similar to the surrounding rock matrix and the clustered distribution of cocoons in only one egg out of the clutch of five eggs. According to the shape, size, and thin wall with surface texture, the cocoons are interpreted as having been produced by wasps. The wasps may have been attracted to the egg because of the presence of scavenging insects feeding on the decaying organic matter, or they may have been attracted to spiders feeding on the scavenging insects. In either scenario, after attacking the insects or spiders inside the sand infillings of the egg, the wasp larvae produced the cocoons described herein. The presence of wasps, which are at the top of the scavenging food webs, suggests that a complex community of invertebrates would have developed around rotten dinosaur eggs.     

Genetically-based variation between two spider populations in foraging behavior

Summary Optimal foraging theory is based on the assumption that at least some aspects of foraging behavior are genetically determined (Pyke et al. 1977; Kamil and Sargent 1980; Pyke 1984). Nonetheless, very few studies have examined the role of genetics in foraging behavior. Here, we report on geographical differences in the foraging behavior of a spider (Agelenopsis aperta) and investigate whether these differences are genetically determined. Field studies were conducted on two different populations of A. aperta: one residing in a desert riparian habitat, and the other in a desert grassland habitat. Data from the spiders' natural encounters with prey demonstrated that grassland spiders exhibited a higher frequency of attack than riparian spiders towards 13 of 15 prey types, including crickets and ants. Grassland spiders also had shorter latencies to attack 12 of 15 prey types, including crickets and ants, than riparian spiders. Subsequently, we reared grassland and riparian spiders under controlled conditions in the laboratory and observed their interactions with prey to determine whether the populational differences we found in the field could be genetic. Again, grassland spiders showed a shorter latency to attack prey (crickets, ants) than riparian spiders. These latencies were not significantly affected by the hunger state or age of the spiders. Finally, we reared a second generation (F2) of grassland and riparian spiders in the laboratory and observed their interactions with prey to determine whether the populational differences in the previous generation were due to genetic effects or maternal effects. As before, grassland spiders exhibited a shorter latency to attack prey (crickets) than riparian spiders. We conclude that the foraging differences we observed between these two populations of A. aperta are genetically determined. These differences probably have resulted from either natural selection acting directly on attack frequency and the latency to attack prey, or natural selection acting on traits which are genetically correlated with these aspects of foraging behavior.     

The parasitoid wasp Eruga unilabiana Pádua & Sobczak,sp. nov. (Hymenoptera: Ichneumonidae) induces behavioral modification in its spider host

Some polysphinctine parasitoid wasps can alter the web building behavior of their host spiders. In this paper, we describe and illustrate a new species Eruga unilabiana sp. nov. and report for the first time, to the best of our knowledge, the interaction between this parasitic wasp and the linyphiid spider Dubiaranea sp. We investigated the wasp's host selection, development, and manipulation of host behavior. We found that most of the parasitized spiders were intermediate‐sized adult females that probably provide sufficient resources for parasitoid larvae and are less vulnerable for parasitoid females than larger host individuals at attack. The cocoon web of Dubiaranea sp. consists of a complex three‐dimensional tangle structure with several non‐stick radial lines that converge at the cocoon. In addition, E. unilabiana individuals construct their cocoons horizontally, which differ from cocoons of the majority of polysphinctine wasps. This study provides important information and discussion to further understand the evolution of parasitoid wasp–spider interactions.     

Morphology and histology of secretory setae in terrestrial larvae of biting midges of the genus Forcipomyia (Diptera: Ceratopogonidae)

Apneustic larvae of the genus Forcipomyia possess unique secretory setae located on the dorsal surface along the body in two rows, one pair on each thoracic and abdominal segment and two pairs on the head. Morphological and histological studies of secretory setae in fourth instar larvae of Forcipomyia nigra (Winnertz) and Forcipomyia nigrans Remm indicate they are modified mechanoreceptors (sensilla trichodea) in which the trichogen cell is a glandular cell producing a hygroscopic secretion. The cytoplasm of the glandular trichogen cell fills the lumen of a secretory seta, which shows one or more pores on the apex. The cytoplasm contains numerous microtubules responsible for transportation of proteinaceous vesicles, and an extremely large polyploid nucleus typical of gland cells. The main role of the hygroscopic secretion is to moist the body and thus facilitate cuticular respiration.     

Dimethoate,fenvalerate and their mixture affects Hylyphantes graminicola (Araneae: Linyphiidae) adults and their unexposed offspring

• 1 Mixtures of organophosphorus and pyrethroid insecticides are widely used to combat resistance in agricultural pests, although few studies have been conducted on the effects of pesticide mixtures on beneficial nontarget organisms.
• 2 In the present study, we exposed adult females (F₀) of Hylyphantes graminicola (Araneae: Linyphiidae) to fenvalerate, dimethoate and their commercially available 1 : 1 mixture (by mass). We investigated the acute toxicity of these pesticides to the exposed adults, as well as sublethal effects on reproduction and acetylcholinesterase and carboxylesterase activity. We also studied the effects of parental exposure on the size, development and enzyme activity of unexposed offspring.
• 3 All three formulations were acutely toxic to H. graminicola, with synergism between dimethoate and fenvalerate leading to greater toxicity in the 1 : 1 mixture than for the two insecticides alone. The sublethal effects of direct pesticide exposure were a reduction in acetylcholinesterase and carboxylesterase activity and a reduction in the number of egg sacs produced by exposed spiders relative to the control spiders. The unexposed offspring of the fenvalerate and mixture exposed spiders were smaller and took longer to mature than the control spiders. Offspring of all exposed spiders also had significantly reduced carboxylesterase activity relative to control spiders.
• 4 We concluded that the effects of parental exposure on the offspring were likely to increase their susceptibility to future pesticide exposures, and reduce the capacity of this spider to serve as a pest control agent.