Whereas spotted-wing drosophila is the black sheep of an otherwise blameless family, the peach fruit fly is one of the Mafiosi of the insect world. By Anna Mouton.
The peach fruit fly — Bactrocera zonata — shares a genus with the notorious Oriental fruit fly — Bactrocera dorsalis. Both belong to the family Tephritidae. About a third of its approximately 4 000 species lay their eggs in soft fruit. Other members of this nefarious bunch include the Mediterranean fruit fly — Ceratitis capitata — and the Natal fruit fly — Ceratitis rosa.
Why does peach fruit fly matter?
Bactrocera zonata does not limit itself to peaches. It has been reported from more than 50 host plants, including several types of pome and stone fruit, as well as citrus. More recently, the peach fruit fly has discovered vegetables, attacking aubergines and tomatoes.
As is the case for other fruit flies, developing larvae of Bactrocera zonata damage fruit directly through feeding, and infested fruit are also more susceptible to decay-causing organisms.
In India, where Bactrocera zonata was first recorded, it causes losses of 25–100% in peaches, apricots, guavas, and figs. In 2005, the annual cost of damage caused by Bactrocera zonata in the Near East was estimated to be EUR 320 million. The European and Mediterranean Plant Protection Organization reported that Bactrocera zonata could outcompete other tephritid fruit flies such as the Mediterranean fruit fly.
Bactrocera zonata does not occur in South Africa. If it were to establish here, it could cause significant losses, especially in stone fruit. Bactrocera zonata is considered a phytosanitary pest by many countries, and its presence in South Africa would undoubtedly have implications for market access.
How do I recognise this fly?
Bactrocera zonata looks a little like a wasp and a lot like several other Bactrocera species.
Adult peach fruit flies are 5–6 millimetres long and have a reddish-brown body. They have bright yellow stripes — called lateral vittae — along the sides of their backs.
Yellow stripes are not special to peach fruit flies. Many closely related flies also have them. Oriental fruit flies have stripes, but their body is usually at least partially black. Melon flies — Bactrocera cucurbitae — have lateral vittae as well as a single stripe — called a vitta — in the middle of their backs.
Entomologists consider many other characteristics when identifying fruit flies. The best way to know whether you have found a peach fruit fly in your orchard is probably to keep an entomologist on speed dial.
How do these flies make more flies?
Female peach fruit flies deposit groups of 3–9 eggs under the skin of fruit. An adult female can lay close to a hundred eggs in a single day and more than 500 in her lifetime.
Eggs hatch after about two days. Larvae feed on the fruit and render them unmarketable. There are three larval stages. Larvae develop inside fruit for about five days and then exit the fruit to pupate in the soil. Adults emerge after about 10 days. These figures are for development at 25 °C — higher temperatures accelerate development.
The optimum temperature range for the development of Bactrocera zonata is 25–30 °C. Development slows or stops at 15–16 °C and all life stages die below 10–13 °C. Bactrocera zonata is adapted to tropical and subtropical conditions and cannot survive extreme cold.
Where did it come from?
The first observation of Bactrocera zonata was from Bengal in India. It occurs throughout India and Pakistan, as well as in other countries in the Middle East and Southeast Asia, including Israel, Thailand, and Vietnam. In Africa, it occurs in Egypt, Libya, Sudan, Mauritius, and Réunion.
Adult peach fruit flies have been known to travel as much as 40 kilometres when they have a following wind. Rapid spread post-introduction was seen in both Réunion and Egypt. In Egypt, the flies were initially detected in East Cairo in 1993, and in West Cairo later that year. By 1997, peach fruit flies were all over the country, including in oases in desert areas.
Bactrocera zonata has attempted immigration to the United States but without success. Detection in California and Florida has so far ended in eradication.
The peach fruit fly has not been recorded from the rest of the Americas, the United Kingdom, Europe, China, Taiwan, or South Africa.
Control of peach fruit fly
Control relies mostly on insecticides mixed with protein hydrolysates in bait-spray formulations.
Male annihilation technique is very effective if carried out on a large scale. Male peach fruit flies are attracted to methyl eugenol which is mixed with an insecticide and deployed in bait stations.
Researchers have identified a parasitoid that could be incorporated into control programmes for Bactrocera zonata.
Bactrocera zonata is sensitive to wet soils and pupae can be controlled by maintaining soil water content at 100% of the field capacity. Orchard sanitation is important, as for all fruit flies.
Fruit that may be infested with Bactrocera zonata can potentially be fumigated or subjected to cold or heat treatments — heat treatments are used for mangoes.
Research conducted by Dr Guy Hallman and colleagues of the United States Department of Agriculture demonstrated that reducing the interior temperature of fruit to 1.7 °C and then maintaining this temperature for 18 days was an effective cold treatment for Bactrocera zonata in oranges.
A study on cold tolerance of six species of Bactrocera, including Oriental and peach fruit flies, found that larvae of Bactrocera zonata were generally more sensitive to cold than those of Bactrocera dorsalis.
Early-warning systems are highly recommended for countries or regions that are currently free of peach fruit flies. Methyl eugenol lures have been reported to attract males over distances of up to a kilometre.