Distribuição na planta, fatores climáticos e parasitismo na dinâmica populacional dos pulgões (Hemiptera; Aphididae) em couve

Abstract

The aphids Brevicoryne brassicae (Linné), Lipaphis pseudobrassicae (Davis) and Myzus persicae (Sulzer) are cosmopolitan pests that cause significant damage in the Brassicaceae family plants. The determination of which biotic and abiotic factors are most important in regulating pests is very important in Integrated Pest Management programs. Thus, this study evaluated the influence of leaf position, rainfall, temperature, parasitism and hyperparasitism in populations of L. pseudobrassicae, M. persicae and B. brassicae in kale, and analyzed which species is the most abundant. The experiment was conducted in field conditions, in Uberlândia MG, from August 2005 to March 2006 and October 2006 to January 2008. The quantification of aphids was done weekly, removing three leaves of each plant part (top, middle and bottom), totaling 101 samples. The leaves were taken to the laboratory for counting live and mummified aphids. Mummified aphids were kept in eppendorf tubes for 30 days and the identification of emerged parasitoids and hyperparasitoids was done. Data analysis was performed by three linear models relating the abundance of aphids and environmental factors. The most abundant species that could be considered the main aphid pests of kale in this region are B. brassicae and L. pseudobrassicae. Furthermore, similar number of B. brassicae was found in the apical portion of kale and it the other portions, differently from the two other species, which were found in greater number in the middle and bottom portions. The combined effect of increased temperature and rainfall had a negative effect on B. brassicae population. As B. brassicae was the only species to exploit intensively the apical plant part, it is concluded that its population was reduced by the mechanical effects of rainfall. Diaeretiella rapae (M Intosh) was the only emerged species of parasitoid on the family Braconidae and Alloxysta fuscicornis (Hartig) was the main hyperparasitoid species. The population of the three aphid species was influenced by the number of emerged hyperparasitoids, showing that this is the key factor regulating the aphids populations. Parasitism was affected only by the population of B. brassicae. This fact can be explained by its greater abundance, small population of M. persicae and low parasitism in L. pseudobrassicae. The relationship of parasitism in the three species with temperature and rainfall was similar to that found in the population of B. brassicae and climatic factors. This indicates that parasitism in the three species appears to have been affected by the most parasitized host, in this case, B. brassicae. Parasitism was low in L. pseudobrassicae (0.8% on average), intermediate in M. persicae (8.5% on average) and high in B. brassicae (16% on average). The low parasitism in L. pseudobrassicae could be a key factor for its pest status of crucifers in the survey region