Notes on Nut Insects
Husk maggot
Identification
Walnut husk maggot (Rhagoletis sp.) is a common pest in Ontario and will
infest nut husks of English walnuts, Japanese walnuts and native black
walnuts. The adult fly, slightly smaller than a housefly, is light brown
in colour. Its wings have a dark cross bar pattern. Husk maggot can be
differentiated from other fruit maggot pests by the wing pattern. See
Figure 1, Wing Patterns of Walnut Husk Fly Compared to Other Common Fruit
Flies. The larvae are a cream colour and grow to 1 cm in length.
Figure 1. Wing Patterns of Walnut Husk Fly Compared
to Other Common Fruit Flies
Period of activity
Female adult husk maggot flies lay eggs in the husks of maturing nuts
in late summer and early autumn. The eggs hatch and larvae burrow into
and feed in the husk, which produces black slimy husks that stain the
nutshell black. Larvae may be seen crawling on husks as they move from
one nut to another. Husk maggot overwinter as pupae in the shallow soil
and decaying litter under trees.
Monitoring
Use yellow sticky board traps to monitor when adult husk maggots emerge
prior to egg laying. See Suppliers of Pest Monitoring Equipment and Biological
Control Agents, on the OMAFRA website.
Management Notes
Husks severely infested with maggot at harvest are very difficult to
remove from the nut, either by hand or by mechanized de-husking machines.
This reduces the crop value. Shell staining makes the nut crop unattractive
and undesirable to growers and consumers. Infested husks may impart off-flavours
through the shell into the nut kernel and spoil -marketability.
The pick up and removal of infested walnuts from the orchard floor as
soon as possible after they fall from the trees can achieve some control
of husk maggot.
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