The smell of footprints in the Queensland fruit fly: signalling to conspecifics attracts eavesdropping predators. (APSF 21016)

APSF 21016 | Amount: $44,480 | Project Leader: V Mendez | Project Period:

A project undertaken at Macquarie University, and supervised by Vivian Mendez.

Cuticular hydrocarbons (CHCs) that cover the cuticle of insects have been extensively studied for their role in preventing desiccation as well as for their function as ‘infochemicals’. Knowledge of cuticular chemical communication, however, mostly comes from social insects; the role of CHCs as infochemicals in non-social insects, such as Queensland fruit fly (Q-fly), has received little attention.

The study of the chemical ecology of tephritid fruit flies has mainly focused on volatile pheromones released by males to attract females at long range. Little is known about cuticular compounds and their role in close contact communication.

Figure 1. Discriminant analysis of principal components on the relative proportions of the 56 compounds from the footprint extracts of Qfly. Retained discriminant functions and principal components were 16 and 5, respectively. Circles represent clusters with 95% inertia ellipses. IF: immature female; IM: immature male; MF: mated female, MM: mated male; VF: virgin female; VM: virgin male.

CHCs have been used as a taxonomic tool to distinguish between fruit fly species, but their biological function remains to be investigated. We recently found that the CHC profile of immature Q-flies is different from the adults and that there is sexual dimorphism in the CHC profile of mature flies (Park et al. 2020). It is hence likely that CHC chemistry in part functions in signalling of sex and maturity.

In pilot observations, we have found that Q-flies leave CHCs behind as they walk on surfaces, and that the chemical profile of the surface where the fly walked matches exactly the fly’s CHC profile. The footprint left by a fly might be used by conspecifics to obtain information about sex and maturity of the fly and by predators to find flies as prey. The concentration of chemical cues might even inform conspecifics and predators about places where flies aggregate on tree branches or fruits.

Figure 2. Behavioural response of immature males, virgin males and mated females of Qfly to the footprint compounds of conspecifics. Asterisk denoted means significantly different from 225s (Signed Rank, p <0.05).

In Q-flies, males and females aggregate in trees at dusk to mate. Q-flies prefer tall trees for mating aggregations (Ekanayake et al 2016) but the mechanism by which a particular tree is selected as the mating arena and found every day at dusk is unknown. We anticipate that CHC footprints have a role as markers of mating arenas in Q-fly (and other fruit flies), and also that females use CHC footprints left by other females on fruit in oviposition decisions. Further, we anticipate that predators make opportunistic use of fruit fly CHC footprints to find prey. The persistence of CHC footprints in nature has not been studied in detail, although there are reports of insect CHC footprints remaining biologically active for as long as 10 days (Rostas 2009).

Low volatility and non-polar structure of fruit fly CHCs suggests that they should be quite persistent. Studying the persistence of CHC footprints in field conditions is critical for understanding to what extent they remain available, or accumulate, in the environment for use by conspecifics and predators, and consequently their relevance in fruit fly biology. In Qfly, our pilot studies indicate that CHC footprints persist for at least five days in laboratory conditions (the longest period tested to date), however, persistence when exposed to field conditions is unknown.

We here seek to investigate how Q-flies use CHC footprints to inform decisions of mating and oviposition, and whether predators use Q-fly CHC footprints to inform hunting. This will be the first study of CHC footprints in a true fruit fly and will lift the veil on an entirely unexplored dimension of fruit fly biology and predator-prey interactions. We will also explore the potential use of footprint chemistry for the management of major fruit fly pests.