Going the distance

Factors influencing dispersal in Oriental fruit flies. By Grethe Bestbier.

“At the moment the Western Cape is an Oriental fruit fly free area, and we want to maintain it like that,” says Prof. Chris Weldon from the Department of Zoology and Entomology at the University of Pretoria. “If you want to keep Oriental fruit flies out of the Western Cape, then you need to know what is driving them, why and where they want to move, and what determines how far they fly.”

Weldon explains that knowledge of dispersal can be used to establish buffer zones. These are areas from which fruit should be excluded to avoid fruit fly invasion. Then again, if the pest does establish in the Western Cape, dispersal ability will determine how large an area needs to be quarantined and treated to eradicate it.

From 2015 to 2017, Weldon led a research project on the dispersal of Oriental fruit flies, studying how far they can fly in relation to physiological and environmental variables. Fly maturity and sex, and the availability of host fruit were studied as relevant physiological factors, while environmental factors included temperature and rainfall. Flies marked with fluorescent pigments were released and recaptured weekly for four weeks, providing valuable information on flight distances and drivers.

Sex and age

The results showed that younger males cover greater distances than older males. Young males tend to disperse to find unrelated females for mating, so as to spread their genes. This behaviour is also seen in other species of the same genus as the Oriental fruit fly.

“The young males have this tendency to want to move away from the place where they emerged,” says Weldon. “There seems to be a pattern where the males disperse when they are young, whereas older males settle down.”

However, this is not the case for females. The distances that females fly is determined by the availability of host plants. Females are stronger fliers when released amongst non-host plants.

“Because females lay their eggs in the developing fruit on host trees, we suspect that they are less likely to move away from host plants. But if they are in a place with no host fruit, the females will need to move away to find somewhere to lay their eggs,” explains Weldon.

Interestingly, the researchers also found that females would not move away from host plants even when these were not in fruit. There is still some uncertainty about what causes this behaviour, says Weldon. “Perhaps the females were using some information from the tree other than host fruit to decide whether to stay put, or perhaps it relates to the environment that those particular trees provide.”

Temperature and rainfall

According to Weldon, South Africa has favourable environmental conditions for Oriental fruit flies to disperse, especially in terms of temperature. His team found that the frequency and duration of flight increases in the optimal range of 24°C to 32°C. When temperatures drop below 20°C, flies become relatively inactive, while very hot conditions — 36°C and above — increase duration of resting.

“The temperature [in South Africa] is only below the threshold of 20°C in the evening or in the early morning. After that, during most of the day, the temperature conditions for flies are great, unless it gets too hot,” says Weldon.

The impact of rainfall is still uncertain. Although there were no clear effects on dispersal, increased rainfall did result in more female recaptures. One possible explanation is that rainfall increases humidity and females prefer humid environments, becoming more active and more likely to be caught. In contrast, as rainfall increased, fewer males were caught.

Weldon says that they don’t yet fully understand these behaviours, but are looking into them. The first step was studying dispersal patterns, while further research will examine the underlying causes.

“Most control tactics require a really good understanding of the biology of the insect. That is where going into the detail comes into play,” stresses Weldon. “If you want to keep the flies out of the Western Cape, then you need to know what determines how far they fly.”

Image: The Oriental fruit fly.