Andreas Gaigl

Atta sexdens (L.) (Hymenoptera: Formicidae) Demography in the Colombian Amazon: An Evaluation of the Palatable Forage Hypothesis

ABSTRACT This study tested expectations of the palatable forage hypothesis for Atta sexdens (L). Literature records on Atta and Acromyrmex nest density in the Neotropics were analyzed and A. sexdens demographical aspects were studied in six transects located along the Colombian Amazon River. Literature research results did not show that Atta and Acromyrmex nest densities are consistently higher in disturbed habitats. In transects, 56 active and 182 inactive nests were found in 2008. Higher active nest numbers and densities were determined for 1992, 1999, and 2006. Nests were present in old forest, in agricultural plot and grassland secondary forest, and absent from open spaces around houses, grasslands, agricultural plots, and a particular sector of old forest. Nest densities were higher in secondary forest. Between 1992 and 2008, the size of disturbed areas increased, whereas nest numbers and densities diminished. Average nest age was higher in secondary forest, but maximum nest age was comparable in the three habitats. On average, nests were superficially larger in old forest. Nests were mostly established in old forest, at sites without fallen trees. Almost all of those currently present in secondary forest were established in old forest before any disturbance occurred. In disturbed habitats, nest inactivity was slightly higher and possibly affected by human control efforts. This study suggested that the palatable forage hypothesis alone did not explain observed demographic patterns. Soil physiochemical variables or microclimatic conditions also are thought to play important roles in determining A. sexdens demography.

Evaluation of three native entomopathogenic nematodes (Rhabditidae) against third instar larvae of Phyllophaga menetriesi (Coleoptera: Scarabaeidae)

The infectivity and pathogenicity of three native entomopathogenic nematodes (Heterorhabditis (HNI 0100 and sp-CIAT) and Steinernema (SNI 0198)) were evaluated in third instar larvae of Phyllophaga menetriesi (Blanchard) under laboratory conditions. Concentrations of 7000 and 13,000 infective juveniles per millilitre were applied to the coleopteran larval host (individuals) that had been placed in plastic cups with 40g of sterile sand at 5.8% (w/w) moisture. The evaluations were done 5 and 10 days after treatment. It was observed that both the percentage of infectivity (74.5%, n = 247), measured in terms of percentage of penetration and the percentage of pathogenicity (10.5%, n = 247) of the three native nematodes were significantly different due to the treatments. Steinernema had the highest values of infectivity (<80%), but no pathogenicity. In contrast, the treatments with Heterorhabditis, despite having the lowest degree of infectivity (52.9%), had the highest mortality values (31.6%). Therefore, the Heterorhabditis strains were considered to be the most promising as biological control organisms within an integrated pest management programme due to their higher degree of pathogenicity. Nevertheless, different strains and species of native nematodes should be evaluated in all the developmental stages of the pest.RésuméLa capacité d’infection et le pouvoir pathogène des trois nématodes entomopathogènes indigènes (Heterorhabditis (HNI 0100 et sp-CIAT) et Steinernema (SNI 0198)) ont été évalués sur des larves de troisieme stada de Phyllophaga menetriesi (Blanchard) en conditions de laboratoire. Des concentrations de 7000 à 13000 juvéniles infectieux/ml ont été appliquées sur les larves du coléoptère hôte, placées dans des gobelets en plastique avec 40 g de sable stérile à 5,8% (w/w) d’humidité. Les observations ont été réalisées 5 et 10 jours après le traitement. On a constaté qu’ aussi bien la capacité d’infection (74,5%; n=247), mesuré en termes de pourcentage de pénétration, que le pouvoir pathogène (10,5%; n=247) des trois nématodes indigènes, sont significativement différents. Steinernema a la capacite d’infection la plus élevée (< 80%) mais pas de pouvoir pathogène. Par contre, les Heterorhabditis qui montrent la capacite d’infection la plus faible (52,9%) ont les taux de mortalité les plus élevés (31,6%). Aussi, les souches de Heterorhabditis sont considérées comme les plus prometteuses comme agent de lutte biologique dans le cadre d’un programme de lutte intégrée contre le ravageur du fait de leur pouvoir pathogène élevé. Néanmoins, différentes souches et espèces de nématodes indigènes devront être testées sur tous les stades de développement du ravageur.