Abstract
Climate-change leads to extreme droughts, but it is difficult to predict how crop pests are affected. Drought may alter behavior of crop pests such as aphids that facilitate transmission of pathogens like Fusarium graminearum, causing Fusarium foot and root rot in barley crops by feeding close to the susceptible base of plants. Ground dwelling generalist predators like Carabid beetles, and wolf spiders are important aphid enemies during the aphid colonization phase in barley crops in Sweden. This study sought to determine the influence of drought on interactions among aphids, aphid predators, and Fusarium infections on spring sown barley (Hordeum vulgare L.) in Sweden. To evaluate the influence of drought on predation of aphids by Carabid beetles and Wolf spiders, the experimental design used was factorial with two factors, i) drought and no drought, ii) predators present and predators absent. Similarly, the experimental design used to assess the influence of drought on interactions between aphids and Fusarium infections on barley sown during spring was factorial with three factors, i) drought and no drought, ii) aphids and with no aphids, iii) Fusarium and with no Fusarium. To evaluate the influence of drought on predation of aphids by Carabid beetles and Wolf spiders, the total number of aphids per cage and the proportion of aphids present on different parts of each barley plant sampled (below ground, 0-1 cm above ground, >1cm above ground on stem, and on the leaves) was recorded. Data was subjected to normality test to determine its distribution and analyzed using generalized linear mixed effects models with the lme function in the nlme package in R 3.4.2. To assess the influence of drought on interactions between aphids and Fusarium infections, the total number of aphids present at 0-1 cm above ground, >1cm above ground on stem, and on the leaves per plant in each pot was recorded. The data was subjected to normality test and analyzed using generalized linear mixed effects models with the lme function in the nlme package in R 3.4.2. Disease scores on the barley inoculated with Fusarium were recorded and the data analyzed using one-way ANOVA test. There was a significant effect of Fusarium inoculation on barley plants. In addition, Fusarium significantly reduced the number of aphids on the whole plant and 0 to 1 cm above the ground. The study concluded that drought increased proportion of aphids (Rhopalosiphum padi L.) below ground. In addition, a strong effect of predation by Carabid beetles and Wolf spiders reduced the number and proportions of these aphids below ground only in dry conditions. This study recommends the need for further studies to establish how drought intensity is likely to affect interactions among pest populations, their natural enemies and plant pathogens.