This study deals with the description, in field conditions, of the oviposition preferences and mortality at the egg stage of the proposed biocontrol agent Anacassis fuscata on its host plant, Baccharidastrum triplinervium, in Southern Uruguay. Egg clusters were classified according to their location on the adaxial or abaxial surface of annual or seasonal leaves. The main tendencies were a very high preference for the oviposition on annual compared with seasonal leaves, and a lack of preference for the adaxial or the abaxial surfaces of the leaf. We propose that this behavior has evolved to create a niche separation between early stage larvae and conspecific adults. Temporal variations in the ovipositing preferences were observed throughout the 2-mo oviposition period, which can be related to changes in the phenology of the host plant. Egg mortality was high in both years studied, and showed variability. The highest incidence was found on the adaxial surface of seasonal leaves, a finding that may be related with an observed low preference for the oviposition on this site. Egg size was smaller in Uruguay than in Argentina and Brazil, which might be explained by the fact that in Uruguay the average number of eggs per cluster is higher. Based on our observations that A. fuscata is a latent species, and that intensive defoliation by larvae does not affect the viability of the host plant, we conclude that even when A. fuscata has features that make it a good candidate for the control of Baccharis and Baccharidastrum, it is not an effective agent for biocontrol under field conditions.
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