From the predator to the prey: a case study of the vulnerability of <i>Harmonia axyridis</i> to aggressive competitors

Harmonia axyridis was originally considered and used as a biocontrol agent throughout the world. But its effectiveness has turned into a serious detriment to ecosystems, as it has become an invasive and harmful species, gradually displacing native coccinellid populations. Its invasive success mostly relies on its voracity, aggressiveness, and dominance in intraguild predation (IGP) interactions. Aggressiveness is a major behavioral characteristic known to influence IGP occurrence and outcome. So, what if H. axyridis were confronted with more aggressive competitors? To understand the vulnerability of H. axyridis to aggressive competitors, we confronted it with an IGP interaction involving two artificially selected lines of the generalist predator Nabis americoferus (one aggressive and one docile) in the presence of extraguild prey (Myzus persicae). Two hypotheses were formulated: (1) the IGP intensity toward H. axyridis is positively correlated with the intraguild predator's aggressiveness, and (2) the IGP intensity decreases in the presence of extraguild prey for both the aggressive and docile lines. Results showed that, as expected, the aggressive line displayed a higher IGP rate on H. axyridis than the docile line, supporting the first hypothesis. The second hypothesis was also supported, since extraguild prey availability led to a decrease in IGP for both lines. Apart from providing insights into the relationship between aggressiveness and IGP occurrence, the discussion focuses on the consequences for H. axyridis in the field as an invasive species and as a biological control agent.