Profile: Olive Fruit Fly

Profile: Olive Fruit Fly

Olive fruit fly is one of the most serious economic pests of olives, but fortunately not yet in Australia.  It doesn’t harm the tree but causes considerable damage to fruit quantity and quality.

Apart from eating the olive fruit pulp, the larvae emerges to leave a hole that allows fungi to enter causing adverse changes to chemical and taste profiles of the olive making olive products inedible.

At the end of autumn both adults and larvae will be found on the olive tree.  The last larvae spend the winter as pupae in the soil. In cold winters adults and pupae will die, but in milder winters both will survive to the following spring.

The adult will then feed on nectar from blossoms and fruits of other plants, predisposing areas with lots of vegetation and fruits to the appearance of the Olive Fruit Fly.

Where summers are dry and hot >35C, the fruit fly activity and survival rate plummets (as the eggs are not viable).  So the microclimatic conditions of the grove will help to explain the year to year incidence of olive fruit fly.

In areas along the coast and mountains where there are lots of rivers, fruit trees and shade, the fly will survive the summer very well.

The female selects fruit that are not water stressed where the eggs will have the best chance of survival.  The number of holes (stings) in the fruit is a measure of fruit fly population, although not all stings will contain an egg.  After the first generation of fly has emerged, the amount of fruit stings will increase exponentially.

Olive Fruit Fly prefer table olive varieties – the thickest, sweetest and least bitter fruit.  This also applies with low to moderate crop loads where fruit will also be thicker and juicier.

Note: A European Union-wide ban on dimethoate, one of the most effective chemical treatments that farmers have against the olive fruit fly will come into force at the end of October 2020.

Is Queensland Fruit Fly (Qfly) a pest of olives?

The Plant Health Australia ‘National Fruit Fly Strategy’ deals with the control of the 2 major fruit fly species present in Australia: Queensland fruit fly Bactrocera tryoni (Qfly) a native of Eastern Australia; and the imported Mediterranean fruit fly Ceratitis capitate (Medfly) which is now endemic to WA; as well as a range of less well known exotic quarantinable fruit fly’s including Olive fruit fly Bactrocera oleae.

Whilst the potential for devastation of the olive industry resulting from an incursion of Olive fruit fly is well known, the status of olives as a host of Qfly is less well known.

It is generally accepted that under high Qfly population pressure OLD fruit fly will ‘sting’ ripe olives, and from time to time we receive reports to this effect.  However stung olive fruit will usually drop from the tree so it is not a commercial problem in terms of contamination of harvested fruit and production of tainted olive oil.

A recent scientific paper has thrown further light on the status of olives as an alternative host for Qfly, allowing Qfly larvae to complete their life cycle in olive fruit – meaning olive groves will need to be included in any national surveillance trapping and control programs:

Olive fruit (‘Olea europaea’ L.) as a host of Queensland fruit fly (‘Bactrocera tryoni’ (Froggatt) in South Eastern Australia

General and Applied Entomology: The Journal of the Entomological Society of New South Wales Volume 47 (Oct 2019)

Dominiak, Bernard C1; Semeraro, Linda2; Blacket, Mark J3; Englefield, Adrian C4; Mellberg, Alicia5

Abstract: There is limited historical information for olive fruit as a host for Queensland fruit fly (‘Bactrocera tryoni’). In 2015, five suspect samples from informal surveys of olive fruit from the Sunraysia district were examined for ‘B. tryoni’ and found positive. Larval and adult identifications were confirmed using both morphological and molecular methods. Olive fruit were found to support and produce between 3.8 and 32.5 adults per kilogram of fruit, which is comparable to some citrus. There is a need to further develop a fruit fly standard for a host susceptibility index or host potential index. If producers are developing a systems approach to manage fruit fly, they need to be aware that olive fruit can act as an alternative host for ‘B. tryoni’.