How adapting to different climates has helped a pest spread across the globe

 

If you have ever bitten into a fruit and been disgusted to find it wriggling with cream-coloured maggots, you have already met at least one member of the fruit fly family.

True fruit flies belong to the fly family Tephritidae, and are quite different from the small Drosophila or “vinegar flies” that hover around and drown in your glass of wine. Unlike Drosophila, which lay their eggs in decaying organic matter such as fruit in a bowl in your kitchen, true fruit flies lay their eggs in ripening fruit that’s developing on the tree.

Once the larvae hatch, they feed on the fruit they were laid in. The damage caused by this larval feeding means that true fruit flies are trouble for fruit growers. Farmers often turn to costly control tactics to try and prevent infestation by true fruit fly larvae, reducing their profits. And fruit infested with true fruit fly larvae can’t be exported. This pest costs fruit farmers and governments well over US$1 billion each year.

The Mediterranean fruit fly, Ceratitis capitata, is among the most destructive true fruit flies in the world. It is native to Africa but is now found in countries around the Mediterranean Basin and Middle East, South and Central America, and parts of Australia and the US. What’s remarkable about this almost global distribution is the wide range of climates that the Mediterranean fruit fly has been able to colonise and thrive in.

My colleagues and I set out to study how this little pest has managed to spread over such wide-ranging environments. We found that the Mediterranean fruit fly is highly adaptable to different environments and can survive extremes in temperature, and water and food availability.

Environmental stress tests

All insects are ectotherms. This means that their body temperature and all life processes – movement, digestion, growth, development and reproduction – are determined by the temperature around them. Water and food availability are also important for survival and growth.

We measured the ability of Mediterranean fruit flies from different climates across South and East Africa to survive high and low temperatures, as well as a lack of water or food. We collected infested fruit from eight sites in South Africa and Kenya, then held the developing larvae and pupae under common environmental conditions in the laboratory. The adult flies were then used in experiments.

First we transferred flies from each site to different temperatures. This is because prior exposure of the Mediterranean fruit fly (as well as other insects) to a warmer or cooler temperature improves their survival when it comes to tolerating extreme temperatures.

We also wanted to know whether closely related populations responded to environmental stress more similarly than populations that were more distantly related. We built a phylogenetic tree based on the genetic fingerprint of each population. Then we compared populations taking into account how closely they were related and the climate they were sampled from.

Our results, recently published in the open access journal Scientific Reports, showed that the Mediterranean fruit fly is highly adaptable.

The eight populations we sampled showed different patterns in their ability to survive high and low temperatures, and lack of water or food. The results lead us to believe that each population adapts differently to its local environment.

In addition, all populations exhibited some flexibility in their environmental tolerance as a result of the temperatures they had experienced before testing. If flies had experienced cooler temperatures before testing, their tolerance of cold temperatures was improved but they were less able to withstand high temperatures. Prior experience of warmer temperatures led to improved tolerance of high temperatures and reduced tolerance of cold temperatures.

An adaptable pest

Our research suggests that the Mediterranean fruit fly has been able to leave its native Africa and become a globally invasive fruit production pest because of its ability to adapt rapidly to new environments.

Its ability to evolve rapidly to different environmental conditions also suggests that the Mediterranean fruit fly will be well suited to cope with climate change.

Short of using pesticides, which are expensive, there are other steps farmers can take to limit the numbers and spread of the Mediterranean fruit fly. Harvested fruit should be inspected and sorted to prevent infested fruit from reaching markets. Temperature or radiation treatments can also be applied to kill larvae in the fruit.

For countries not currently affected by the Mediterranean fruit fly, it is vital to prevent its entry and establishment by enforcing stringent quarantine regulations.

In both cases, government agencies and grower collectives need to provide support required to limit the economic damage caused by this adaptable pest.

Christopher Weldon, is a Senior Lecturer in Entomology, at the University of Pretoria. This article was first published in The Conversation of 2 July 2018. Click here for the original article.

Dr Christopher Weldon

July 10, 2018

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Researchers
  • Professor Chris Weldon
    Professor Christopher Weldon is an Associate Professor in Applied Entomology at the University of Pretoria (UP). He completed his undergraduate studies at the University of Newcastle in Australia, graduating with a Bachelor of Environmental Science (honours) and the Dean’s Medal. He has been pursuing research at UP since he was appointed as a lecturer in Applied Entomology in 2012.

    Prof Weldon says his field of research would contribute to the betterment of the world because human health and livelihoods often intersect with the activities of insects. Using a foundation in insect behaviour, ecology and physiology, his research aims to address these interactions, whether negative or beneficial, with a focus on flies but also insect pests of horticulture. Results from these studies inform sustainable management practices that can reduce reliance on pesticides but may also facilitate the beneficial use of flies.

    He leads a research group of postgraduate students at honours, master’s and PhD level. “We pursue projects on the traits that make fruit flies destructive and invasive pests; the behaviour and movement of fruit flies that damage our fruit for consumption and exports; the biology and management of mosquitoes and other blood-feeding flies that affect human and animal health; and the use of flies for bioconversion of organic waste to promote a sustainable, circular economy,” Prof Weldon explains.
    New research that the professor and his research group have been working on over the past 18 months has been the practical interventions that can be used by hotels to reduce mosquito abundance, the nuisance of mosquito bites and the potential spread of diseases. Such practical interventions will improve the experience of hotel guests and encourage tourism.

    Prof Weldon and his group have also done research on how body condition and environment affect fruit fly attraction to lures and movement in the field; the diversity, abundance, and damage caused by thrips in avocado and macadamia orchards; and how to improve surveillance of invasive pests of citrus. All this work is being done in partnership with South African horticulture industries as well as international organisations to improve the productivity and competitiveness of South African horticulture.
    His advice to school learners or undergraduates who are interested in his field is to read widely about the diversity and importance of insects. There are plenty of interesting and reputable sites featuring that kind of content, Prof Weldon says, such as Entomology Today, as well as great books on insects in southern Africa. He also suggests they join a society like the Entomological Society of Southern Africa or the Lepidopterists’ Society of Africa to learn more about insects from experts and have opportunities to work with them. “Register for an undergraduate degree programme in Entomology like UP’s BSc Entomology, he adds, “and be sure to take the right subjects in school to be admitted.”
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