Egg parasitoids are known worldwide for being very efficient in combating a large number of agricultural and forestry pests. These insects, by parasitizing pest eggs, prevent their hosts from reaching the larval stage, in which they cause damage to crops. Parasitoids belonging to the genus Trichogramma (Hymenoptera) have a wide geographic distribution and play an important role as natural enemies of numerous lepidopteran pest species (butterflies and moths) in different agroecosystems. In recent decades, parasitoids of the genus Trichogramma They have been used as control agents to suppress populations of these pests in several countries and in the most varied crops, including vegetables.
In Brazil, there are records of the occurrence of 26 species of Trichogramma, being that Trichogramma pretiosum is the most widely distributed species, having already been reported in the states of Amazonas, Ceará, Espírito Santo, Goiás, Minas Gerais, Mato Grosso, Mato Grosso do Sul, Paraíba, Paraná, Pernambuco, Rio de Janeiro, Rio Grande do Sul, Santa Catarina, São Paulo and the Federal District. It is also the species that parasitizes the largest number of hosts, including naturally parasitizing lepidopteran pests that attack tomato crops, such as the tomato moth. Tuta absoluta, the small and large drills, Neoleucinodes elegantalis e Helicoverpa zea, respectively, the army caterpillar spodoptera sp., in addition to the recently introduced species in Brazil Helicoverpa armigera, considered limiting factors for the crop and causing great losses to producers.
However, one of the major obstacles to using this natural enemy to control tomato insect pests is the fact that large quantities of pesticides continue to be used to control pests and diseases in the crop. Due to the importance of species of Trichogramma as natural enemies of several insect pests, studies on the impact of agrochemicals on these organisms are of fundamental importance. These studies aim to generate information that assists in decision-making in Integrated Pest Management (IPM) programs, in the maintenance of these organisms in agroecosystems, in reducing environmental impact, as well as in reducing risks to human health.
The selectivity of agrochemicals to beneficial organisms allows the compatibility of chemical and biological control methods, notably in tomato cultivation, the target of a large number of applications of chemical products to control insect pests and diseases.
Selectivity studies
In several countries, selectivity tests have become mandatory, which requires the use of internationally approved methods, with the aim of standardizing techniques for selectivity studies, allowing comparison of the results obtained.
In Brazil, there is currently a Selectivity Research Group, accredited by the National Council for Scientific and Technological Development (CNPq), since 2011, to adapt to Brazilian conditions the methodologies already established and recommended by the “International Organization for Biological and Integrated Control of Noxious Animals and Plants (IOBC), West Palaearctic Regional Section (WPRS)" for selectivity studies, as well as electing standard beneficial organisms for selectivity studies in Brazil. Furthermore, it will also be up to this Research Group to disseminate the information obtained from selectivity tests through scientific articles in specialized journals, workshops, lectures and discussions between the academic and technology diffusion sectors.
Several studies on the selectivity of agrochemicals used in tomato cultivation to natural enemies have already been carried out in Brazil. The vast majority provide information about the effects of the compounds on the egg parasitoid T. pretiosum, an important tomato pest control agent (Table 1).
These studies showed that, in general, abamectin, acetamiprid, cartap, chlorpyrifos, deltamethrin, lambda-cyhalothrin and methoxyfenozide were the most toxic compounds to this parasitoid, having been classified as moderately harmful and/or harmful (classes 3 and 4, respectively ), according to the IOBC/WPRS toxicity scale. On the other hand, B. thuringiensis, chlorfluazuron, chlorothalonil, cyromazine, flubendiamide, iprodione, lufenuron, novaluron, tebufenozide, teflubenzuron, thiamethoxam and triflumuron proved to be harmless to T. pretiosum, being categorized in class 1.
It is important to highlight that, among the various agrochemicals evaluated in these studies, the active ingredient flubendiamide proved to be harmless (selective) at all stages of crop development. T. pretiosum, demonstrating the possibility of joint use of this agrochemical and the parasitoid, in the fight against an important pest recently introduced in Brazil, the H. armigera. This active ingredient belonging to the diamide chemical group has an innovative mode of action, causing interruption of muscle contraction, paralysis and death of treated insects. This pesticide is recommended for the control of caterpillars in cotton, corn, soybean and tomato crops, demonstrating great efficiency.
However, the use of this and other agrochemicals in pest control must be carried out in a way that does not allow the development of resistance by pests, including ensuring that these compounds maintain their efficiency in combating pests over the years.
Final considerations
It appears that several studies on the selectivity of agrochemicals used in tomato cultivation to the parasitoid T. pretiosum have already been developed in Brazil. However, the continuous synthesis and commercialization of new compounds with insecticidal properties, as well as the increasing demands of national and international consumer markets, make it necessary to frequently study the impact of these compounds on this organism, so that the biological and chemical control methods and minimize the use of agrochemicals in pest control.
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