Deterring egg predators with faecal strings is more efficient when eggs are aggregated

Abstract Predation can be a great source of mortality for insect eggs. Thus, female insects use multiple strategies to reduce it. Two common strategies are to aggregate eggs, which may function to satiate egg predators, and various deterrence strategies, such as the addition of faecal strings on top of the egg. We tested the hypothesis that these two strategies work best in combination, where egg aggregation may strengthen the effect of deterrence strategies. We compared egg predation by the predator Paederus riparius both within the species Galerucella lineola (cleaned eggs versus eggs with faecal strings), and between the species G. lineola and G. sagittariae (where the latter species naturally lack faecal strings). Our findings revealed a robust treatment interaction, where faecal strings and egg density affected egg survival when exposed to Paederus riparius . Egg survival increased with egg density for eggs with faecal strings but was constantly low on eggs without faecal strings (cleaned G. lineola eggs or G. sagittariae eggs). This study emphasised that a combination of insect traits may be needed to defend insect eggs against their natural predators.