A tale of two flies

How Oriental fruit flies differ from Mediterranean fruit flies. By Grethe Bestbier.

The Mediterranean fruit fly has long been established across South Africa and now a new kid on the block has appeared: the Oriental fruit fly. While both Mediterranean and Oriental fruit flies cause their fair share of damage, the latter is a different type of trouble. Known for its aggression, competitiveness and rapid reproduction, the Oriental fruit fly might pose the bigger risk to South Africa’s deciduous fruit industry.

Different journeys but similar life histories

The Oriental fruit fly (Bactrocera dorsalis) and the Mediterranean fruit fly (Ceratitis capitata) underwent different journeys to South Africa. Originally from Asia, the Oriental fruit fly was first discovered on the African continent in Kenya in 2003. From there, the pest spread south. In 2013, it invaded South Africa. Today, Oriental fruit fly is officially established in the northern and northeastern parts of the country, but not in the Western Cape or Langkloof.

Mediterranean fruit fly, on the other hand, is established over a wider area. It is believed to originate from tropical Africa and invaded Europe more than 150 years ago. From there it spread across the globe, including to South Africa. Currently, Mediterranean fruit flies are found in most parts of South Africa.

Both Oriental and Mediterranean fruit flies have a typical fruit-fly life cycle: the female and male mate; the female lays eggs under the skin of fruit; the larvae hatch and feed on the fruit; the final larval stage jumps into the soil where it pupates; the adult emerges from the soil. This process takes about a month under optimal conditions of 25°C and slows down in winter. Fruit flies do not become dormant during cold months, but overwinter as adults or larvae. Because fruit flies are polyphagous and attack a wide variety of fruit, there is usually an available host, no matter the season.

The pit bull versus the chihuahua

The Oriental fruit fly is a tropical insect that prefers tropical fruits like mangoes, but it can also infest deciduous fruit, including nectarines, plums, apples, and pears.

“We wanted to know whether Oriental and Mediterranean fruit flies could use the same hosts,” says Dr Welma Pieterse, an entomologist with the Department of Agriculture, Land Reform and Rural Development. “The conclusion is that both fly species can complete their life cycles on various deciduous hosts. In fact, Oriental fruit flies can do it more efficiently.”

Oriental fruit flies easily outcompete other flies. Pieterse says that while observing both species for her research, the Oriental fruit flies’ hostility was striking. “They are incredibly aggressive. They pushed and shoved each other,” she says. “The Mediterranean flies mostly kept out of the way while the Oriental flies patrolled the fruit.”

It is not only their aggressive nature that makes the Oriental fruit fly the pit bull and the Mediterranean the chihuahua in this fight. The former is also a better invader in terms of reproductive rate.

“An invasive insect wants to fit in as many generations in a year as possible, so being able to reproduce quickly is advantageous,” explains Prof. Pia Addison of the Department of Conservation Ecology and Entomology at Stellenbosch University.

Pieterse’s research found that various factors enable Oriental fruit flies to reproduce faster than Mediterranean fruit flies. Oriental fruit flies live longer as adults, so can lay eggs for longer and thus generate more offspring. They also attempt more oviposition and lay more eggs than Mediterranean fruit flies. Female Oriental fruit flies lay around fifty eggs over ninety days — about twice as many as Mediterranean fruit flies.

Oriental fruit flies have other strategies as well. When the females probe fruit with their ovipositors, they deposit pheromones that deter other flies from infesting that host. Lastly, Oriental fruit-fly larvae hatch earlier than Mediterranean fruit-fly larvae, which gives them a competitive edge. The earlier larvae hatch, the better their chances in the fruit.

Oriental fruit fly’s apple fetish

While nectarines proved to be the best host for both fly species in laboratory studies, apples were surprisingly popular with Oriental fruit flies. “We weren’t expecting Oriental fruit flies to do so well on apple, because it is far removed from a tropical fruit,” says Addison. “But it did really well. The reproductive rate was high on apple compared to Mediterranean fruit fly.”

Pieterse observed both species on apples for ninety days in the laboratory. By the end of the experiment, the Oriental fruit flies were still very active and laying eggs. Apples seemed to extend the Oriental fruit fly’s lifespan, especially compared to the Mediterranean fruit fly. Relative to other fruit, however, apple is not the number one host.

“Apple is definitely not their primary host, but they will most likely be able to use it as a survival host. Because it is a thick and sturdy fruit, pupae can possibly survive in it over the winter. Then, when more suitable temperatures and crops appear, it can live on in the preferred hosts. In that way, the population is sustained,” says Pieterse.

Addison believes that the Oriental fruit fly’s potential to establish in deciduous fruit production areas in the Western Cape is very high. “Climatic models, the Oriental fruit fly’s ability to compete with Mediterranean fruit fly, and its suitability for deciduous hosts, indicate that it could do well.” However, she warns that these are predictions based on a controlled laboratory study. “We don’t know how it will respond to a Mediterranean climate. But, as we speak, those insects are busy adapting, and we know that insects adapt quickly.”

“Early detection and elimination of Oriental fruit flies are of utmost importance,” emphasises Pieterse. “Monitoring is crucial, because on deciduous fruit they can sustain their population. To avoid establishment in the Western Cape, we must be extra careful here. Monitoring systems, traps, and people who check it regularly, are important to know what is going on.”

Image: The Oriental fruit fly.