Effect of nitrogen on the preference and performance of a biological control agent for an invasive plant

The nitrogen content of plants can have a positive, negative, or neutral effect on herbivore populations and thus, determining the nature of this response is important for predicting the outcome of a biocontrol program targeting weeds. In this study, we tested the effect of fertilization of houndstongue (Cynoglossum officinale L.) on both the individual and population level attributes of Mogulones cruciger Herbst, a classical biocontrol agent of houndstongue. In a randomized field experiment, fertilization significantly increased weevil populations in the first trial and resulted in a marginal, but still positive, effect in the second trial of the experiment repeated a year later. Insect response to fertilization paralleled the host plants’ growth response in both trials. In laboratory experiments, where other confounding factors such as plant size were experimentally eliminated, nitrogen fertilization increased the amount of leaf tissue consumed by 24% and increased the fecundity of individual weevils by 25%. The results of these experiments show that fertilization is an efficient means of increasing the number and potential fecundity of biocontrol agents mass reared in a cultivated field setting. They also show that soil nitrogen can be an important factor in the population ecology of biocontrol agents in the field, and that this response can be predicted by the responses of individual insects in the laboratory.