Walnut Husk Maggot

Common pest in Ontario walnut orchards. Husk maggot will infest nut husks of Carpathian (English) walnuts, Japanese (heartnut) walnuts and native black walnuts. Husk maggot is most common infesting nuts of wild black walnut trees that grow along fencerows, in forests and in managed forest plantations. Fall is when infestations are most easily observed.

Maggot larvae feed in the husk of walnuts. The larvae are not able to eat through the hardened shell that holds the nut meat. Therefore, larvae cannot cause direct feeding injury to the edible kernel inside the shell. Husk maggot larvae do cause indirect damage to the nut kernel and affect the appearance of walnuts when marketed in-the-shell.

While still on the tree, infested husks begin to decompose quickly staining black where larvae are actively feeding within the husk. Husks that are severely infested with maggot larvae by harvest time are very difficult to remove from the nut, either by hand or by mechanized de-husking machines. Shell staining makes the nut crop unattractive and undesirable to growers and consumers.

Infested husks may impart off-flavours in the nut meat, where juices of rotting husk and larvae excrement marinate the inner kernel for several weeks prior to harvest. Although the larvae can't feed directly into the kernel, juices from outside the hardened shell can seep through. Affected nut kernels may not be entirely healthy to consume as food. Husk maggot infestations can therefore seriously limit chances for marketing walnut nut crops.

Biology of walnut husk maggot

The biology of walnut husk maggot is similar to other fruit maggot pests such as apple maggot, blueberry maggot and cherry fruit fly. The adult husk maggot fly is slightly smaller than a housefly and is light brown in colour. The wings are clear with dark cross bar pattern. The larvae are cream colour and grow to 1 cm in length.

Figure 1. An adult Walnut Husk Maggot fly lays eggs into the ripening husk of a native black walnut. Black spots show where eggs have been deposited.

In late summer to early autumn, female adult husk maggot flies lay eggs in the husks of maturing nuts. The eggs hatch and larvae burrow into and feed in the husk, producing black slimy husks that stain the nut shell black. Larvae may be seen crawling on husks as they move from one nut to another.

Figure 2. Black slimy staining can spread quickly over the nut shell as larvae feed and grow inside the husk.

Soon after ripened nuts fall, the larvae leave the rotting husk and borrow into the ground where they pupate in shallow soil during winter. In the fall, squirrels often move infested walnuts a distance from parent walnut trees enabling husk maggot to spread to new areas.

Figure 3. Mature larvae of husk maggot

Growers can monitor for adult husk maggot flies using yellow sticky board traps. Traps should be placed in walnut orchard trees along scrubby fencerows or near forests where adult maggots may be active in wild walnut trees. Insecticide sprays should be applied when adults are observed on traps and before adult females lay eggs. Control is not possible after the eggs hatch and larvae have burrowed into the husk.

Currently, Guthion Solupak is the only insecticide registered for control of walnut husk maggot. Growers of tree nuts would benefit by having other insecticides registered to control husk maggot. For husk maggot control, days-before-harvest needs to be somewhat short for registered insecticides since Walnut Husk Maggot is a late season pest. Insecticides that are currently used to control apple husk maggot would be equally useful for controlling walnut husk maggot. Early spring curculio and codling moth are also economic pests of all walnut species grown in Ontario and growers would benefit with registered insecticides as alternatives to, or in addition to Guthion.

Figure 4. Walnuts free of husk maggot larvae (left) are much lighter in colour than walnuts blackened (right) by husk maggot larvae.

| Top of Page |