Dichelops melacanthus (Dallas, 1851) (Heteroptera: Pentatomidae) no sistema plantio direto no sul de Mato Grosso do Sul: flutuação populacional, hospedeiros e parasitismo

Abstract

This work was done in order to verify population fluctuation, alternative hosts, and the natural occurrence of parasitoids in eggs and adults of the green belly stink bug, Dichelops melacanthus (Dallas, 1851), in No-Tillage System in the South Region of Mato Grosso de Sul State. Weekly samples were taken - from September 2005 to December 2006 – on soybean, second season corn crop, on wheat and on volunteer corn plants, in the middle of straws and on weeds. All the field observations were done in areas of Ponta Porã, Itaporã and Dourados, in Mato Grosso do Sul State, Brazil, where were also collected forty adults and their egg masses to observe parasitism in the lab. The insect was detected at the three experiments throughout the year, but peaks above 1.0 stink bug/m2 were observed more frequently in Ponta Porã, from February to December 2006. In Ponta Porã was also registered, soon after the soybean harvesting - in the middle of the remaining straw - in the presence of volunteer corn plants, the highest of all the peaks 4.1 (±1,45) insects/m2 in April 2006. In Itaporã, the population density reached the magnitude of 1.0 (±0,38) D. melacanthus/m2, in November 2005, also on volunteer corn plants. Approached of the same level in December 2006, at the initial vegetative growth stage of the soybean in the presence of volunteer corn plants and reached the uppermost level 1.5 (±0,53) individual/m2 in September and 1.5 individual/m2 (±0,26) in November 2006, both on volunteer corn plants. As for Dourados, more than 1.0 (±0,31) stink bug/m2 was only registered in September 2006, on volunteer corn plants; approached of the same level in October and November of the same year, on volunteer corn plants, and on soybean at early vegetative growth stage associated with volunteer corn plants. By and large, the highest population peaks occurred near the soybean harvesting; in the middle of the remaining straw - soon after the soybean harvesting – mostly in the presence of volunteer corn plants; at the initial growth stages of the second season corn crop and of the volunteer corn plants. The same occurred during the entire permanence of the wheat crop in the field, mostly at its initial development stages. The contrary happened during the soybean vegetative and early reproductive growth stages, except when they were in the presence of volunteer corn plants. It was also observed that when the second season corn crop and the volunteer corn plants have developed, the population became lower. Parasitism exerted in adults by three different species of Tachinidae occurred in almost all months of the year. Individuals collected in Itaporã, in March 2006, did not exhibit parasitism. However, 53% of the insects collected in November were parasitized. The highest level of parasitism exerted in the stink bug D. melacanthus eggs - by the Scelionidae Telenomus podisi Ashmead (1893) – were 38,4 31,5 and 45,0%, in Ponta Porã; 42,3%, in Itaporã and 46,1%, in Dourados. Among the weeds, the four favorite alternative hosts are Cenchrus echinatus L., Chloris gayana Kunth, Panicum maximum Jacq. and Commelina benghalensis L.