Oviposition distribution of the boll weevil, Anthonomus grandis (Coleoptera: Curculionidae), in cotton fields under different infestation pressure

NUSSENBAUM AL; FOGAR; SIMONELLA; BONACIC KRESIC; DEVESCOVI; RUSSO; LECUONA,; SALERNO; DF SEGURA

Congreso; Congreso Brasilero de Algodon; 2015

The choice of an oviposition site has a direct impact on the reproductive success of insects. This is particularly important for species that developed in confined patches for which immature stages cannot escape. Females of Anthonomus grandis prefer to oviposit in squares (flower buds). Even though multiple ovipositions can be found in nature, usually only one immature stage reaches the adult stage per square. The objective was to study the costs associated with laying eggs in previously infested squares. Squares from plants and fallen squares were collected in cotton plots in Saenz Peña, Chaco (Argentina). Squares were transfer to the laboratory where it was measured the number of oviposition (sealed punctures) and the size. The squares were incubated at 25°C and 70 ± 10 % HR and darkness until adult emergence, then sex and weights of adults obtained were recorded. In order to evaluate the larval elimination due to competition in the square, squares with two and three ovipositions were dissected and the larval stage and if it was alive or dead was recorded. A total of 600 infested squares were incubated for adult emergence. Adult weights of each sex, obtained from squares with different infestation levels, were compared by means of analysis of covariance (ANCOVA) with a square size as a covariate. The sex ratio (number of females / total number of adults emerged from squares with different number of ovipositions) were compared by means of chi-square homogeneity test. From a total of 600 infected squares were obtained 447 adults, 39% from one puncture squares and the rest from multiple puncture squares (up to 7 ovipositions). Only in 2 cases (both of infestation level 2) more than one adult emerged, and the square with 7 punctures gave no adult alive. A correlation between the weight and size of the square was found. When this relation is considered, significant differences were found between the weight of adults emerged from squares with different numbers of punctures for both males and females (ANCOVA females: Number of punctures F(4,177) = 6.03; p < 0.0001; Square size (covariable) F(1,177) = 45.64; p < 0.00001; ANCOVA males: Number of punctures F(4,227) = 5.32; p < 0.0004; Square size (covariable) F(1,227) = 563.65; p < 0.00001). Adults emerged from squares with three punctures were lighter than adults emerged from squares with one or two punctures for both females and males. Mean adult weight of adults emerging from squares with four and five punctures followed the same trend, however few adults were obtained and therefore the variability is high, probably blurring the differences. The proportion of females was higher with increasing number of oviposition (Chi-square = 8.51; gl = 3; N = 156; p < 0.036, test of homogeneity). With respect to the larval elimination, in most cases a single larva stage 3 and 2 were found. The only square where two large third stage larvae were observed, they were divided by a thick wall, so that any point came into contact.The main cost of laying eggs in previously infested squares was only a single individual reaches adult per square (except one case where two adults were obtained) as a result of competition. The results of dissections to evaluate how is the larval elimination could be indicated that the removal occur between larva 1 and 2. Other costs associated with using infected buds can relate to the fitness of the adults obtained. In particular, adults emerge from buds with the most ovipositions result in a lower weight (regardless of the size of the square). This difference in the weight may have an impact on the fitness of adult, since it is known that weevils least significant decrease food and oviposition. Regarding the sex ratio, interestingly it found that the greater the number of oviposition the proportion of females increases. Females of A. grandis might recognize signals associated with the presence of conspecific oviposition to avoid the larval competition. Knowledge of the behavior of females and identification of these signals could provide a basis to develop control methods against this important pest.