Drosophilid Melting Pot: African Fig Fly Meets Spotted-Wing Drosophila in the U.S.
By Douglas G. Pfeiffer, Ph.D.
In 2012, fruit entomologists in Mid-Atlantic and New England states began to notice a new drosophilid fruit fly in our fruit plantings. It helped that we were already paying close attention to vinegar flies with the recent invasion and spread of spotted-wing drosophila (Drosophila suzukii), a disastrous pest of berry crops and cherries that had first reached Virginia and states northward in 2011. So, the advent of a reddish-brown drosophilid with spectacular, paired silver and black stripes along the head and thorax was hard to miss. In addition to grape clusters, large numbers of the flies were also seen in other hosts, such as tomatoes that had been otherwise injured.
This new species was identified as an invasive species originally from West Africa, the African fig fly (Zaprionus indianus). This week, several colleagues of mine at Virginia Tech (one of whom is now at Colorado State University), along with a fellow researcher at Animal Genetics, Inc., have published a profile of the African fig fly, its range and interactions with spotted-wing drosophila, and management options in the open-access Journal of Integrated Pest Management.
African Fig Fly Expands Its Range
Like spotted-wing drosophila from Asia, African fig fly has become a cosmopolitan world traveler. From its native West Africa, it has spread through most of Africa and much of India, Austria, France, Italy, Spain, Saudi Arabia, Brazil, Uruguay, Panama, and Mexico. It is a highly competitive species, and it disrupts native drosophilid communities in newly invaded areas. African fig fly made its New World debut in 2005, when it created problems for the fig industry in Brazil. The first North American discovery of African fig fly was in Florida in 2005, following an introduction separate from the South American introduction. However, evidence from both Virginia and Pennsylvania suggest it was present in both states at least a year before our detection. In the North America, the geographical distribution in winter appears limited to the deep South, but by late summer flies are found in New England, Wisconsin, Minnesota, Ontario, and Quebec.
African Fig Fly in Fruit
When we brought Virginia grape clusters to the lab expecting to rear out spotted-wing drosophila, often a large majority of adults represented African fig fly. This was surprising because an examination of the oviscapt (the flies’ egg-laying anatomy) did not show the large, serrated structure typical of spotted-wing drosophila.
African Fig Fly Interacts With Spotted-Wing Drosophila
We decided to delve further into the role of African fig fly in grapes, comparing development of both species in grape berries and evaluating potential interspecific competition. We created varying densities of larvae in both artificial media and several varieties of wine grapes. In order to control for the total number of individuals while evaluating the effect of African fig fly presence on spotted-wing drosophila, we compared treatments of equal numbers of spotted-wing drosophila and African fig fly with treatments that included only spotted-wing drosophila larvae equal in number to the total combined maggot load in the competition treatment (i.e., two African fig fly and two spotted-wing drosophila versus four spotted-wing drosophila alone). We showed that spotted-wing drosophila developmental time was often lengthened, and pupal size reduced, when in the presence of African fig fly. Under some conditions, survival of spotted-wing drosophila was also reduced in the presence of African fig fly. Hence, interspecific competition was more important than intraspecific competition.
The question remained on how African fig fly got into the grape berries in the first place, since it lacks the armored, serrated oviscapt of spotted-wing drosophila. We found that an African fig fly female will sometimes deposit her eggs near, or even in, the oviposition hole produced by a spotted-wing drosophila female. In effect, African fig fly can follow spotted-wing drosophila into a host fruit and outcompete the original colonizer, a partially developed form of kleptoparasitism.
Prospects for African Fig Fly as a Pest
Spotted wing drosophila is a notoriously difficult target for naturally occurring parasitoids because of its effective immunological system. African fig fly is at least as recalcitrant in this regard.
African fig fly may pose a problem for soft-fruited plants in two ways. Some berry crops may be susceptible to oviposition; successful oviposition attempts have been seen in both strawberry and raspberry. In addition, we have shown that African fig fly is able to take advantage of oviposition sites of another invasive species, spotted-wing drosophila, and impose interspecific competition on that species. This competition is more pronounced in smaller-fruited varieties. There may also be potential impacts on population dynamics of other drosophilid species—a topic for further studies!
Journal of Integrated Pest Management