Time is quickly running out for growers to hang walnut husk fly traps in their orchards for monitoring adult populations of this damaging pest.
University of California IPM guidelines for walnuts set June 1 as the deadline for putting up these traps in coastal areas and June 15 in inland areas. Usually, adults emerge from their overwintering pupal stage between late June and early September and as early as mid-May in coastal areas. Peak emergence is usually from mid-July to mid-August.
However, with an early crop season like this, WHF and other insects are also likely to be on a faster schedule. In fact, at least one Sacramento Valley trap had caught its first adult WHF of the season by the first week of June – much sooner than usual.
Although black walnut is the preferred host, WHF also can infest any variety of English walnut. Eureka, Hartley, Franquette, Mayette, Chandler, and Tulare are particularly susceptible, say UC Cooperative Extension farm advisors
About the size of a housefly, the WHF features a yellow spot just below the area where the wings are attached and a dark triangular band at the tip of the wings. It’s posed a serious threat to walnut growers along the coast and in the Sacramento Valley for some time. Now, it’s becoming more of a problem for growers in the San Joaquin Valley, as well.
The first signs of an infestation are small stings caused by females depositing eggs in groups of about 15 below the surface of the husk. Within five days, these eggs hatch into white maggots that later become yellow with black mouth parts.
Feeding on the husk for three to five weeks, these maggots turn the inside of the husk mushy and black. The outer skin of the husk usually remains intact, but its fleshy parts decay and stain the nut shell. These stains cannot be removed by normal bleaching procedures, and the nut is, therefore, unsatisfactory for in-shell sale. A husk fly infestation early in the season (late July to mid-August) leads to shriveled and darkened kernels, increased mold growth, and lower yields. Late infestations do little damage to the kernels but may stain the shells and make hull removal difficult.
Timely treatment can help limit such damage. As described in the UC IPM Guidelines for Walnuts (www.ipm.ucdavis.edu), it begins with the use of unbaited yellow sticky traps (Pherocon A [apple maggot] NB [no-bait], super-charged with ammonium carbonate) to attract adult flies for monitoring population levels.
Hang traps in the orchard as high as possible within an area of dense foliage on the north side of trees. If not hung high enough, they won’t accurately detect the first female with eggs.
Use at least two traps per 10 acres and place the traps in orchard hot spots: large shaded trees, trees growing in damp areas or near black walnut trees, and trees that were damaged by walnut husk fly the previous season.
Monitor traps at least twice a week, and, preferably, three times a week to avoid damage before the first treatment is applied. Write down the catch each time. As soon as flies are caught or whenever there is a sudden increase in trap catches, monitor for eggs.
Not every orchard requires treatment for walnut husk fly every year. When chemical treatment is needed, precise timing is critical. Correct timing is not the same in every orchard and varies depending on insecticide and monitoring method used.
More information on methods for monitoring WHF adults and eggs and treatment recommendations are available in the online UC IPM Guidelines for Walnuts (www.ipm.ucdavis.edu)
Husk flies are not a problem after husk split and treatments are not necessary if harvest will occur within three weeks. Growers with previous severe late damage from this pest may want to use ethephon to hasten maturity and husk split.