Variation in food availability influences prey‐capture method in antlion larvae

Abstract 1. Larvae of a Myrmecaelurus sp. are unique among antlions because they have two prey‐capture methods; they either ambush prey at the surface, or dig pit traps that prey fall in to. It was hypothesised that larvae will use the capture method that maximises their net rate of energy gain, which will be influenced by food availability (encounter rate) and by past energy inputs (body condition). 2. Costs were estimated by measuring resting and activity metabolic rates and determining the duration of pit maintenance at various encounter rates with ants that served as prey. Benefits were estimated from the energy gained per ant captured at different encounter rates. 3. Net energy gained was higher with a pit than without one, and was influenced more by the differences in prey capture rate between the two capture methods, and less by the differences in energy costs associated with each method. The proportion of larvae that constructed pits was higher when they were in intermediate body condition than when in good or in poor body condition. 4. Thus, the use of one capture method or the other depends on a combination of the influences of past net energy gain and the antlion’s most recent change in encounter rate with prey. Ambushing without a pit may serve as a default when physiological constraints limit the larvae’s ability to invest in pit construction and maintenance, or when larvae are sated, and saving the energy of pit construction and maintenance is worthwhile.     

Migration patterns and functional groups of spiders in a desert agroecosystem

Abstract 1. Arthropods living in annual crops suffer mortality caused by agricultural practices. Therefore, migration from surrounding habitats is crucial to maintain populations of natural enemies of insect pests in crops. In desert agroecosystems there is a pronounced contrast between managed and unmanaged habitats, where irrigated and fertilised crops are islands of productivity in an arid matrix. This contrast could either enhance or inhibit movement of natural enemies between the landscape components. 2. The importance of the surrounding arid habitats as a source for spiders in crops was examined in the Negev desert of Israel. Spiders were sampled in both arid natural habitat and adjacent wheat fields using pitfall traps and visual searching. In addition, spiders in wheat fields were sampled throughout the winter cropping season using emergence traps at increasing distances from the field edge. Stationary and movable emergence traps were used to distinguish between residents and migrant species. 3. The spider assemblage in the wheat was dominated by three families: Linyphiidae, Theridiidae, and Gnaphosidae. Spider sampling in both natural arid habitat and adjacent wheat fields enabled four functional groups to be recognised that differed in habitat preference, movement patterns, and population dynamics. Thirty‐three per cent of collected individuals were classified as crop residents whereas more than 50% were classified as migrants from the surrounding habitats. These findings suggest that the surrounding habitats influence spider assemblage composition in the fields, in spite of the marked contrast in habitat structure and productivity. 4. Spider assemblages in the wheat fields were dominated by migrant species arriving from the surrounding arid habitats. Migrant spiders inhabited the crop throughout the cropping season. The combined contribution of resident and migrant functional groups may act to prevent insect pest outbreaks in this desert agroecosystem.