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California Walnuts: More Heat Will Change Pest Dynamics – Be Alert

Owen Taylor
By Emily J. Symmes, Sacramento Valley Area IPM Advisor June 18, 2017

California Walnuts: More Heat Will Change Pest Dynamics – Be Alert

The impacts of such an unusually wet winter and prolonged spring rain events on insect and mite activity are still developing. This article summarizes some notable observations thus far and specific pest management considerations as we move into summer.

Codling Moths

The first codling moth biofix generally ranged from early- to mid-April in 2017 in the Sacramento Valley (many of the same orchards observed first biofix in late March the previous two years). As this goes to print, we are nearing 1060 degree days (DD) in most blocks and are on the lookout for the second biofix.

The first codling moth flight this year (1A and 1B peaks) were reported to be quite high in many orchards throughout the region, and applications targeting one or both of these flight peaks were common. Keep in mind that a good codling moth program can help minimize early-season infestation by navel orangeworm, so keep a close eye on your trap counts and the numbers of codling moth-infested dropped nuts in your orchards.

Although wet winters can cause increased overwintering mortality of navel orangeworm, significant survival was apparent in walnut mummies examined this spring (March 2017). Bear in mind that your neighbors may not have been able to execute their normal NOW sanitation programs last winter either, and these may be a source of populations moving into your orchard as the season progresses (in addition to any carry-over populations you may have in your orchard). Prior to husk split, NOW will go back into mummies and into this-season’s damaged nuts (codling moth-infested and blighted nuts).

Scale

Significant populations of frosted scale were evident in a number of orchards throughout the Sacramento Valley this year. It is not entirely clear why this pest, which it typically present only at low levels, has increased so dramatically in recent years. Research is underway examining this phenomenon, as well as best practices and timings for effective population reduction. Frosted scale produces honeydew when feeding (walnut scale does not).

This favors growth of sooty mold, which increases the chances for sunburn damage, so keep an eye on this if your orchard was heavily impacted by frosted scale this spring. Frequent rains throughout winter and early spring limited the ability for many growers to get into the orchards to apply dormant/delayed-dormant scale treatments, necessitating applications targeting the spring crawler stage.

Crawler emergence was observed late in the second week of May 2017 (compared to the first week of May 2015 and last week of April 2016), with peak crawler activity noted approximately two weeks later. This highlights the importance of monitoring the populations, rather than applying treatments based on “typical” calendar timing (which would have been too early this year to target peak crawler activity). Our research trials this year are examining efficacy of different treatment materials and timings specifically for frosted scale (walnut scale has been the focus of research in recent years).

If treatments were applied for frosted scale (either dormant/delayed-dormant or crawler), monitor the populations next dormant period looking for the overwintering nymphs (photos and more information at  ipm.ucanr.edu/PMG/r881300111.html). Frosted scale has only one generation per year, so the effects of this season’s spray program will be best observed at this time, and as next spring’s populations begin to develop.

Conversely, walnut scale populations have dropped off in many orchards, likely due to effective management programs over the past few years. We observed walnut scale crawler emergence during the last week of May 2017 (compared to first and second weeks of May in 2015 and 2016, respectively). Another reminder the importance that treatment timings should be supported by field observations of pest activity each year.

Husk Fly

Walnut husk fly traps just beginning to hit the week of June 5 in Butte and Sutter counties. No activity noted as of that time in our trapping locations in Tehama county. Remember that WHF treatment decisions should be made on a site-specific basis and take the time to hang traps high in the tree canopy – this will provide better accuracy in detecting activity. If trapping indicates the presence of treatable WHF populations, all insecticides should be applied with a bait (i.e., molasses, Nu-Lure®, Monterey Insect Bait®).

The exception is GF-120 which contains its own bait. For low- to moderate-populations, coverage is not critical and low-volume and/or partial coverage applications (e.g., alternate row) of bait with insecticide can be effective. However, in high population orchards with extensive previous damage, high-volume, full coverage, and/or multiple applications of bait with insecticide may be necessary to achieve adequate control.

If you miss a timing and are observing fresh stings, full cover neonicotinoids that have some ovicidal (egg-killing) activity mixed with an adulticide will provide partial control of eggs if applied immediately after stings are observed. Generally, a short-residual insecticide-plus-attractant will kill walnut husk fly for 10 days.

With the egg development period added to this time, there is about 3 weeks of protection after an application (GF-120 treatments often must be applied more frequently).

Spider Mites

Spider mite activity remains low at press time, but will likely begin to pick up with sustained warmer temperatures. Prophylactic May applications of abamectin, while still favored by many almond producers, have shown to be less effective and economically-viable in most walnut orchard situations. Treatments should be based on thresholds of spider mites and their natural enemies (particularly predator mites and sixspotted thrips).

In general, the goal is to manage the ratio of predators-to-spider mites (not just spider mite numbers alone) to achieve a balance in which predators can provide free control services. Also consider the impacts of other pesticides on spider mite and predator populations (organophosphates and pyrethroids are highly detrimental to spider mite natural enemies and often result in spider mite flare-ups).

More information on treatment thresholds for mites and predators in walnuts is available at: ipm.ucanr.edu/PMG/r881400111.html. Best practices for getting the most out of your miticide in walnuts include choosing the right material for the job (i.e., those softer on predators if they are present, desired residual activity and pre-harvest intervals, quick and effective knock-down if needed, etc.) and obtaining optimal coverage (high volume, slow speed).

Owen Taylor
By Emily J. Symmes, Sacramento Valley Area IPM Advisor June 18, 2017