Flower Thrips and Odd Almond Foliar and Stem Growth

Cameron Zuber, Orchard Crops Farm Advisor, Merced and Madera Counties

David Haviland, Entomology and Pest Management Advisor, Kern County

Almond growers commonly observe the condition of shoot terminals as an overall indicator of tree health and vigor. Long internodes with healthy green leaves are seen as an indicator of high vigor and tree health, whereas shortening internodes, and small or discolored leaves are a cause for concern. This summer we received multiple calls from growers with trees that exhibit good health and vigor, but where the terminal ends of stems have become stunted with wavy and odd-looking leaves. This damage is sometimes caused by a relatively unknown pest of almonds: western flower thrips (Frankliniella occidentalis).

Figure 1. Photos of tips of leaf shoots in June 2023 showing shortened internodes and stunted leaves.

Western flower thrips have rasping-sucking mouthparts. They are most known for feeding on the flowers on a wide range of plants, but they can also feed on really soft plant tissues, such as the new growth at the terminals on an almond tree. When this occurs, internodes become increasingly shorter and new leaves get smaller (Figure 1). Observations made in Kern County in previous years have shown that as soon as the thrips are removed, the internode length and leaf size respond by returning to normal.

There is a direct correlation between how soft the tissue is, and the thrips ability to feed on it. Trees with rapidly-expanding vegetative growth are at highest risk due to that softer tissue. This is most common in young trees that are overfertilized, overwatered, highly vigorous varieties, or varieties on highly vigorous rootstocks. The risk of thrips feeding decreases significantly as tree begin to bear fruit and carbohydrate sinks shift from vegetative to reproductive growth, or as the season progresses and trees and their leaves become hardened off.

Thrips damage should not be confused with zinc deficiency, which can result in similar symptoms. However, symptoms of zinc deficiency typically show up later in the season, like after harvest, when the risk of thrips feeding decreases.

Western flower thrips should not be confused with sixspotted thrips (Scolothrips sexmaculatus). Sixspotted thrips is a predatory species of thrips that feeds almost exclusively on spider mite and are typically found on mature leaves feeding on mites and mite eggs within webbing. Adults are easily recognized due to the presence of three distinct black spots on each of the wings (Figure 2).

Figure 2. Adult sixspotted thrips (Scolothrips sexmaculatus). Bottom photo shows sixspotted thrips feeding on a spider mite (bottom photo source Jack Kelly Clark, UC IPM Program).

The best way to monitor for western flower thrips is to tap the ends of impacted stems into the palm of the hand or a clipboard. Then look for small, cylindrical, yellow-brown organisms running around. If you are able to observe the western flower thrips closely under magnification, they have long cylindrical bodies, fringed wings with lots of hairs, and black setae extending from the tip of the abdomen (Figure 3).

Photos of western flower thrips (Frankliniella occidentalis) collected in June 2023 from an experimental orchard in Parlier, CA. Top photo of thrips in hand after tap impacted end of stem into palm. Bottom photos of thrips collected into a glass jar, frozen, air dried, and then viewed using a microscope with 4.5 magnification.

In almonds, western flower thrips are suspected to come from the foothills as the resident vegetation dries. In years that the vegetation dries early (i.e., April to early May), western flower thrips may feed on the tender surface of hulls, causing them to have a purplish color. This superficial damage is of no concern as the inner kernel is left undamaged. In wetter years, like 2023, foothill vegetation did not dry until late May or June, hulls were sufficiently hardened off before the thrips migration and damage occurred. However, the relatively cool and humid spring extended the period of time almond leaves and new growth remained succulent and susceptible to feeding by the thrips before they hardened off when temperatures spiked in early July.

Research projects to document impacts of western flower thrips on crop growth or quality have not been conducted. However, it is presumed that stunting to new vegetative growth, particularly on young trees, has the potential to produce smaller trees, which could reduce yields in the future. For that reason, some growers have resorted to making insecticide treatments when the observe levels of stunting become unacceptable for them.

In general, treatments for western flower thrips are rarely warranted as the thrips coming from the foothills eventually fades away, die, or move to other locations in search of preferred places to feed, such as flowers. However, there are certainly cases where treatments are warranted, particularly in May or June if the stunting on new growth is excessive.

The most effective insecticides for western flower thrips are in the chemical class called spinosyns. This includes the active ingredients spinetoram (e.g., Delegate) and spinosad (e.g., Success or Entrust). If considering a treatment for western flower thrips, please understand that these products are also highly effective against the predatory sixspotted thrips that serves as the most important natural enemy of spider mites. Please weigh the benefit of controlling plant-feeding thrips against the detriment caused by reducing populations of predatory thrips.

If you are interested in more information on how to identify western flower thrips, please see this UC IPM page on identification tips for thrips that are relevant in citrus.

Special thanks to Jhalendra Rijal (Area IPM Advisor, Stanislaus, San Joaquin, and Merced Counties) and Houston Wilson (Specialist, Department of Entomology, UC Riverside) for their help with the initial identification of this thrips as well as insight used in this article.

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