Melon Fly

Melon flies could end up costing California farmers over $4.5 billion if they ever get a toe-hold in the state.

Melon flies (Bactrocera cucurbitae) are a type of fruit fly. Native to India and Asia, melon flies were first seen in Hawaii in the late 1800’s. They have now become a devastating pest on the Islands. Quarantine stations have worked long and hard to prevent this pest from entering the Continental U.S. The melon fly was first seen in California in 1956, and several other times since, but whenever melon flies are identified stateside, eradication programs immediately go into affect. These programs use pheromones to attract male melon flies. These males are then sterilized and released. This messes up melon fly breeding. So far, this method has been effective. So, why would a gardener care, if the pest isn’t even here? Because maybe it is.

Adult melon fruit fly (USDA) Public Domain

Melon fly host plants

It would probably be easier to list the plants that are not seen as food by melon flies, but it is important to know where to look, and to know what to watch for, so here’s the fruit fly menu of favorites from your garden:

• fruit trees - apple, avocado, fig, guava, mango, orange, papaya, passion fruit, peach, pear
• cucurbits - cantaloupe, chayote, cucumber, gourds, melons, pumpkins, squash, watermelon
• nightshade family - eggplant, peppers, tomato
• legumes - beans, cowpeas
• brassicas - cauliflower, mustard
• grapes
• okra

Melon fly description

The size of a house fly, melon flies are mostly orange or yellow and brown with a pale black T-shape on the abdomen and distinct wing patterns. Wings are clear with a large brown spot at the tip and a brown stripe along the back edge and along the base. Melon fly antennae also have an especially long third segment. Melon fly larvae (maggots) are creamy white, without legs, somewhat flattened at the back end. Maggots are less than 1/2 an inch long. Pupae are somewhat smaller than the maggots, held in a protective case that can be dull white or red, or brownish yellow. Eggs are very tiny, white, and somewhat elliptical.

​Melon fly lifecycle

A single female melon fly can lay 1,000 eggs. Eggs are laid on young fruit and tender new stems, which will provide food for newly hatched maggots. Eggs that have been laid under the skin of fruits, or in host plant stems, flowers, and exposed roots, will hatch and the feeding damage begins. There are three larval stages, or instars. After feeding continuously, mature maggots drop to the ground, where they burrow into the top inch of soil and enter a pupal stage. There can be 8 to 10 generations a year.

​Melon fly damage

During the heat of the day, adult melon flies rest on the shady undersides of leaves. When temperatures are more comfortable, they feed on nectar, decaying fruit, sap, and bird poop. [Keep in mind, as these pests fly from one food source to another, they can be carrying pathogens from the bird poop to your fruit crop.] Tunneling and feeding create points of entry that allow bacterial, fungal, and viral diseases to enter. Generally, the fruit ends up rotten and inedible.

Melon fly control

Unfortunately, there are not any effective controls available to the home gardener. You can certainly rake up the soil under and around potential host plants, to spot, remove, and report any pupal cases you find, and be sure to quarantine new plants. Currently available insecticides have not been found to work against melon flies.

If you think you see a melon fly, please make every effort to capture or kill it. Then call the CA Pest Hotline at 1-800-491-1899, or your local Department of Agriculture, to report it. Only by working together can we protect commercial agriculture and our own gardens from the melon fruit fly. And don’t smuggle fresh fruit or produce across state lines. There’s a lot more at stake than you might think.