Spider mites are considered indirect pests in almonds, in the sense that they do not feed directly on the harvested product. Rather, they cause injury to plants by sucking cell contents from foliage. Signs of feeding injury include leaf stippling, yellowing, and dropped leaves. High populations of mites can also be recognized by webbing on leaves and tree terminals (Image 1). Significant spider mite injury can become economic crop damage in almonds in subsequent seasons in the form of reduced vegetative tree growth and crop reduction. In addition, excessive leaf drop can interfere with harvest operations and nut drying in the current season. The most effective spider mite management programs are focused on integrating multiple tactics including cultural practices, biological control, and miticide applications when needed.
Properly-irrigated, vigorous trees are less susceptible to spider mite damage. Ensure that trees are not stressed due to inadequate irrigation, fertilization, or other factors. Reduce dusty conditions by oiling or watering roadways and, where possible, maintaining ground cover.
Spider mites have a suite of natural enemies that can occur to varying degrees in the orchard environment. The most impactful of these biological control agents in California almond and walnut orchards are typically predator mites (Image 2) and sixspotted thrips (Image 3). When we think of biological control in practice in most orchard environments, we are largely discussing conservation biological control—in other words, with respect to the beneficial predators naturally-occurring in the orchard, “don’t starve them, don’t kill them.” This means that we have to be willing to tolerate some level of food source in the orchard to maintain predator populations. Food sources may come in the form of other mite species early in the season (e.g., European red mite, brown almond mite), as well as subeconomic populations of spider mites themselves throughout the season. It’s also critical to understand the impacts of all miticide and insecticide applications on natural enemies (link to resources containing this information to follow).
Managing spider mites in almonds in recent years has typically taken one of two general approaches in conventional orchards: applying a prophylactic early-season treatment or utilizing threshold-based treatment timings (later season) and conservation biological control. Both have pros and cons, and each method can be used to successfully manage spider mites. Which is “better” depends on a number of factors in your particular orchard operation, the typical abundance of natural enemies and how effectively they are conserved, and may vary depending on the environmental conditions year-to-year. Considerations for each method are summarized below.
Early abamectin treatments (May), once on the rise in many almond orchards, seem to be waning in popularity over the past few seasons. Growers and PCAs are increasingly reporting concerns about efficacy and resistance-management considerations. When applied properly and at the appropriate time if spider mite populations necessitate treatment and natural enemies are not abundant, abamectin can be an effective miticide and these early treatments can control mites into summer. Below are some key considerations regarding the most effective and responsible use of abamectin:
- Abamectin functions as a nerve toxin that must be ingested by mites. Once applied, the material must move into the leaf tissue, where it can then be picked up by feeding mites. This translaminar movement of the material works best prior to leaf hardening and when leaves are mostly free from dust and other residues. Applications before leaf hardening can be quite effective.
- Applying abamectin after leaf hardening (i.e., with hull-split sprays) may seem like an inexpensive insurance policy, even if the effectiveness of the material at this timing is greatly reduced. However, bear in mind two additional issues: this is the time when natural enemies tend to be more abundant if preserved early in the season (more on that below) and, from a resistance-management standpoint, two applications of the same active ingredient within the same season is not advisable.
- Abamectin is highly toxic to spider mite natural enemies, particularly sixspotted thrips and predator mites. Use of abamectin early in the season may contribute to later season spider mite flare-ups due to reduction or elimination of these beneficials in the orchard by direct toxicity and/or by reducing their food source (spider mites, European red mites, brown almond mites).
- Without beneficials to at least slow a mite flare up as the abamectin wears off (expect 60 days of activity if applied properly and at the tight time), spider mite populations can jump up to dangerous levels in just a couple of weeks in summer heat and water stress. Juggling irrigation/sprayer access, harvest prep activities, and crew availability to spray a sudden mite flare-up can mean the fix to a fast-moving problem isn’t fast enough, and that can mean dropped leaves at harvest. Lots of dropped leaves at harvest can mean slow drying nuts, slow nut pickup, longer water shut off and more orchard water stress that can translate to future yield loss.
- In years where spider mites are slow(er) to develop, “May sprays” of abamectin may be of very little value, as additional later-season sprays often become necessary regardless of early-season intervention, and natural enemies are unnecessarily disrupted. Weigh the pros and cons of the inexpensive insurance policy in treating below-threshold populations vs. destruction of natural enemies (FREE control) and consider how overuse of a particular chemistry over time can increase the likelihood of resistance development. Best to use practices that help maintain all of the tools in the toolbox so that they are available and effective when particular situations call for it.
- For a very good summary article on the uses (and misuses) of abamectin in almonds written by UCCE Entomology Advisor David Haviland, visit: com/2013/04/12/managing-mites-in-almonds-with-abamectin/.
- A blog post from Franz Niederholzer on continued mite monitoring and management after a May abamectin treatment can be found at: com/almonds/insects-mites/what-to-if-you-applied-abamectin-to-almonds-in-may/.
Threshold and Biological Control Approach
With this method, spider mites are treated once economic thresholds are reached (not before) and the overall goal is to maintain a balanced ratio of natural enemies-to-spider mites that will allow the beneficials to help suppress spider mite populations.
- Monitoring and treatment thresholds take into account the abundance of both the pest spider mites and their key natural enemies (predator mites and sixspotted thrips). For more details, refer to the decision tree (Image 4) and visit ucanr.edu/PMG/r3400211.html.
- As noted above, the basic tenet of conservation biological control (maintaining “good bugs” in our orchards to help control the pests) is “Don’t Starve Them & Don’t Kill Them.” Early season destruction of natural enemies and/or their food sources will likely mean that they will not be present, or not present in enough numbers at the right time, to provide measurable impacts later in the season when we need them to help fight flare-ups.
- Admittedly, predators alone may not be sufficient to keep spider mites below economically-damaging levels, and miticides may be needed based on your site-specific monitoring when thresholds are reached. Know which predators are present and choose materials accordingly. Using a miticide that is softer on beneficials helps keep them around to suppress spider mites missed by the pesticide. This link contains a table of almond pesticides and their impact on beneficials, including predatory mites and sixspotted thrips: ucanr.edu/PMG/r3900311.html.
- Best practices for getting the most out of your threshold-based miticide application 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.), obtaining optimal coverage (high volume, slow speed), and applying with oil or the recommended adjuvant.
- At the recent Almond Conference, Kern County UCCE Entomology Advisor David Haviland reported the dramatic impacts sixspotted thrips populations had on reducing spider mite levels in three orchards across Kern County in 2017 (available at com/growers/programs-and-events/almond-conference > select “Insect Pest Management Update”).
For more information on spider mite management (in both almonds and walnuts), with particular focus on monitoring and treatment thresholds, please see my article in the May 2017 issue of West Coast Nut and visit www.sacvalleyorchards.com.