Effect of natural vegetation management practices on Prays oleae (Bernard) and its parasitoids

Abstract: The olive moth (Prays oleae Bernard) is generally considered as the most serious pest of olive trees in northern Portugal. In this region, there is a rich and diverse parasitoid complex of this pest. Spontaneous vegetation may provide food, alternative hosts and shelters for these parasitoids. This study investigated the effects of different spontaneous vegetation management practices on the moth’s anthophagous generation and its parasitism. A total of 14 and 15 olive groves with different spontaneous vegetation management practices (herbicide application, tilling, and conservation of vegetation) were sampled in 2011 and 2013, respectively. In 2012, the pest population levels were extremely low probably due to the serious drought. In each grove, 10 olive trees were randomly selected and 20 olive moth larvae per tree were collected. Larvae were kept in laboratory at controlled conditions and emerging moths and parasitoids were recorded. Generalized Estimation Equations (GEE) with a Logit link function and exchangeable correlation structure were used to fit the response variables. HeatmapFit was used to assess the quality of the models. The estimated probability of emergence of olive moth in 2011 (range: 0.15 to 0.20) was significantly lower than in 2013 (range: 0.55 to 0.70) but did not differ among different management practices. In contrast, the estimated probability of parasitism in the groves with spontaneous vegetation (0.50) was significantly higher than in the groves with herbicide application (0.36) in 2011. The parasitism experienced a dramatic decrease in 2013 and was significantly lower in the groves with spontaneous vegetation (0.045) than in the tilled groves (0.13). Ageniaspis fuscicollis (Dalman) accounted for the majority of the parasitism and followed by Elasmus flabellatus (Fonscolombe). These results suggest that the presence of spontaneous vegetation has probably increased the abundance of parasitoids and conservation of spontaneous vegetation, combined with other field management practices, may be considered as an important strategy for the control of the olive moth. Nevertheless, the significant decrease of the olive moth population suffered from the 2012 drought had also seriously reduced the abundance of parasitoids. This fact emphasizes the value of maintaining an equilibrium between the abundance of natural enemies and pests.