David Haviland, UC Cooperative Extension, Kern Co.

Over the past month I have received numerous phone calls and texts asking different versions of the same question: When almond prices are low, how can a grower cut costs?

This question is often a tricky one to answer because in almond production, you have to spend money to make money. Trying to save money by reducing nitrogen or water costs can lead to yield losses and lower returns, just as skipping a fungicide spray can lead to losses from disease.  The same can be true for the management of arthropod pests such as navel orangeworm, spider mites, peach twig borer, and leaffooted bug.

However, in the case of arthropod pest management, not all insects and mites need to be treated every year, and there are cases where almond growers have a history of spraying more than is necessary, especially at the timing referred to as ‘May sprays’.

Peach Twig Borer

In the Sacramento Valley, the term ‘May spray’ had its origins with peach twig borer (PTB). “May” sprays referred to the timing at which insecticide applications should be made, based on degree-day models, to control PTB if they had not already been controlled during the dormant season or at bloom.  However, despite its historical significance as an almond pest, damage by PTB has become almost irrelevant throughout California for trees that are bearing fruit.  Most growers, including those who capture hundreds of PTB in traps, typically report very low to negligible levels of PTB damage in their official USDA kernel quality assessments.  These assessments, of course, are what counts.

There are currently two main theories for why PTB is not the problem it used to be.  The first is that modern-day precision irrigation practices allow for uniform shell expansion, leading to a good shell seal.  When there is a good shell seal, PTB tends to feed outside the shell on the inside of the hull.  This is in comparison to the historic use of flood irrigation, where wet-dry cycles during shell expansion caused cracks and splits in the shell that facilitated PTB access into the marketable kernel.  The second theory relates to improved biological control.  Historically, dormant and in-season applications of organophosphates reduced populations of all ant species.  However, modern-day use of selective ant baits allows for control of pestiferous ant species, such as southern fire ant, while allowing the survival of the beneficial native gray ant.  This species of ant feeds on PTB larvae, especially while they overwinter inside hibernacula on the trunk.  Reductions in the use of organophosphates (Lorsban^®, diazinon, etc.) and pyrethroids (Brigade^®, Warrior^®, Asana^®, etc.) have likely also benefited species of parasitoids that attack PTB.

For growers that are unsure about whether or not they need a “May” spray for PTB, the UC Statewide IPM Program recommends evaluating shoot strikes in late April.  If it is easy to find four strikes per tree on a mature tree, a treatment may be warranted, though my personal opinion is that threshold should be increased for reasons previously described.  Non-bearing trees, of course, are a completely different story because unlike bearing trees, where the primary economic damage is feeding on the kernels, in nonbearing trees the primary concern is the impact of shoot strikes on scaffold and shoot development.

Spider Mites

Another common target for “May” sprays in the Sacramento Valley is the two-spotted (and possible Pacific) spider mite.  May sprays for spider mites were invented in the early 2000s by almond growers in the lower San Joaquin Valley. At that time in Kern County, it was common for almond orchards to become defoliated by June in the absence of intervention, and it was quickly determined that newly-registered abamectin (Agri-Mek^®, etc.) was most effective through its translaminar activity if applied in May before leaves hardened. Ten years later, as generic abamectin products became available and active ingredient prices dropped, May sprays became adopted more widely throughout the entire San Joaquin Valley. Adoption eventually reached a point that ‘preventative’ mite sprays in May became the industry standard practice.

Fast forward to 2020 and things have totally changed. It is nearly impossible to find mite-induced defoliation in a bearing almond orchard until after hullsplit, and biological control of spider mites is at an all-time high, thanks primarily to ‘greener’ production practices that promote sixspotted thrips. Recent research projects in the lower San Joaquin, upper San Joaquin, and Sacramento Valleys have all shown that this natural enemy is present and important.  In the Sacramento Valley, sixspotted thrips in some locations have displaced predatory mites as the most important mite predator.  Overwintering adult sixspotted thrips have been shown to become active in the Sacramento Valley in the beginning of May, at the same time that spider mite populations sometimes increase.

In an effort to maximize biological control, miticides should only be applied in the spring if a treatment threshold of 40% of leaves infested has been reached, and there are less than 2 sixspotted thrips in two Pherocon predator traps (Trécé, Inc) within a week, or less than 3 sixspotted thrips in four traps.  Surveys have shown that in most orchards most years, sixspotted thrips density far exceeds this threshold at the time May sprays are needed.  The exceptions are typically cases where the grower has done something that disruptions biological control, such as a pyrethroid spray for leaffooted bugs. It is time for almond growers to make ‘preventative’ miticide sprays obsolete. Abandoning this practice will also help save on costs in a tight year.

Navel Orangeworm

May sprays for NOW have always been problematic.  The most common insecticides for lepidopteran worms, including methoxyfenozide (Intrepid^®) and chlorantraniliprole (Altacor^®), do not kill overwintering larvae and pupae that are still in the mummy, do not kill adults that fly in the spring, and can only provide control from the time eggs are laid until the new larva passes below the mummy surface.  Unfortunately, the first NOW flight occurs over a long period of time such that even a perfectly timed spray will only kill a small segment of the first-generation eggs and larvae.  This also assumes that coverage is excellent, as eggs laid into cracks and crevices without insecticide residue have a good chance for survival.  May sprays also do not directly protect kernels of the current year’s crop, including from moths immigrating into the orchard from neighboring tree crops as hull split begins.

Cost-wary growers who plan to make one spray for navel orangeworm should apply it at the beginning of the hullsplit. If a second spray is needed, it should be applied towards the end of July as residues from the first spray break down, pollinizers are splitting, and the third flight is approaching.  In the Sacramento Valley, especially in locations and years where NOW only has three flights (compared to four flights each year in the San Joaquin Valley), the need for a “May” spray is extremely rare.

 Leaffooted Bug

Insecticide applications for leaffooted bugs in the spring are only needed sporadically.  Cases where treatments are warranted most commonly follow mild winters in orchards near excellent overwintering sites.  Key examples include areas that contain plants with foliage, such as urban landscapes, Cyprus trees or citrus orchards; or lots of debris, such as riparian areas along streams or rivers.  From March to May, monitor almond orchards for the presence of leaffooted bugs, gummosis associated with a puncture mark on the almond hull, and aborted nuts.  If found at levels that are not acceptable, consider a treatment.  Otherwise, save your money.

Interactions

After monitoring individually for PTB, NOW, leaffooted bug, and spider mites, the verdict in most cases will be that no insecticide applications are needed in April or May. However, when exceptions for one of these pests occur, growers should make sure they get the best bang for their buck.  If spraying for PTB, choose a product that is also effective against NOW.  If spraying for NOW, consider applying it at a timing that is optimal for PTB.  If spraying for leaffooted bug, consider the relative merits of different options: pyrethroids are inexpensive and effective but kill natural enemies, abamectin has short-lived effectiveness but is safe on most natural enemies other than sixspotted thrips, and clothianidin (Belay^®) is more expensive and has short effectiveness but preserves natural enemies.  If applying a miticide, weigh your options between using abamectin (least expensive but toxic to sixspotted thrips) or a more expensive miticide not known to impact natural enemies that may help reduce the risk that another miticide treatment is needed later in the season.

When all things are considered, limiting “May” sprays have great potential to serve as a low-hanging fruit for cost-wary almond growers in 2021.  For more information about making ‘May spray’ decisions, consult the UC IPM Pest Management guidelines for Almonds or contact your local UC Cooperative Extension office.