Fatores envolvidos na escolha do hospedeiro pelo caruncho do feijão: Zabrotes subfasciatus (Coleoptera: Chrysomelidae: Bruchinae)

Abstract

The oviposition behavior, common in endophytic species, determines the diet of several insects. As in the case of the weevil Zabrotes subfasciatus (Coleoptera: Chrisomelidae: Bruchinae), member of the subfamily Bruchinae, considered one of the biggest pests of beans stored in warehouses, as it lays its eggs and feeds on several species of Leguminosae. Recent work shows that there is a hierarchy in the choice of grain that will serve as food for their descendants, being: beans > soy > lentils ≥ chickpeas. However, the factors that determine this choice are still unknown, even though they are of great importance for agriculture and the economy. Several characteristics of the seeds may be responsible for determining this choice. However, the determining factor must be perceived by the female before ovipositing. The size and texture of the seed does not seem to be decisive. In this work we test the hypothesis that volatile compounds emanating from seeds are the determinants of host choice by females of Z. subfasciatus. For that, we aim (a) identify the volatile compounds that are emanated by different grains (beans, soy, lentils and chickpeas) and recognized by the females during the round period, prior to oviposition; (b) and to analyze the participation of the antennae in the recognition of these substances through the comparison between females and males. Initially, females of Z. subfasciatus up to 24 h of age were placed in an arena of choice for multiple simultaneous tests. In 4 peripheral petri dishes, one type of grain (lentil, soy, beans or chickpeas) was placed per plate; the fifth plate was empty (control). Therefore, more specific tests were performed, in a multiple paired arena. However, in this case, two peripheral petri dishes received the same type of grain for grains vs. control and grains vs. grains disputes. The observed hierarchy was: beans ≥ soybeans> chickpeas> lentils. Subsequently, as they have to choose between: tegument, reserve and meristematic tissues, whole grain, humidified whole grain and a seedless control, for the 4 grains form, in order to identify which part of the grain has compounds recognized by the females. No difference was observed. To identify the volatile compounds, present in beans and chickpeas, the grains were submitted to aeration testing, followed by gas chromatography analysis coupled with mass spectrometry. Were found 10 substances in beans and 17 in chickpeas. Ethylene benzene (styrene) stood out as a very abundant compound in beans. the compounds contribution in the choice of the grain has yet to be investigated. In the analysis to assess the participation of antennas on the choice of grains, females and males were divided into four distinct groups: (1) weevils without the right antenna, (2) weevils without the left antenna, (3) weevils without both antennas and (4) control (weevils with both antennas) and placed in a multiple paired arena. The presence of at least 1 antenna, both in females and in males, was determining for the weevils to be directed towards the plate containing some type of grain. The same was not observed in Z. subfasciatus which, without two antennas, were unable to identify the location of the grains. However, only females showed a preference for beans over chickpeas. Males, on the other hand, demonstrated greater sensitivity to the presence of grains when using the right antenna when compared to the left antenna. Analyzes of the antennae of females and males, by Scanning Electron Microscopy (SEM), show sensory elements potentially involved in the capture of olfactory information, and the existence of sexual dimorphism. Females have a large amount of basiconic sensilae, know respond to plant volatiles. Males have more trichoidal sensillae, with mechano- and chemoreceptor function, which normally respond to pheromones. Together, these results demonstrate a female preference for beans; that volatile compounds are present throughout the seed; that chemical composition presented by the grains has styrene highlighted in bean seeds and hexanal together with octane highlighted in chickpea seeds; that the antennae are fundamental for the recognition of these compounds by the insects, but only the females show preference among the different grains; and that the antennae present different sensory elements in males and females of the species, and physiological lateralization may occur in the antennae of the males. Ongoing experiments should reveal the scope of participation of the compounds identified in the seeds as infochemicals involved in the selection of hosts for oviposition and development in Z. subfasciatus.