Effects of advanced age on olfactory response of male and female Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae)

Highlights

• Olfactory response in B. tryoni declined with increasing age in both sexes.

• Male attraction to cue-lure declined dramatically after 12 weeks of age.

• Females’ strong attraction to guava-juice declined gradually after 6 weeks of age.

• In females the decline was primarily caused by a drop in exploratory activity.

• In males the decline was caused by reduced exploration and selective orientation.

Abstract

Olfaction is an essential sensory modality of insects which is known to vary with age. In short-lived insects odour response generally declines rapidly with increasing age, but how increasing age affects the olfactory response of long-lived insects is less known and there may be different life-time patterns of olfactory response. Here, we examine the effect of age on olfactory response and exploratory activity of a long-lived tephritid fruit fly, Bactrocera tryoni from sexual maturity (3 weeks) to advanced age (15 weeks). Males were tested against a male-specific attractant, cue-lure, which is associated with courtship and sexual selection in this species; while females were tested against guava-juice, a highly attractive oviposition host fruit odour. Trials were done in the laboratory using a Y-tube olfactometer at three weekly intervals. The probability of olfactory response of both males and females to tested odours declined with age. Males retained a constant attraction to cue-lure until 12 weeks of age, but then showed a significant drop in olfactory response at 15 weeks. However, females showed the highest attraction to guava-juice odour until six weeks of age and declined gradually thereafter. The change on odour response over time can be associated with an age-related change in initial locomotor activity for females as there was no change, over the life of the experiment, in selective female orientation to the odour source once flies started exploring within the olfactometer. However, for 15 week-old males, there was a simultaneous drop in both locomotor activity and selective olfactory orientation. The consistent attraction of male to cue-lure might be related to life-long reproductive activities of males, as males are thought to mate continuously during life. On the other hand, females’ highest attraction to guava-juice odour in early life followed by a gradual decline might be linked with their oviposition rate which peaks in early life.

Introduction

For insects, olfaction is an essential sensory modality for the perception of stimuli from the environment (Krieger and Breer, 1999, Touhara and Vosshall, 2009). Olfaction plays a key role in behaviours such as the location of food, mates and oviposition substrates, as well as enemy avoidance (Lebreton et al., 2017, Wyatt, 2014). An insect’s response to odours may vary depending on physiological factors such as feeding and nutritional status, mating status and sexual maturation (Gadenne et al., 2016, Jin et al., 2017, Lemmen-Lechelt et al., 2018). For example, insects that are deprived of food generally have a higher attraction to food odours (Wäckers, 1994); while virgin insects are more responsive to odours from mating sites and partners than are mated insects (Jin et al., 2017, Sollai et al., 2018). An individual’s age is another factor that strongly affects the odour response of an insect (Gadenne et al., 2016, Reyes et al., 2017).

In insects, age-related changes in olfactory responses are linked to sexual maturation, as well as ageing (Gadenne et al., 2016, Martel et al., 2009). For example, in early life, insects are more attracted to food odours while, after sexual maturation, they may be more attracted to cues from mating partners (Klowden, 1990). Additionally, an insect’s response to the same odour may change with age, for example, females of the blowfly, Phormia regina Meigen, show increasing olfactory sensitivity to swormlure-4 and 1-hexanol during the first week of adult life (Crnjar et al., 1990). In contrast to increasing olfactory sensitivity, an insect’s response to odours can be negatively affected by ageing once past some optimal age (Gadenne et al., 2016, Iliadi and Boulianne, 2010). For instance, the antennal olfactory sensitivity of tsetse flies, Glossina morsitans morsitans Westwood, to various tested volatiles started declining after only five days of age (Otter et al., 1991), while the perception of aversive odours diminished in eight week old Drosophila melanogaster Meigen (Cook-Wiens and Grotewiel, 2002). For D. melanogaster, Cook-Wiens and Grotewiel (2002) attributed the reduction of olfactory response with increasing age to both the development of age-related defects in the olfactory system and associated parts of the brain, as well as a decline in locomotor activity.

Tephritid fruit flies (Diptera: Tephritidae) are globally significant insect pests of agriculture, affecting most horticultural crops (Norrbom et al., 1999). Female fruit flies oviposit into host fruits, where resultant larval feeding causes fruit damage (Fletcher, 1989, May, 1958, Sutherst et al., 2000). The behaviour and ecology of tephritid fruit flies are strongly tied to olfaction, including the pheromonal attraction of conspecifics during mating (Landolt et al., 1985, López-Guillén et al., 2011), location of host fruit by females for oviposition (Eisemann, 1980, Jang et al., 1999, Liu and Zhou, 2016), and for male tephritids of the tribe Dacini for the location of plant secondary chemicals which are linked to male sexual selection (Kumaran et al., 2013, Raghu, 2007, Shelly, 2010). The management of pest tephritids relies heavily on the manipulation of this chemical ecology, using olfactory attractants for surveillance and lure-and-kill controls (Aluja, 1996, Koyama et al., 2004).

Despite the implications for management, and simply for better understanding tephritid biology, studies of ageing and changing olfactory responses in tephritids are limited. Multiple studies have investigated the effect of ageing from adult emergence to sexual maturity [which occurs on average from 10 to 20 days after emergence, (Clarke, 2019)] on the response of male Dacini to male-specific lures, such as methyl eugenol (ME) and cue-lure (Kamiji et al., 2018, Metcalf and Metcalf, 1992, Weldon et al., 2008, Wong et al., 1991). Most of these studies have found that lure attraction is dependent on the male becoming sexually mature, although in some species males may begin to respond prior to maturation (Wee et al., 2018). Fitt (1981), in the only study of its type, recorded the response of Bactrocera opiliae (Drew and Hardy) to ME significantly past sexual maturation and found no change in responsiveness of males up to 12 weeks of age. Outside of the Dacini male lures, age-related olfaction studies in the Tephritidae are rare. The response of Ceratitis capitata (Wiedemann) to synthetic food attractants was investigated until 40 days of age, where it was shown that sugar-fed flies showed higher response at middle age (10–25 days age) while protein-fed flies showed higher response at a young age (1–5 days age) with a declining trend afterwards (Kouloussis et al., 2009). For Anastrepha obliqua (Macquart), antennal responses to male lure and host fruit volatiles were observed until 20 days of age and it was found that older flies exhibited lower antennal response compared to younger flies (Reyes et al., 2017). While such studies are valuable, and with the exception of Fitt (1981) for males, there are no studies of which we are aware that evaluate olfactory responses of both male and female tephritid fruit flies to olfactory cues until an advanced age, as has been done for Drosophila (Cook-Wiens and Grotewiel, 2002) and other insects (Gadenne et al., 2016). This is despite the fact that some tephritids can be very long-lived (Carey et al., 2008), with Dacini tephritids, the focus of this paper, surviving 11 weeks on average in the laboratory (Clarke, 2019) and over 28 weeks as over-wintering adults (Clarke et al., 2019).

Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) is an Australian endemic insect which is a key pest of the Australian horticultural industry (Clarke et al., 2011, Sutherst et al., 2000). Sexually mature Bactrocera tryoni males are attracted to cue-lure [4-(p-acetoxyphenyl)-2-butanone] (Meats and Hartland, 1999), while mature females are attracted to volatiles released from ripening host fruits (Cunningham et al., 2016, Eisemann, 1980). Cue-lure is widely used for quarantine surveillance, monitoring and field control of B. tryoni (Dominiak et al., 2003, Lloyd et al., 2010, Reynolds et al., 2016, Stringer et al., 2017), while fruit-based odours are being used as the basis for female traps (Cunningham et al., 2016). Bactrocera tryoni can be long-lived, with some individuals surviving in excess of 20 weeks in controlled laboratory studies (Balagawi, 2006), but there is no knowledge of how olfactory responses in the fly change once they become sexually mature, which occurs at approximately 21 days for wild flies (Dalby-Ball and Meats, 2000).

Given the importance of olfaction in the management of B. tryoni, this study evaluates age-dependant olfactory responses and exploratory activities of the fly from sexual maturity to advanced age (15 weeks). Specifically, we observed the effect of age on olfactory response and exploratory activity of male and female flies in the presence of cue-lure and guava-juice odour, respectively. We hypothesized that: (i) exploratory activities will be higher in younger flies compared to older flies; (ii) the flies’ response to odours will be higher at a younger age compared to old age; and (iii) younger flies will take less time to choose the odour source compared to old flies.