Hull split is one of the most critical decision-making periods of the year for PCAs that that manage spider mites in almonds.
At this time, hot weather, dusty conditions, tree stress due to crop load, and reduced irrigation can all cause significant outbreaks of spider mites. Hull split is also often the last opportunity to utilize miticides in a spray program due to PHIs, as well as an opportunity to get a free ride through the field with a navel orangeworm spray.
However, not all fields require hull split miticide sprays. In many cases, beneficials are sufficient to keep spider mite populations in check. Some of the most common predators include six-spotted thrips, mite destroyer beetles, minute pirate bugs and lacewings.
Predatory mites can also be very effective, though there have been many observations that their numbers have been declining over the past few years, especially in the lower San Joaquin Valley.
• Deciding if a treatment is needed
Monitoring and treatment guidelines can be found within the University of California Pest Management Guidelines for Almonds (www.ipm.ucdavis.edu).
Treatment decisions are made based on the percentage of leaves with mites present on them. If no predators are present, treatments should be made if about 25 percent of the leaves have mites on them.
If predators are present, treatments can be held off until about 50 percent of the leaves are infested. As with any monitoring program the sampling accuracy increases as the number of trees sampled and number of areas in the field sampled increase. The UC guidelines that can be found online take this into account.
PCAs should also take into account additional information such as drought stress to the trees and mite history within the block. At hull split it is also wise to adapt the thresholds a little to consider that hull split may be the last opportunity to get into the field and spray before harvest begins.
Choosing a miticide
There are several miticides to choose from at hull split. The most commonly used include Acramite, Envidor, Fujimite, Kanemite, Oil, Omite and Zeal. Each of these products can do well at hull split, depending on what you want to accomplish.
For general use at hull split, Fujimite and Envidor provided the best overall control in University of California trials over the past three years in Kern County.
Plots treated with these products either did not, or rarely had mite populations return to pretreatment levels for the duration of the trial. Decisions between using these two products generally relate to pest population and price. Fujimite is more expensive, but can knock down mites quickly if they are getting out of control. It is the closest replacement that exists for the niche previously held by Omite. Compared to Fujimite, Envidor is less expensive and provided similar residual control in trials, but is slower acting and should be used at a normal treatment threshold and not as a product to try to put out fires.
The next best miticides in our trials were Acramite and Zeal, which provided excellent knockdown of mites for two to three weeks. Compared to Fujimite and Envidor, mite populations were similar for the first two to three week period, but were then quicker to return to pretreatment levels in about three to four weeks. In the case of Zeal, this is likely due to a reduced ability of the product to enter into the leaf at hull split compared to early in the year when the leaves are softer.
Another consideration with these products is that out of the top four products in our trials, Acramite is considered to be the safest to predatory mites. All four of the top products are relatively safe to predatory insects.
The other products evaluated were Kanemite and 2 percent 415 oil. Both treatments provided good knock-down of mites, but little residual control. The best fit for Kanemite in almonds is where very quick knockdown of mites is needed from a product with a very short PHI. The best fit for oil is to assist in mite suppression when it is used as a surfactant at about 1 percent v/v, or as a stand-alone treatment at 2 percent v/v where mite suppression is needed during the season or near to or between harvests.