Predation activity of two winter-active spiders (Araneae: Anyphaenidae, Philodromidae).

Anyphaena accentuata and Philodromus spp. are cold adapted and winter-active spider species. Their predation activity was investigated at constant temperatures between -4 and 30°C. The lower temperature threshold for Anyphaena was -3.7°C, while that of Philodromus was -1.2°C. At 1°C the latency to capture and prey consumption was significantly shorter in Anyphaena than in Philodromus. The capture rate increased with temperature and was maximal at 15°C in Anyphaena and at 30°C in Philodromus. At 30°C, the latency to the capture was significantly shorter in Philodromus than in Anyphaena whose mortality significantly increased.

Comparing growth patterns among field populations of cereal aphids reveals factors limiting their maximum abundance.

Cereal stands in central Europe are commonly infested with three species of aphids that may become serious pests. With increasing abundance, the proportion of a particular species in the total aphid population may remain constant, suggesting a density-independent exponential growth, or the proportion can change, suggesting density-dependent constraints on growth. The constraints that affect particular species, and thus their relative abundance, were studied. The proportionality between maximum abundances of the cereal aphids was studied using a 10-year census of the numbers of aphids infesting 268 winter wheat plots. For two species their abundance on leaves and ears was compared. With increasing aphid density the maximum abundance of Rhopalosiphum padi (Linnaeus) remained proportional, but not that of Sitobion avenae (Fabricius), which was constrained by the smaller surface area of ears compared to leaves. There was no evidence of inter-specific competition. Maximum abundance of R. padi and Metopolophium dirhodum (Walker) on leaves did not change proportionally as the proportion of M. dirhodum decreased with increasing overall aphid density. This decrease was probably caused by the restricted distribution of M. dirhodum, which is confined to leaves, where space is limiting. No change in proportion between populations was detected when the average densities were below 0.54 aphids per leaf or ear. Non-proportional relationships between aphid populations appeared to be due to spatial constraints, acting upon the more abundant population. Detecting the limitation of population growth can help with the assessment of when density-independent exponential growth is limited by density-dependent factors. This information may help in the development of models of cereal aphid population dynamics.

Host plant age and population development of a cereal aphid, Metopolophium dirhodum (Hemiptera: Aphididae).

The sudden decline following the peak in population abundance of aphids on crops of small grain cereals is attributed to the joint effect of natural enemies and plant senescence. To distinguish between these causes, a four year experiment was established in which the numbers of Metopolophium dirhodum (Walker) infesting spring wheat plots sown from April to June at c. 14 day intervals were determined. Aphid abundance in replicates sown at successive dates peaked within a period of 5-9 days (106-171 day degrees above a base temperature of 0 degrees C) although their sowing dates varied by 62-97 days (727-1106 day degrees). At the time of the aphid population peaks, plants in the different sowings differed in age (11-99 days), developmental stage (stage 15-65 on the Zadoks scale), leaf nitrogen content and shoot mass. Maximum abundance of M. dirhodum decreased with sowing date because the time available for its population increase was shorter on late than early sowings. The abundance of M. dirhodum on spring wheat was similar to its abundance on winter wheat. After reaching peak abundance, aphids declined in numbers within 3-7 days. The effect of host plant ageing on the M. dirhodumdecline thus appeared small. Natural enemies (largely mycoses), and timing of alata production may have contributed to the aphid decline.

Thermal requirements for development of aphidophagous Coccinellidae (Coleoptera), Chrysopidae, Hemerobiidae (Neuroptera), and Syrphidae (Diptera): some general trends.

The sum of effective temperatures (SET) and lower development threshold (LDT) were established for eggs and/or pupae of central European populations of 20 species of chrysopid, coccinellid, hemerobiid, and syrphid predators of aphids. LDT ranged between 5.6° and 12.2°C, SET between 38.3 and 140.9 day degrees (dd), with broad overlap among stages and taxa. When LDT was plotted against SET, the data for both eggs and pupae were scattered along a single regression line which predicted a 0.47°C decrease in LDT per 10 dd increase in SET (r=-0.77, p<0.001). A regression calculated from published data from all over the world predicted a 0.24°C/10 dd decrease in LDT, and the data were more scattered (r=-0.38, p<0.01). This is perhaps the first report on the functional relationship between LDT and SET at the interspecific level. The species and stages differed in typical development length (VDL) and in the extent of its deceleration by low temperatures (DD). DD increased with increasing VDL, but the relative effect of low temperature on development length (DD/VDL ratio) reflected thermal adaptations consistent with the life history of the species. Polyvoltine species were less affected by low temperatures than monovoltine species, particularly the thermophilic ones.

Behavioral regulation of developmental cycle in Pyrrhocoris apterus L. (Heteroptera: Pyrrhocoridae).

During the warm season the larvae of Pyrrhocoris apterus select in their natural habitats places with temperatures which do not exceed 24°C on average even at the hours of maximum sunshine. At the same time other places of their habitats have temperatures up to 50°C. This active behavioral selection of the microclimate regulates the speed of P. apterus development and entertains the monovoltine cycle in climatic, conditions of Central Europe. A less rigorous choice of the microclimate, even when higher temperatures were tolerated and the development was not disturbed by them, would cause the disturbance of the monovoltine cycle and would probably endanger the survival of the population.

Action of photoperiod at cold in diapausing Chrysopa carnea females (Chrysopidae, Neuroptera).

To reveal the effect of photoperiod on insects under low temperatures, dormant females of Chrysopa carnea were exposed for 3 months (November-January) to cold (7°±2.5°C) and to long- or short-day conditions (18L:6D or 12L:12D). They were thereafter activated under short-day conditions at 25°C.The samples differed considerably in the incidence of ovipositing females, duration of pre- and post-oviposition period, and fecundity. All parameters evidenced a considerably higher reproductive activity of insects chilled under short-day conditions. It was thus confirmed that adults of C. carnea perceive photoperiod at cold. The action of short photoperiod at cold does not impair the subsequent oviposition; by contrast it promotes future reproduction, apparently also due to photoperiod changes. Although short-day photoperiod inhibits the activation at warm temperatures, it enables diapause development under cold temperatures.