The Impact of Pesticides - Academy Publish

Selectivity of pesticides to beneficial insectsAmong the beneficial insects of agronomic importance in the soybean crop, thenatural biological control agents are noteworthy for their contribution in maintainingthe pest populations below the economic threshold levels. However, although manyexamples of successful biological control indeed exist, the chemical control is stillessential to ensure good yield. The pesticides such as insecticides, fungicides,herbicides, and acaricides, represent important tools for crop management and playa significant role in the success of agricultural production. The most suitableproducts for use in the Soybean-IPM, however, are those that combine a goodcontrol of target pests with a minimal impact on the activity of natural enemies(beneficial arthropods). The integration of chemical products with biologicalcontrol, in most cases, is crucial for the success of agriculture and reduction ofnegative impacts of their use. Hence, the selectivity of the insecticides to naturalenemies is of great importance and should always be considered when choosing thebest product to be used.It is important to emphasize that in stressing the importance of the use of selectivepesticides in agriculture one should not considered only the insecticides andacaricides, but also the herbicides, fungicides, plant growth regulators, foliarfertilizers and other chemicals that might be applied over the top of plant canopy.These other products are most often neglected in this matter of selectivity tobiological control agents and may also have negative effects on beneficialarthropods.It is important to point out that the ways in which an agrochemical may or may notbe selective, is not a unique one and all of them must be considered whenclassifying a product as selective or harmful to a specific natural enemy. Theselectivity, in general, may be physiological and/or ecological; and both arecomplementary and very important for the balance of the agro-ecosystem.Physiological selectivityThe physiological selectivity is inherent to the chemical compound and is related tothe tolerance of a natural enemy when it is subjected to direct contact with thatspecific product, with its residues, or by its ingestion. This type of selectivity is dueto physiological differences among the species involved regardless if they are pests,predators, parasitoids, pathogens or pollinators, and can be determined by theconcentration of a product that results in good control of the pest, without affectingthe beneficial arthropods. This selectivity is, for example, what occurs with somephosphatic insecticides when there is a reduction in the penetration of the tegument,or an increase in the degradation of toxic molecule through the enzymatic system ofa beneficial insect.In this context, it is important to emphasize that there are different intensities ofselectivity, which means that the pesticides are not only classified as selective orharmful. The degree of physiological selectivity of a product is usually expressed bythe ratio of the median lethal dose (LD 50 ) to the pest and the natural enemy or therelationship between the dosages recommended for the control of the pest and theLD 50 to the natural enemy. Therefore, if a curve of mortality of the pest and of thenatural enemy is drawn for a given product, the dose to obtain the best selectivitycan be defined as the point on the curve where the difference between the mortalityof the pest and of the natural enemy is the maximum.It is often difficult to achieve the optimal level of physiological selectivity. Many ofthe insecticides act on the central nervous system and there is a great similarity inthe transmission of nerve impulses, not only among the different insect ordersAcademyPublish.org - The Impact of Pesticides177