Most insects mount a potent antimicrobial defence upon contact with microbes or microbe-associated pattern molecules. Using a combined set of methods for analysis of insect innate immunity, we report here that piercing of the pea aphid Acyrthosiphon pisum with a bacteria-contaminated needle elicits lysozyme-like activity in the haemolymph but no detectable activities against live bacteria. Confirming these results, we found no homologues of known antimicrobial peptides in our cDNA library generated by using the suppression subtractive hybridization method or in over 90,000 public expressed sequence tag (EST) sequences, but lysozyme genes have recently been described in A. pisum. Interestingly, we discovered that production of viviparous offspring was significantly accelerated upon wounding. Therefore, we postulate that aphids may increase terminal reproductive investment and limit antibacterial defence in response to a threat to their survival.