Toxicity of insecticides to the sweetpotato whitefly (Hemiptera: Aleyrodidae) and its natural enemies

Efficient chemical control is achieved when insecticides are active against insect pests and safe to natural enemies. In this study, the toxicity of 17 insecticides to the sweetpotato whitefly, Bemisia tabaci (Gennadius), and the selectivity of seven insecticides to natural enemies of this insect pest were evaluated. To determine the insecticide toxicity, B. tabaci adults were exposed to abamectin, acephate, acetamiprid, cartap, imidacloprid, malathion, methamidophos, bifenthrin, cypermethrin, deltamethrin, esfenvalerate, fenitrothion, fenpropathrin, fenthion, phenthoate, permethrin and trichlorphon at 50 and 100% of the field rate (FR), and to water (untreated control). To determine the insecticide selectivity, adults of Encarsia sp., Acanthinus sp., Discodon sp. and Lasiochilus sp. were exposed to abamectin, acephate, acetamiprid, cartap, imidacloprid, malathion and methamidophos at 50 and 100% FR, and to water. Groups of each insect species were exposed to kale leaves preimmersed in each treatment under laboratory conditions. Mortality of exposed individuals was recorded 24 h after treatment. Cartap and imidacloprid at 50 and 100% FR and abamectin and acetamiprid at 100% FR showed insecticidal activity to B. tabaci adults. Abamectin at 50 and 100% FR was the least insecticidal compound to the natural enemies Acanthinus sp., Discodon sp. and Lasiochilus sp. The present results suggest that abamectin at 100% FR may decrease B. tabaci field populations but can still be harmless to predators. Implications of these results within an integrated pest management context are discussed.     

Plant extracts from the Mediterranean zone of Chile potentially affect soil microbial activity related to N transformations: A laboratory experiment

Abstract

Available soil N can be lost through ammonia volatilization, nitrate leaching to surface and ground waters, and nitrous oxide emission under intensive systems of agricultural production. Research has shown that phenolic compounds isolated from certain plants influence N cycling in soils, which results in a decrease of N losses and represents a key issue for environmental protection worldwide. In this study, water-soluble and ethanol-soluble compounds were extracted from the leaves, bark and roots of certain tree and shrub species in the Mediterranean zone of Chile, with the aim of measuring their effects on N transformation, soil respiration, soil microbial biomass and urease activity. The studied species were Acacia caven Mol, Quillaja saponaria Mol., Bacharis linearis (Ruiz & Pav.) Pers. and Pinus radiata D. Don. Plant extracts were applied to soil (Haploxeralf) fertilized with urea. The extracts obtained from the roots of A. caven in water, bark of A. caven in ethanol, bark of P. radiata in ethanol, leaves of P. radiata in water and roots of B. linearis in water showed a reduction in the soil nitrification rate. The bark of A. caven in ethanol and bark of P. radiata in ethanol extracts showed a reduction in urease activity. In conclusion, only the bark of P. radiata in ethanol extract affected all the biological parameters considered, reducing soil nitrification, urease activity, microbial biomass and carbon dioxide emissions from the soil, these being attributed to high levels of phenolic content in the extract. However, further research is necessary to explain the mechanisms involved.     

Interaction of biological nitrogen fixation with sowing nitrogen fertilization on common bean in the two seasons of cultivation in Brazil

Sowing nitrogen (N) fertilization can limit biological nitrogen fixation (BNF) reducing common bean yield. The aim of this study was to evaluate the effect of sowing N fertilization plus inoculation on the growth and yield of plants in the two seasons of cultivation in Brazil. In the dry season, N fertilization and inoculation promoted a greater shoot dry weight and higher pod number and yield than only inoculated. In contrast, in the rainy season, this treatment promoted no increase in shoot dry weight and yield compared with the inoculated alone. The number of nodules was greater for the inoculated alone treatment, but nodule weight was not affected by N fertilization in either season. Therefore, sowing N fertilization and inoculation can be an agronomic practice to achieve a higher common bean yield in the dry season, while in the rainy season, the inoculation without N fertilization can support a high yield at a lower cost.