To control whiteflies on soybean crops in an effective and economically viable way, it is necessary to quantify the occurrence and density of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) on the leaflets. Estimating the number of B. tabaci cm‐2 on leaflets is difficult, because its distribution pattern on the various parts of the plant canopy and on the leaflet surface is unknown. The aim of this study was to evaluate the distribution of B. tabaci nymphs on soybean plants and leaflets, under greenhouse and field conditions. One hundred soybean plants infested with all nymph stages were randomly selected in a greenhouse, and 25 in a field. Of each plant, a trifoliate leaf of the middle third of the plant’s height was selected and its central leaflet was collected (greenhouse experiment), or a trifoliate leaf of each third layer (upper, middle, and lower), of which the left, central, and right leaflets were collected (field experiment). The collected leaflets were divided into 32 sections (1 cm2 per section), arranged in an array of eight rows and four columns to count whitefly nymphs. The Morisita index (Iδ), the negative binomial parameter k, and the dispersion index (I) were calculated for each leaflet, using the number of nymphs as variable. The highest population densities of whitefly nymphs were found in the middle third of the soybean plants. In leaflets from the middle third, the nymphs concentrated in the middle and bottom parts of the leaflets, whereas in the upper and lower thirds of the plant, they were randomly distributed on the leaflets.

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The whitefly Bemisia tabaci (Gennadius, 1889) is a major pest species in soybean, leading to severe economic losses on this crop due to the difficulties involved on its management. Previously restricted to the Middle-west and Southeast regions of Brazil, whitefly infestations have steadily increased in the Southern state of Rio Grande do Sul, the third biggest soybean growing region of the country. Control failures and scarcity of updated information have led Brazilian soybean growers to raise excessively the number of sprays per crop season, increasing control costs and jeopardizing the long-term sustainbility of this strategy due to selection of resistant strains and potential harmful effects on the environment. The aim of this work was to evaluate the performance of different chemical insecticides in the control of B. tabaci nymphs and adults on soybean crops in two different sites, under the field conditions faced by the growers in the state of Rio Grande do Sul. The most efficient treatment for the control of B. tabaci adults was cyantraniliprole + lambda-cyhalothrin, at the doses of 100 + 7.5 g a.i. ha-1, which provided 65% of average control efficiency. As for nymph control, the most efficient treatment was acetamiprid + pyriproxyfen, at the doses of 60 + 30 g a.i. ha-1, which resulted in 67% of whitefly control in average. Two sequential sprays beginning at the infestation onset are recommended in order to enhance control efficiency.

Stink bugs are a major concern for pest management in soybean crops. With agricultural frontiers expanding in Brazil and cultivation techniques being heavily intensified, stink bug populations have become increasingly dispersed and hard to control, causing severe economic losses to soybean growers across the country. Chemical insecticides known as neonicotinoids, organophosphates and pyrethroids currently represent the main control strategy for this pest, being often mixed together in order to enhance control efficacy and prevent resistance development. Each of these chemical groups is characterized by a different mode of action inside the insect’s body, which determines if the insecticide will provide a fast knockdown effect or a long residual control effect. The aim of this work was to evaluate the knockdown and residual control effects delivered by these groups of insecticides under field conditions and during two cropping seasons, both in isolated and combined use, determining the most efficient strategy for chemical management of stink bugs on soybean crops. The pyrethroid lambda-cyhalothrin (250 g L-1) had the best knockdown effect, while the neonicotinoid imidacloprid (700 g kg-1) provided the longest residual control. The highest control efficacy was obtained with the combination of lambda-cyhalothrin + thiamethoxam (106 + 141 g L-1), which resulted in 84.8% of stink bug control.