Summary1. A host specialist parasitoid is thought to have greater efficiency in locating hosts or greater ability to overcome host defence than a generalist species. This leads to the prediction that a specialist should locate and parasitise more hosts than a generalist in a given arena. The work reported here tested these predictions by comparing the host‐searching behaviour of Diadegma semiclausum (a specialist) and Cotesia plutellae (an oligophagous species), two parasitoids of larval Plutella xylostella.2. Both parasitoids employed antennal search and ovipositor search when seeking hosts but D. semiclausum also seemed to use visual perception in the immediate vicinity of hosts.3. Larvae of P. xylostella avoided detection by parasitoids by moving away from damaged plant parts after short feeding bouts. When they encountered parasitoids, the larvae wriggled vigorously as they retreated and often hung from silk threads after dropping from a plant.4. These two parasitoids differed in their responses to host defences. Diadegma semiclausum displayed a wide‐area search around feeding damage and waited near the silk thread for a suspended host to climb up to the leaf, then attacked it again. Cotesia plutellae displayed an area‐restricted search and usually pursued the host down the silk thread onto the ground.5. Diadegma semiclausum showed a relatively fixed behavioural pattern leading to oviposition but C. plutellae exhibited a more plastic behavioural pattern.6. The time spent by the two parasitoids on different plants increased with increasing host density, but the time spent either on all plants or a single plant by D. semiclausum was longer than that of C. plutellae. Diadegma semiclausum visited individual plants more frequently than C. plutellae before it left the patch, and stung hosts at more than twice the rate of C. plutellae.7. The results indicated that the host‐location strategies employed by D. semiclausum were adapted better to the host's defensive behaviour, and thus it was more effective at detecting and parasitising the host than was C. plutellae.

This study aimed to determine factors that influence the detection of DNA of Plutella xylostella L. (Lepidoptera: Plutellidae) in the gut contents of arthropod predators when the polymerase chain reaction is used to amplify a diagnostic fragment of the gene coding for cytochrome oxidase subunit I. The effects of temperature, time since feeding, subsequent food intake, sex, weight, and species of predator on prey detectability were studied in the laboratory. Three types of predator were studied: the spider Venator spenceri Hogg. (Araneae: Lycosidae), a bug with sucking mouthparts, Nabis kinbergii (Reuter) (Heteroptera: Nabidae), and a coccinellid with chewing mouthparts, Hippodamia variegata (Goeze) (Coleoptera: Coccinellidae). In all experiments, the detectability of prey DNA was negatively correlated with time post‐feeding. The duration of detectability differed among the predator species. The time calculated for median detection success at 20 °C ranged from 49.6 h in V. spenceri to 36.1 h in N. kinbergii and 17.1 h in H. variegata. In H. variegata, but not in V. spenceri, the rate of detection decreased with increasing temperature. Subsequent food intake did not affect the detectability of DNA of P. xylostella in V. spenceri. In H. variegata, sex and weight of the predator did not influence detection of prey DNA. In addition, this study uncovered potential sources of error caused by detection of prey DNA following secondary cannibalistic and intraguild predation. The results provide essential information for the interpretation of prey detection data from field‐collected predators’ gut contents.

Siekmann, G., Tenhumberg, B. and Keller, M. A. 2001. Feeding and survival in parasitic wasps: sugar concentration and timing matter. -Oikos 95: 425-430.Sugar consumption can increase the longevity and lifetime fecundity of many species of parasitic wasps. Consequently, for these insects the availability of sugar sources in the field is important for their reproductive success. As sugar sources can be highly variable in quantity, space and time, the chances of finding a sufficient amount of sugar to increase longevity might be very low. Therefore, the reward from a single feeding event can be critical for the forager's fitness. We measured the longevity of the parasitoid Cotesia rubecula after a single honey meal differing in sugar concentration (25, 47, 86% w/w) and timing (day of emergence and 24 h later). Survival was analysed with Cox's Proportional Hazards Model. The risk of starving to death in sugar-fed wasps was reduced by 0 -73% in comparison to unfed wasps, depending on sugar concentration and timing. Longevity was significantly increased by sugar concentration and by feeding later in life. Our results suggest that in the field, adult C. rubecula has to locate food at least once per day to avoid starvation. Many species of parasitic wasps can increase their reproduction through feeding on supplemental sugar sources (Leius 1961a, b, Syme 1975, Chippendale 1978, England and Evans 1997, Heimpel et al. 1997a. Sugar feeding usually results in an increase in longevity that is often positively correlated with a higher fecundity (Wolcott 1942, Leius 1961b, Sahragard et al. 1991, Hagley and Barber 1992, Morales-Ramos et al. 1996, Heimpel et al. 1997a. Successful sugar foraging is considered to be a critical factor that influences the lifetime reproduction of parasitic wasps in the field (Wäckers and Swaans 1993, Sirot and Bernstein 1996, England and Evans 1997, Jacob and Evans 1998, Heimpel et al. 1998), but little has been done on the estimation of realistic lifetimes of sugar-feeding wasps when sugar is not constantly and easily available. In many habitats foods and hosts of parasitic wasps are spatially separated or not available at the same time, resulting in rare food encounters. G. Siekmann, B. Tenhumberg and M. A. Keller, Dept of Applied andTaking into account the spatial and temporal variability of sugar sources in the field we address the specific question of how much a single sugar meal affects survival in adult Cotesia rubecula (Marshall) (Hymenoptera: Braconidae). The estimation of longevity and reproduction of parasitic wasps in the field should not rely on data obtained from cage experiments with ad libitum sugar conditions. Considering the different qualitative, spatial and temporal scales at which sugar sources occur in the field (Jervis et al. 1993), this would lead to a drastic overestimation of the lifetime reproduction of wasps. The quantity and quality of sugar sources in the field vary as they can be obtained from various sources such as floral and extrafloral nectar, other plant materials ...

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Predators are important biotic factors in the population dynamics of the diamondback moth, Plutella xylostella. A specific DNA marker was developed to detect P. xylostella in the gut contents of two polyphagous predators, Nabis kinbergii and Lycosa sp. A distinct 275-bp product was amplified by polymerase chain reaction (PCR) from the internal transcribed spacer (ITS-1) of the ribosomal gene of P. xylostella, but not from 11 other arthropod species collected from Brassica fields in South Australia. Fortuitously, the primer set could also amplify DNA products from two species and three varieties of Brassica plants, with the fragment size about 600 bp. When N. kinbergii was analysed after feeding a single fourth instar P. xylostella, 67% of individuals were positive with the 275-bp PCR product up to 16 h after feeding. Likewise, the PCR product was detected in 80% individuals of Lycosa sp. up to 72 h after feeding on a single fourth instar P. xylostella larva. Initial tests of samples collected from the field showed that the predation incidences for both N. kinbergii and Lycosa sp. determined by the 275-bp fragment corresponded to the density of P. xylostella in the field.