Chemical and Physical Fecal Defense Mechanisms in Insects
Biology 342 Fall 08
The adaptive value for these behavior lies in their ability to either deter predators, avoid predators entirely, or block mechanisms of predation. Frass chains allow the larvae to avoid predators completely, while fecal shields deter predators and, to a lesser extent, give the larva a way to physically obstruct predation
Frass chains are considered a predator avoidance strategy. By suspending themselves away from the leaf, larvae can avoid arthropods walking on the leaf surface (Machado and Freitas 2001). While this may not have evolved as a direct response to predation by ants, it has an adaptive value in diminishing larval predation. Machado and Freitas give an experimental demonstration of the survival value of frass chains against predators in the natural larval habitat (Machado and Freitas 2001).
Fecal shields are a predator deterrence strategy. The beetles emit chemical compounds, such as saponins and steroidal alkaloids, in their feces that have toxic effects on predators. Authors show that generalist predator ants, such as Formica subsericea, will be disuaded from predating on beetle larvae with frass shields (Vencl et al. 1999). Once an ant has been in contact with a shield, it will move away from the larvae and begin to clean itself vigorously. There are a variety of chemical compounds in the beetle's feces that is obtrusive and toxic to the ant, including molecules suchh as steroidal glycoalkoloid derivatives, dioscin metabolites, fatty acids, and phytol derivatives (Vencl and Morton 1998).
The demonstration by Schaffner and Muller of attraction of the parasitoid Lemphagus pulcher to Lilioceris lilii (Fig. 4) fecal shield extracts adds another aspect to the fecal shield's adaptive value as a defense mechanism (Schaffner and Muller 2001). While the presence of a unique series of chemical compounds in the fecal shield may act as a deterrent to one species, its presence in close association with a particular species makes it a readily exploitable cue for parasites or predators that prey preferentially on the fecal-shield bearing species.
Fig 9. Lilioceris lilii larvae with fecal shield (Photo © Roy Anderson).