Difficult, chemical lures main control method, else avoid planning susceptible crops during fruit fly months
Fruit flies are the horror of commercial and backyard gardeners. Not only do they cause extensive damage to many fruiting plants, but they are hard to control due to their biology and lifecycle. The best route for the organic gardeners is to simply avoid planting host crops during the most active months of the fruit flies lifecycle (high summer, Jan-Mar in South Africa).
The Mediterranean fruit fly (or medfly), Ceratitis capitata, originated in sub-Saharan Africa and quickly spread throughout the Mediterranean regions. Globally it has become a problematic invasive species worldwide. It belongs to the Tephritidae family of the Diptera order or ‘true flies’, where the second pair of wings has been reduce to ‘halters’ (spoon-shaped wings) used for balance during flight.
A similar species, native to southern & eastern Africa, is the Natal fruit fly, Ceratitis rosa. It has displaced the Mediterranean fruit fly in several regions and can be more economically damaging that the Mediterranean fruit fly. It is similar looking to the Mediterranean fruit fly, but is slightly larger and has differently patterned wings and black spots on its thorax.
The species we have a problem with is the Cucurbit Fruit Fly, Bactrocera cucurbitae. It is a widespread problem of many plants in the Cucurbitaceae family, which includes squash, cucumbers and melons.
Both the Medflies and Natal fruit flies are polyphagous, meaning they feed on several varieties of fruiting plants. They prefer softer or thin skinned fruits, since the lay their eggs underneath the skins of fruits.
Here is a list of common host plants of importance for the backyard gardener:
We never had a big problem with the fruit flies, they usually attacked the apricots growing wildly in the veld and most of my produce was spared. During our high drought years 2014-2016, our spring rains where very late we have had an invasion of the Cucurbit fruit fly on my squash plants in the pumpkin patch, causing a 90% fruit loss of our second cropping round in January-March of 2014. We had some losses in the main garden as well, but the largest point of invasion was in the pumpkin patch. The cucurbit fruit fly is slim and larger than the Medfly. It is red with yellow spots on it abdomen. Uncharacteristically, it has non-iridescent black eyes and clear wings. Here is a photo of one ovipositioning. I also include some the damage it caused to the cucumber and squash fruits as well as some tomatoes and peppers. The most tell-tale sign is the wrinkling and bending of the fruit at the point of ovipositioning of the female flies. After which the larvae devastate the fruit from within.
In optimum conditions, the Medfly can complete its lifecycle (egg, larvae, pupae and adult) within 21 days. Whereas in cooler periods, below 10oC (50oF), the lifecycle either ceases or takes up to 100 days to complete. Females can lay up to 300 eggs during their life time. This effectively ensures a robust population that is challenging to control. Ovipositioning occurs underneath the skin of the fruit, where clusters of 75 eggs are laid. Larvae hatch after 1.5 – 3 days and immediately begin to feed and tunnel in the flesh. Larvae are typical white maggots between 7-11 mm in length. Larvae mature in 10-26 days depending on the host plant, after which they drop from the fruits to pupate in the surrounding soil. Pupae are 4 mm cylindrical shaped ‘pods’ resembling a cereal grain. Adults emerge in 6-13 days. Adults die within 4 months if no food is available (fruit, honeydew or plant sap). Some adults may overwinter cool temperatures or survive for up to 6 months in adverse conditions. Adult fruit flies are small (6 mm) and can disperse up to 20 km to seek a suitable host plant. Global fruit product trade has transported and introduced fruit flies to many new areas as well as re-introducing fruit flies to pre-exising populations.
Fruit flies cause extensive crop damage by the initial ovipositioning of females and the feeding of larvae. Both expose fleshy parts of the fruit which quickly become infected with secondary bacterial/fungal rot. The initial oviposition point causes a pitting of the skin and fruit, this makes the fruit grow at an angle (bend at the pitting point), thereafter larvae feeding and rots make the fruit soft and sunken. Most fruits cannot be rescued and the best remedy is simply removing the host plant before larvae fall from the fruits to pupate in the soil (resulting in re-infestation next season).
When it comes to fruit fly invasions, there is little to do on the organic side as most products available in South Africa are nasty chemicals or expensive lures based on pheromones. We decided not to fight the invasion, after failed attempts at making my own lures and bagging each fruit individually was much too laborious. Thus, we plant a huge amount of summer squash early in the season and rip them all out after their first cropping in December. Larger winter squash (Butternuts, Flat white boers and Hubbards) are more resistant as their skins are very hard to pierce, if smaller fruit become infected – soak them in water for 2 days to drown any larvae and throw away. Fruit fly populations are reduced drastically when there are no suitable host plants, therefore not providing any are the best way to control fruit flies organically.
One type of biological control of many pest species, especially flies, is to release sterile males into the environment. Males from a fly species are radiated to make them sterile. Afterwards they are released into the environment to compete with wild-type males and mate with females. No offspring are produced from sterile males and hence the population decreases with each successive fly generation and the release of sterile males each season. Unfortunately, irradiated males are less fit than wild-type males and often compete at a diminished capacity.
This technique has been successfully used on screw-fly, Cochliomyia hominivorax, a pest of cattle. It has also been implemented for the Medfly and Mexican fruit fly, Anastrepha ludens. This technique has been extended to several human pathogenic carriers such as:
Anopheles mosquito, carrier of Malaria,
Tsetse fly, carrier of Sleeping sickness,
Aedes mosquito, carrier of filariasis, dengue and yellow fever.