Reasons for the number of insects decrease of the Samara Region (on the example of leaf beetle populations)

Abstract

The survival mechanisms of leaf beetles (as typical phytophagous insects, surrounded by a corrosive environment: unstable, rapidly changing climate, decreasing forage, direct action of predators, ecto- and endoparasites, parasitoids, pathogens helminthiasis and entomophtorous, other pathogenic organisms) were studied during 1974-2016 in the Samara Region. 25 background species of chrysomelid beetles (approximately 240 individuals) were used as model objects. In addition to the natural fluctuations of the insect population, 21 causes of sudden increase in the number of local populations of leaf beetles were revealed, as a result of the influence of negative environmental agents, including abiotic (8), biotic (12) and anthropic (1 reason). From 5% to 90% of the leaf beetles population die as a result of unfavorable wintering conditions, sharp fluctuations in temperature and humidity, wind and precipitation; 2 reasons (concerning the resources of the forage base), 2 reasons (caused by the imbalance of the age and sexual structure of the population and therefore low reproductive potential), 2 reasons (determined by the fierce competition of insects and larval cannibalism) and 6 reasons (associated with the influence of predators, parasites and pathogens) are able to withdraw from 2% to 96% of the livestock (eggs, larvae, pupae and adults) of the population. It is concluded that many agents affects the number of insects.