The arthropod fauna of a winter wheat field.
Abstract
On Rothamsted farm in southern England, the predatory and parasitic arthropod population of part of a winter wheat field alternating with fallow, during the years 1967-74, depended for its food on the plant-feeding species, especially aphids, cecidomyiids and stem-, leaf- and shoot-borers, which invaded the growing crop in early or late spring. Litter formed on the soil surface from dead leaves and provided food and shelter for the saprophagous larvae of small flies such as Lonchoptera lutea Mg. and Scaptomyza pallida (Zetl.) (Parascaptomyza pallida). Larvae, pupae and all aphid morphs were hosts for hymenopterous parasites, which in turn may have been secondarily parasitised. Unprotected plant feeders attracted predators. The catch in the emergence cage, therefore, consisted of the successful insects and spiders.The numbers of the different species caught, varied each year, and the index of diversity alpha was large (110.6, 63.3, 84, 85.9) in 1967, 1969, 1970 and 1971 but dropped to 39.8, 50.9 and 44.9 in 1972, 1973 and 1974 when fewer insects were caught. In 1968, the year when many aphids were caught, alpha was 27.5. Of the weather factors affecting the final emergence of the two small species of Diptera L. lutea and S. pallida there was a negative correlation with sunshine in June. With two-factor analysis of weather conditions in June and emergence of L. lutea, there was a correlation at the 5% level, negatively with June sunshine and warm days and positively with rainfall and days with rain, showing the importance of moist conditions for larval development.Although more arthropods emerged from the crop than from the soil, some insects such as Megaselia spp., Tachydromia arrogans (L.), Anapausis sp. and some Platypalpus spp. were commoner away from plant cover. Many cecidomyiids also developed in organic matter in the soil. The commonest surface moving carabids, Harpalus rufipes (Deg.), Pterostichus melanarius (Ill.) and Agonum dorsale (Pontoppidan), were not equally active every year. Other species such as Nebria brevicollis (F.) and Loricera pilicornis (F.) were absent some years, L. pilicornis in 1971 and Nebria brevicollis in 1971 and 1973. Bembidion lampros (Hbst.) was uncommon in July.From May to July in 1973 and 1974 the two commonest cereal aphids on the winter wheat plants were Macrosiphum avenae (F.) (Sitobion avenae) and Acyrthosiphon dirhodum (Wlk.) (Metopolophium dirhodum), but numbers dropped in July. Parasitism by hymenopterous parasites reached 100% in the centre of the crop in both 1973 and 1974. The winter wheat crop formed an ecosystem, varying in composition each year throughout the eight-year period.