Olfactory preferences of gravid femaile stable flies Stomoxys calcitrans L. (Diptera : muscidae) and their fitness consequences

Abstract

Oviposition decisions are a prominent example of behaviour directly affecting the fitness, abundance, distribution, and population dynamics of holometabolous insects. Due to eggs being immobile and adult insects often not practicing biparental and/or maternal care, gravid females, when ovipositing, should select substrate(s) that maximise fitness of their offspring. Studies have revealed that this selection is influenced by biotic (intra and interspecific competition, parasitism, larval experience, etc…) and abiotic (olfactory cues, visuals cues, substrate physiochemical properties, etc…) factors. However, in the stable fly, Stomoxys calcitrans L. (Diptera: Muscidae), a cosmopolitan blood-feeder that transmits several pathogens to animals, the influence of these factors in gravid female oviposition decisions is still unknown. Therefore, in this thesis, I examined oviposition decisions by S. calcitrans, associated fitness consequences for offspring, the olfactory cue(s) involved, and the influence of biotic factors in egg-laying decisions. I demonstrated (Chapter 2) that S. calcitrans prefer to lay on donkey and sheep dung over camel, cow, elephant, giraffe, and zebra dung or grass. I showed that this preference was related to the good nutritional value of these substrates that consequently led to the best performance of S. calcitrans offspring. Furthermore, I identified β-citronellene and (R)-(-)- carvone, from donkey and sheep dung, respectively, as the olfactory cues responsible for observed preference-performance behaviour. In the laboratory, β-citronellene and carvone elicited the strongest oviposition behaviour in S. calcitrans. I confirmed this in the field by showing that traps baited with these compounds also caught more gravid S. calcitrans. To develop a better understanding of oviposition site selection in S. calcitrans, I went on to show (Chapter 3) that S. calcitrans avoided substrates with conspecific larvae, heterospecific (Musca domestica) larvae, or the mite, Macrocheles muscaedomesticae. This avoidance behaviour was associated with fitness costs to offspring incurred in the presence of competition and parasitism. Competition with conspecific larvae led to significant changes in wing morphology (Chapter 4), with flies reared in a group of 5 having larger wing centroid size, wing length, wing width, wing area and wing loading compared with those reared in a group of 25. Also, these parameters were higher in flies developed in donkey and sheep dung in comparison with those grown in camel and cow dung. Preferred dung may lead to adults with better dispersal capabilities. Larval experience does not affect oviposition decisions by female S. calcitrans because they did not prefer to oviposit on their developmental substrate. This ruled out the Hopkins’ host selection principle in S. calcitrans and verifies the importance of innate rather than learned oviposition preferences in this species. Overall, the results reported in this thesis have enabled me to generate a fundamental knowledge of S. calcitrans oviposition decisions. Also, I have identified novel candidate attractants that could enhance trap catch of S. calcitrans to improve surveillance and limit transmission of pathogens. Furthermore, this thesis opens new research avenues in identifying repellents from conspecific and heterospecific larvae, which if packaged with the identified attractants could be used in a push-pull control system against S. calcitrans.