Potential of Mealybugs Infestation, Planococcus spp. (Hemiptera: Pseudococcidae), in an Agroforestry System in Coffee Crops

The use of arboreal species with economic and environmental values can add value to the coffee activity. However, this association requires a detailed knowledge since trees can be a source and refuge of pests and/or the modified environmental conditions, as shading, can affect the incidence of phytophagous arthropods. On the other hand, a diverse agrosystem may have positive impacts where the natural enemies can find refuge, additional food as nectar and pollen, and extra preys, increasing the natural control of pests on coffee (Venzon et al., 2014; Tomazella, 2016).

Among these insects, the mealybugs are considered key pests, especially the citrus mealybug, Planococcus citri (Risso), and the pacificus mealybug, Planococcus minor (Maskell), (Pseudococcidae), which constitute a threat to the coffee plants since they attack flower buds and fruits causing heavy fruit drop (Santa-Cecilia & Souza, 2014). In spite of the diversity of plants that colonize (Williams & Granara de Willink, 1992), these mealybugs may showa certain preference for a host or to have their development and reproduction favored in certain plants due to their nutritional quality.

The insect host selection and recognition process includes several steps, such as habitat and host location, host acceptance and feeding and/or breeding (Le Rü et al., 1995b). For this, olfactory, visual, gustatory and tactile stimuli are used, as well as the humidity and intensity of the environment light (Heard, 2000; Powell et al., 2006).

Several species of scale insets exploit a limited number of plants, however, they may occasionally occur in other hosts even being not suitable for their development. Mealybugs of the genus Planococcus, Phenacoccus and other scale insects have sensilla in the antenna with contact and olfactory functions (Salama, 1971; Koteja, 1980; Le Rü et al., 1995b; Calatayud & Le Rü, 2006) and it has been hypothesized that they use these structures for the host selection. However, the efficiency to use these sensory organs is limited due to the fact they are apterous (females and males in the first instars) with reduced mobility. Some authors even consider olfactory stimuli of limited value for host location by insects of the order Hemiptera and, such stimuli, would act only at short distances (Backus, 1988). The infestation of new plants would be mainly a passive process, circumscribed to neighboring plants, dispersion by wind, tools used by men or the use of infested plants coming from nurseries. This would result in localized infestations. In contrast to olfactory stimuli as a mechanism to locate a host, other stimuli may exert some action in the selection (or rejection) of a plant (Le Rü et al., 1995b). Olfactory and contact chemoreceptors are present at the apical end of the labium of mealybugs and can be used in the host selection by detecting the stimuli at the leaf surface (Le Rü et al., 1995b; Calatayud & Le Rü, 2006). These stimuli may be more important than the olfactory ones present in the antenna. Contact chemoreceptors seem to be used by sucking insects as already verified for aphids. However, it is still under discussion the true role played by volatiles as stimuli for host location in homopteran insects (Powell et al., 2006). The host selection by tasting the plant contents through the gustative sensilla present in the cibarium cavity of the alimentary canal is determinant in other sucking insects (Powell et al., 2006) and we expect to be similar in mealybugs. Unfortunately to our knowledge there is not studies regarding the presence of gustatory sensilla in the cibarium of mealybugs but we can assume to be similar to other sucking insects.

All these factors may influence the host selection process, however, food preference and plant quality, reflected in the nutritional value, will finally influence the reproduction and the capacity to host the insect.

Thus, the objective of this work was to determine if tree species used in association with coffee plants are potential hosts of two species of mealybugs and by consequence representing a threat to the crop. These studies will ensure abetter understanding of the interaction of mealybugs and arboreal species in shaded coffee plantations. The following hypotheses were tested: (a) P. minor and P. citri mealybugs exhibit dietary preference for coffee plants and have, in this host, better conditions for their development and reproduction; (b) both mealybugs can reproduce in the tested tree species; (c) both mealybugs show olfactory preference for certain plants.