Olive Fruit Fly Project

Project Leaders:  Gaylord Desurmont, Marie-Claude Bon  

Background

The olive fruit fly, Bactrocera oleae Rossi (Diptera: Tephritidae),is a widespread pest that feeds exclusively on wild and cultivated olives (Daane and Johnson 2010). The olive fruit fly is found almost anywhere cultivated olives are grown or wild olives naturally occur. The olive fruit fly was recently found in Southwestern China far to the east of all previously known records (Bon et al., 2015). The fruit fly likely originated in sub-Saharan Africa, where the wild olive Olea europaea cuspidata is found and from which the domesticated olive O.europaea europaea was derived (Nardi et al, 2005; 2010). In California, the olive fruit fly has spread to all commercial olive growing regions since first being detected in 1998. Due to the extensive occurrence of non-commercial olive trees throughout California, and low profit margin for both oil and table production olives, research into classical biological control has been a major focal point for controlling this fly (Johnson et al., 2006; Daane et al., 2011; Hoelmer et al., 2011).

The olive fruit fly Bactrocera oleae

Project

As part of a collaborative program initiated in 2002 with the Californian Department of Food and Agriculture (CDFA) and the University of Berkeley (CA), EBCL has carried out extensive foreign exploration in the Mediterranean region, Africa, and Asia for natural enemies which resulted in the discovery of at least five parasitoid species as potential biological control agents for the olive fruit fly in California (Daane et al., 2008; Hoelmer et al., 2011; Yokoyama et al., 2011).  Two populations (South Africa and Kenya) of one of these agents, the larval parasitoid Psytallia lounsburyi (Braconidae) which has been mainly mass-reared at EBCL (Chardonnet et al., 2019) have been successfully released and established in California (Wang et al., 2013; Daane et al., 2015).  Unexpectedly, genetic studies showed that the P. lounsburyi of South African origin predominantly established in California over the Kenyan origin, although it was less released, suggesting that it might be better adapted than the Kenyan origin to the ecological conditions found in Central and Northern California (Bon et al., 2017).

Counting olive fruit flies emerged from infested olives

The genetic features and the microbiomes of these populations might be important factors triggering the success (or the relative success) of their adaptation in California. The two populations were shown to be genetically distant and differently infected by the bacterial endosymbiont, Wolbachia sp. (Cheyppe et al., 2011). Importantly, we do not know what are the key genes or the key endosymbiotic microorganisms underlying such differences in their adaptation. Therefore, a comprehensive evaluation of these key genes and their microbiomes will help to underpin the understanding of the efficacy of this wasp as a classical biological control agent. 

Another parasitoid, P. ponerophaga, was collected by CABI Pakistan for EBCL and is currently reared at EBCL. Compared to P. lounsburyi, it presents different ecological and behavioral characteristics: P. lounsburyi does not need the presence of olives initiate the oviposition process but is highly host-specific; on the other hand, P. ponerophaga is highly stimulated by the presence of olives to start the oviposition process but its level of host-specificity remains to be determined. Identifying the key cues (visual and chemical) that initiate and condition the foraging and oviposition behavior of these parasitoids, as well as estimating the likelihood of these parasitoids to attack non-host species in presence and absence of these cues, would have immense value to evaluate and compare P. lounsburyi and P. ponerophaga as biological control agents and possibly improve their use in the field. The experimental frame of such study could be applied to other braconid species, as braconids of fruit flies in general have been of high interest for biological control practitioners worldwide.

Illustration of the rearing method of P. ponerophaga used at EBCL.
The central ball contains C. capitata larvae covered by a parafilm layer.
Olives are used to stimulate the parasitoids to land and eventually inspect the ball.

An alternative management strategy that has not yet been applied to the olive fruit fly system is the use of augmentoria (singular: augmentorium) to reduce pest numbers while enhancing parasitoid numbers in olive fields (Klungness et al., 2005; Jang et al., 2007). An augmentorium is a tent-like structure where infested fruits can be deposited. The augmentorium is covered with a net whose mesh size is small enough to sequester the adults of the pest that emerge from the infested fruits, preventing them to enter the field, while being big enough to allow parasitoids emerging from infested fruits to escape, thus enhancing their numbers in the field and their pest suppression action in the field. This approach has been used with success to control fruit-infesting tephritid flies in several tropical islands, including Hawai’i and La Reunion Island. The low cost and low maintenance associated with this strategy, coupled with its easiness to use and overall effectiveness have made it a popular management strategy among growers. Recent collaborative research between EBCL USDA ARS and USDA ARS Hilo (Hawai’i) has demonstrated the potential for application of this method to the olive fruit fly system (Desurmont et al. 2022, Journal of insect science).

Test of different meshes conducted to identify the optimal mesh size of an augmentorium
adapted to the olive fruit fly system