Efeito da umidade do substrato na sobrevivência e na infectividade de juvenis de vida livre de Heterorhabditis amazonensis (Nematoda, Heterorhabditidae)

Abstract

The present study was motivated by the fascination and interest regarding the biology of the juvenile phase of parasitic nematodes. The juvenile stages of entomopathogenic nematodes (EPNs) are very similar to those of other groups of parasitic nematodes, except for the occurrence of symbiosis between their larvae and bacteria. These juveniles have infective capacity and the continuity of their life cycle depends on surviving at the external environment, finding and infecting new hosts. However, little is known about the interaction of infective juveniles (IJs) of these nematodes and the soil characteristics, among which moisture content stands out for its strong influence on the biology of these animals. Therefore, the aim of this work was to investigate the influence of soil moisture on the survival and persistence of infectivity of IJs of Heterorhabditis amazonensis Andaló, Nguyen & Moino Jr, 2006, a native Brazilian specie. This influence was evaluated in two independent studies. The first experiment aimed to compare the number of living IJs recovered from soil samples of two experimental groups. The evaporation group was subjected to natural evaporation after an initial moisture content of 80% and the control group was maintained at constant moisture level of 80%. Among the results were: (1) the continuing infectivity of all the IJs recovered; (2) the high mortality in the first three days in both groups; (3) this high mortality occurred under moist substrate conditions and (4) the fact that the conservation of moisture in the control group during the experiment was not reflected in a significant advantage for the survival of the juveniles in this group. In the second study, the same comparison was made during 30 days for IJs in constant substrate moisture conditions of 100, 75 and 30%. The most notable results were: (1) the persistence of infectivity in all the IJs; (2) the rapid mortality in the first three days of observation, of 88, 79 and 59%, respectively; and (3) the fact that the driest soil condition, of 30%, was most favorable for the survival of the IJs, with significantly higher numbers until the 30th day of observation. In both studies, the humidity levels refer to the percentages in relation to the saturation point, and the substrate used was previously sterilized and completely dried and the soil samples were maintained at 25 °C, 90% RH. The infectivity of the larvae recovered at both experiments was tested in larvae of the beetle Tenebrio molitor Linnaeus, 1785. The technique to recover the juveniles was developed based on the literature. The results permit inferring that dry conditions and fine granulometry of soil

are favorable to the survival of IJs of this specie of EPN and the high mortality in the first three days appears to be a characteristic of this specie and does not depend on the substrate’s moisture content. Finally, several scenarios may come out from the dynamic interaction between the moisture and other soil conditions. The relative importance of moisture to the biology of these animals possibly will vary at each resulting scenario.