John S. Fenlon

Detection of secondary predation by PCR analyses of the gut contents of invertebrate generalist predators

Predation by generalist predators is difficult to study in the field because of the complex effects of positive and negative interactions within and between predator species and guilds. Predation can be monitored by molecular means, through identification of prey DNA within predators. However, polymerase chain reaction (PCR) amplification of prey DNA from predators cannot discriminate between primary and secondary predation (hyperpredation), in which one predator feeds on another that has recently eaten the target prey. Here we quantify, for the first time, the potential error caused by detection of prey DNA following secondary predation, using an aphid–spider–carabid model. First, the aphid Sitobion avenae was fed to the spider Tenuiphantes tenuis and the carabid Pterostichus melanarius, and the postconsumption detection periods, for prey DNA within predators, were calculated. Aphids were then fed to spiders and the spiders to carabids. Aphid DNA was detected in the predators using primers that amplified 245‐ and 110‐bp fragments of the mitochondrial cytochrome oxidase I gene. Fragment size and predator sex had no significant effect on detection periods. Secondary predation could be detected for up to 8 h, when carabids fed on spiders immediately after the latter had consumed aphids. Beetles tested positive up to 4 h after eating spiders that had digested their aphid prey for 4 h. Clearly, the extreme sensitivity of PCR makes detection of secondary predation more likely, and the only reliable answer in future may be to use PCR to identify, in parallel, instances of intraguild predation.

A spider population in flux: selection and abandonment of artificial web-sites and the importance of intraspecific interactions in Lephthyphantes tenuis (Araneae: Linyphiidae) in wheat

Lepthyphantes tenuis, a small sheet-webbuilding linyphiid spider is one of the most abundant spider species of cereal fields in Europe. In the present study we examined the process of web-site selection and web-site tenacity by adult females of this species in a winter wheat field. Spiders were selective in their choice of web-site. Different immigration rates into various manipulated web-sites, in field and laboratory, suggested that structural support and suitable micro-climate (high humidity) are the most important factors in the selection. Small holes dug in the ground were the most favoured web-sites. Web-site occupation was influenced by the presence of other conspecific spiders. Territorial contests occurred between spiders attempting to occupy the same web, these almost invariably led to the take-over of the web when the intruder was heavier. Interference, but also a certain level of tolerance, between spiders within the same web-site but in different webs was suggested by direct and indirect evidence. Many holes supported two or even three spiders in vertically stratified webs. Leaving probability of marked spiders was significantly higher in multiply occupied holes than in holes with a single web. Comparison with the results of a no-interference stochastic model showed that multiple occupancy in nature is less frequent than predicted by the model. There was further evidence for weak extra-web-interference between spiders in that multiple occupancy was even less frequent and overall occupancy was lower in web-sites which were packed close to each other. However, a level of tolerance for crowding is shown by the fact that closely packed hole colonies supported a spider density 13 times higher than in natural web-sites in the field. A marking experiment was carried out to gain information on web-site tenacity (i.e. the length of time a spider spends in a web-site) and abandonment. The average duration of tenacity was less than 2 days. A random loss function gave a good fit to the tenacity distribution and suggested that spiders abandoned web-sites randomly with a fixed leaving probability of c. 0.5. Individual webs were often used consecutively by more than one spider, and some spiders built more than one web in the same web-site. Calculations showed that abandonment is the most frequent leaving mode, while take-over by contest between spiders and disappearance due to destruction were some-what less frequent and equally likely modes of ending tenacity. It is suggested that the apparent contradiction between the selectiveness and competitiveness of spiders for web-sites and the relatively short tenacity observed can be resolved by hypothesising that spiders leave websites soon because they apply the strategy of spreading risk: spiders by frequently moving from one web-site to another distribute their reproductive efforts across several localities. This hypothesis is further supported by changes in web-site preference and ballooning behaviour at the onset of the reproductive stage in L. tenuis.

The effect of temperature on the functional response of Phytoseiulus persimilis (Acari: Phytoseiidae).

Environmental variables, such as temperature, are important in determining the efficiency of biological control in ornamental crops. This paper examines the effect of temperature on the functional response of adult female Phytoseiulus persimilis to eggs of the spider mite, Tetranychus urticae. The functional response was determined using a new functional response assay technique with plant stems as an arena, rather than leaf discs. The use of plant stems allows the influence that plant structure has on predation to be incorporated into the assay. Control assays were also used (without predators) to estimate natural losses of prey. The data were analysed using a binomial model, with the use of Abbots formula to correct for the losses in the controls. A combined equation to describe the effect of temperature and prey density on the predation rate of Phytoseiulus persimilis was derived. The results showed that more prey are eaten as the temperature increases from 15 °C to 25 °C, but the number of prey eaten then declines at 30 °C, although not to the levels seen at 20 °C. The implication of these results for biological control in ornamental crops, where the temperature can often exceed 30 °C, is discussed.

Phytoseiid mites in protected crops: the effect of humidity and food availability on egg hatch and adult life span of Iphiseius degenerans, Neoseiulus cucumeris, N. californicus and Phytoseiulus persimilis (Acari: Phytoseiidae)

The effect of relative humidity on egg hatch success for Iphiseius degenerans, Neoseiulus californicus and N. cucumeris was described by a binomial model with a parallel slope. The shape of the response differed for Phytoseiulus persimilis and a model with separate parameters gave a significantly better fit. Fitted response curves showed that I. degenerans, N. cucumeris,N. californicus and P. persimilis were ranked by decreasing tolerance to low humidity, with egg mortalities of <0.5, 3, 12 and 16% respectively at 75–80% RH at 20 °C. Egg stage duration for I. degenerans and N. cucumeris was unaffected over the range 60–82% RH. For N. californicus and P. persimilis egg duration was significantly longer at 60 and 70% than for either 82 or 90% RH. No effect of relative humidity was found on the mean life span of adult females when food was available continuously to the mites. N. californicus lived significantly longer (58 days after the first egg was laid) than the other species. No significant difference was observed in mean life span between adult females of I. degenerans and N. cucumeris (25 and 28 days respectively). The mean life span of adult female P. persimilis (19 days) was significantly shorter than the other species. In the absence of both food and water, the survival of adult female mites was reduced to 2–4 days. Survival time was at least doubled when free water was available in the absence of food. Mean survival of adult female mites with water but without food was 10 days for N. cucumeris, 18 days for N. californicus, 6 days for P. persimilis and 4 days for I. degenerans. Survival of adult female N. cucumeris and N. californicus was increased significantly, to 20 and 22 days respectively, when fungal hyphae were present along with water but in the absence of other food.

Influence of temperature on the infectivity of entomopathogenic nematodes (Steinernema and Heterorhabditis spp.) to larvae and pupae of the vine weevil Otiorhynchus sulcatus (Coleoptera: Curculionidae).

The susceptibilities of early and late instar vine weevil larvae and pupae to three species of entomopathogenic nematodes, indigenous to the UK, were tested in a series of bioassays. Steinernema kraussei (isolates L017 and L137), S. feltiae (the commercial product Nemasys®) and Heterorhabditis megidis (the commercial product Nemasys® H, reared both in vivo in Galleria mellonella larvae and in vitro), were tested at 6, 10 and 18°C for 2 weeks (early instars of O. sulcatus) or 3 weeks (late instars and pupae of O. sulcatus). Nematodes were applied to over 3800 larvae or pupae and there were over 400 untreated controls. Each insect was examined subsequently to determine mortality, and parasitised specimens were dissected to establish whether adult nematodes had developed. Differences in pathogenicity between H. megidis reared in vitro and in vivo were demonstrated. S. kraussei (L137) was consistently the most virulent nematode isolate at low temperatures. The results revealed a significant (P < 0.001) effect of temperature on small larvae of O. sulcatus, but also showed differential levels of mortality, not due to nematodes, for both small larvae and pupae. The use of Abbotts correction for control mortality is challenged and the validity of competing risks theory examined.In einer Reihe von Biotests wurde die Anfalligkeit von fruhen und spaten Larvenstadien sowie Puppen des Russelkafers Otiorhynchus sulcatus gegenuber drei in UK einheimischen entomopathogenen Nematoden untersucht. Steinernema kraussei (isolate L017 und L137), S. feltiae (Handelsprodukt Nemasys®) und Heterorhabditis megidis (Handelsprodukt Nemasys® H, beide in vivo an Larven von Galleria mellonella und in vitro gezuchtet) wurden bei 6, 10 and 18°C fur zwei Wochen (fruhe Stadien von O. sulcatus) oder drei Wochen (spate Stadien und Puppen von O. sulcatus) gepruft. Uber 3800 Larven oder Puppen wurden mit Nematoden behandelt, daneben gab es uber 400 unbehandelte Kontrollen. Anschliessend wurde jedes Insekt untersucht, um die Mortalitat zu bestimmen. Parasitierte Exemplare wurden aufprapariert um festzustellen, ob sich adulte Nematoden entwickelt hatten. Zwischen in vitro und in vivo kultivierten H. megidis konnten Unterschiede in der Pathogenitat festgestellt werden. S. kraussei (L137) war bei niedrigen Temperaturen durchgehend das virulenteste Isolat. Die Ergebnisse zeigten eine signifikante (P < 0.001) Wirkung der Temperatur auf kleine Larven von O. sulcatus. Sie zeigten fur kleine Larven und Puppen aber auch unterschiedliche Mortlitatsgrade an, die nicht auf Nematoden zuruckgingen. Die Anwendung von Abbotts Korrektur zur Prufung der Mortalitat wird kritisch hinterfragt, die Gultigkeit der Theorie der “competing risks” wird gepruft.

Supplemental food affects thrips predation and movement of Orius laevigatus (Hemiptera: Anthocoridae) and Neoseiulus cucumeris (Acari: Phytoseiidae)

Two experiments were done to examine the predation of thrips, and the movement of Orius laevigatus Fieber and Neoseiulus cucumeris (Oudemans) in the presence and absence of two supplemental food sources, pollen and the fungus Trichoderma viride. The presence of pollen led to a 55% reduction in predation of the thrips by N. cucumeris and a 40% reduction in thrips predation by O. laevigatus, in experiments using single predators. The presence of fungus had no significant effect on thrips predation by either of the natural enemy species. Movement of the natural enemies was examined in a multiple predator experiment, and this showed that O. laevigatus was more likely to remain on the plant in the presence of thrips and when supplemental food, either pollen or fungus, was present. For N. cucumeris, there was no association between the presence of thrips and the mite, with the majority of the mites being found on the leaves where pollen was present. Although the single and multiple predator experiments were done at different times, the indications are that the predation rates of the N. cucumeris do not differ greatly between the two experiments, suggesting that there may be a potential interference effect between the mites, which is not present for O. laevigatus. The significance of these results for the use of supplemental food sources in biological control is discussed.

Risk assessment with time to event models

Introduction to Time to Event Methods, M.C. Newman and M. Crane Time to Event Analysis of Standard Ecotoxicity Data, M. Crane and A. Grosso Applying Time to Event Methods to Assess Pollutant Effects on Populations, M.C. Newman and J.T. McCloskey Time-Concentration-Effect Models in Predicting Chronic Toxicity from Acute Toxicity Data, F.L. Mayer, M.R. Ellersieck, G.F. Krause, K. Sun, G. Lee, and D.R. Buckler Just How Much Better is a Time to Event Analysis? P.M. Dixon Using Time to Event Modelling to Assess the Ecological Risk of Produced Water Discharges, C.C. Karman Time to Event Analysis in the Agricultural Sciences, J.S. Fenlon Time to Event Analysis in Ecology, B.F. Manly Time to Event Methods in Engineering, A. Kimber Can Risk Assessment be Improved with Time to Event Models? M. Crane, P.F. Chapman, T. Sparks, J. Fenlon, and M.C. Newman Conclusions, M. Crane, P.F. Chapman, and M.C. Newman

The relationship between dietary specialism and availability of food and water on cannibalistic interactions among predatory mites in protected crops.

The interplay between dietary specialism, the tolerance of food and water stress and level of cannibalism is likely to be important in determining the outcome of biological control using inundative releases of multiple natural enemies, such as phytoseiid mites in protected crops. The dietary specialist, Phytoseiulus persimilis, with a short immature development time (4–5 days) when plentiful food was available had a low ability to survive without food (5 days), even with access to water. The dietary generalists, Neoseiulus californicus, N. cucumeris and Iphiseius degenerans, had longer immature development times (by up to 2 days) than P. persimilis. Survival ability differed amongst the generalist species when they were starved but provided with constant access to water. Both N. californicus and N. cucumeris survived the longest (8–10 days) and I. degenerans survived the shortest period (4 days). No negative intra-specific interaction between immatures was observed with P. persimilis when food was available and in the absence of food this species tended to starve rather than act cannibalistically. Both N. californicus and N. cucumeris showed a low degree of cannibalism between immatures, either when food was available, or when starved but given access to water. Even when food was available survival of I. degenerans fell by 30% in 4 days and remained at 60–70% for 3 further days; survival continued to decline rapidly when they were starved but provided with water. This indicates that immatures of I. degenerans could either feed on dead conspecifics or that they were capable of a degree of cannibalism. Adult females of P. persimilis did not feed on conspecific eggs even when deprived of food but provided with water. Adult female N. californicus and N. cucumeris did feed on conspecific eggs but at a low level (<1 egg per day), which occurred only after 48 h starvation. Although egg cannibalism occurred more consistently with adult female I. degenerans than with other mite species it was at a low level (<1 egg per day). If the tendency to cannibalism, not just of eggs but with more susceptible life stages such as larvae, is reduced when water is available freely this could be important in determining the interactions that occur under natural conditions.

Do functional traits improve prediction of predation rates for a disparate group of aphid predators

Aphid predators are a systematically disparate group of arthropods united on the basis that they consume aphids as part of their diet. In Europe, this group includes Araneae, Opiliones, Heteroptera, chrysopids, Forficulina, syrphid larvae, carabids, staphylinids, cantharids and coccinellids. This functional group has no phylogenetic meaning but was created by ecologists as a way of understanding predation, particularly for conservation biological control. We investigated whether trait-based approaches could bring some cohesion and structure to this predator group. A taxonomic hierarchy-based null model was created from taxonomic distances in which a simple multiplicative relationship described the Linnaean hierarchies (species, genera, etc.) of fifty common aphid predators. Using the same fifty species, a functional groups model was developed using ten behavioural traits (e.g. polyphagy, dispersal, activity, etc.) to describe the way in which aphids were predated in the field. The interrelationships between species were then expressed as dissimilarities within each model and separately analysed using PROXSCAL, a multidimensional scaling (MDS) program. When ordinated using PROXSCAL and then statistically compared using Procrustes analysis, we found that only 17% of information was shared between the two configurations. Polyphagy across kingdoms (i.e. predatory behaviour across animal, plant and fungi kingdoms) and the ability to withstand starvation over days, weeks and months were particularly divisive within the functional groups model. Confirmatory MDS indicated poor prediction of aphid predation rates by the configurations derived from either model. The counterintuitive conclusion was that the inclusion of functional traits, pertinent to the way in which predators fed on aphids, did not lead to a large improvement in the prediction of predation rate when compared to the standard taxonomic approach.

Egg Distributions of Insect Parasitoids: Modelling and Analysis of Temporal Data with Host Density Dependence

A simple numerical procedure is presented for the problem of estimating the parameters of models for the distribution of eggs oviposited in a host. The modelling is extended to incorporate both host density and time dependence to produce a remarkably parsimonious structure with only seven parameters to describe a data set of over 3,000 observations. This is further refined using a mixed model to accommodate several large outliers. Both models show that the level of superparasitism declines with increasing host density, and the rate declines over time. It is proposed that the differing behaviours represented by the mixed model may reflect a balance between behavioural strategies of different selective benefit.