Sudan Gyawaly, UCCE IPM Advisor, Butte, Colusa, Sutter-Yuba, Glenn, and Tehama Counties
Jhalendra Rijal, UCCE IPM Advisor, Stanislaus, Merced and San Joaquin Counties

Brown spots caused by feeding damage from true bugs can be graded as a defect and so are an increasing concern for almond growers. These pests include various species of stink bugs and leaffooted bugs, which feed on many crops and commonly found in tree fruit and nut orchards, tomato fields, and other crops. So, depending on the cropping systems around almond orchards, these insects may be present in or migrate into the orchard throughout the season, and season-long -monitoring is critical. All these bugs have piercing-sucking mouthparts, and their infestations in almonds can lead to nut abortion, nut drop, gumming, and/or brown spots on the kernel. This article discusses key true bug pests that infest almonds and important considerations for their management.

Leaffooted bug. Leaffooted bugs are the commonly known true bug pests in almonds. Although they may be present in orchards in small numbers throughout the season, the most significant damage occurs during the earlier part of the season (from March to May). Their feeding early in the season results in nut abortion, significant nut drop, and defective kernels (i.e., gummy nuts). Leaffooted bug infestations on almonds after the shell hardening result in brown spots on the kernel or wrinkled and misshapen kernels. 

Native stink bugs. There are at least four species of native stink bugs that can be present in almond orchards. These species include green stink bug, red shoulder stink bug, Uhler’s stink bug, and consperse stink bug. However, the green stink bug is the most common, causing most brown spot damage in almonds. Native stink bugs, including green stink bug, are usually a problem in almonds later in the almond maturity, primarily due to migration from other hosts such as tomato, corn, wheat, and other crops near almond orchards. The green stink bug can produce gummy fruits even after the shell hardening. Recent observations showed that about 1/3rd of those gummy fruits result in kernel damage, mostly brown-spotted kernels, during harvest.

Invasive brown marmorated stink bug. While the brown marmorated stink bug (BMSB) is present in some urban and residential areas, it has not yet widely spread to almond orchards in the Sacramento Valley. BMSB is already established as a significant pest in many orchards in the Northern San Joaquin Valley, where it is prevalent. Early-season infestations by BMSB in almonds can cause nut abortion and drop, similar to those caused by leaffooted bugs. They can also penetrate the shell and damage kernels even after shell hardening, causing brown spots on the kernel or lea

ding to wrinkled, misshapen kernels. The key factor driving high levels of BMSB damage is the presence of overwintering sites such as woodlots, human-made structures and other hosts in the landscape, as these bugs prefer to move from one host to another throughout the season.

Boxelder bug. In addition to the large true bugs discussed above, boxelder bug can become an economic pest if an orchard is in an area that has other host plants that are common in riverbanks or similar places. Boxelder bug damage has been observed in several almond orchards in the Northern Sacramento Valley in recent years. Boxelder bug infestations before shell hardening reportedly cause nut drops and kernel damage, so their presence in high numbers early in the season can be a concern. This insect has much shorter mouthparts than large bugs, which likely won’t cause kernel damage after shell hardening.

Monitoring and treatment decisions for true bug pests in almonds. 

While effective traps and lures are available for invasive brown marmorated stink bugs, effective traps and lures are not available for other stink bugs in almond orchards. The lack of monitoring tools, especially for green stink bugs, makes it difficult to detect and estimate the pest pressure in the orchard, making management decisions difficult. For leaffooted bugs, a new trap and lure have recently become commercially available. This can be a good detection tool, especially in the early season, as migrating insects can be intercepted in these traps when adults are moving into almond orchards. True bugs are sporadic almond pests, and they do not have action or economic thresholds. So, the best way to minimize their damage is by conducting regular orchard scouting and taking timely action for control. Starting in mid-March, monitor the orchard weekly for signs of true bug activity, including dropped nuts, nuts with gummosis, and adult bugs, their nymphs, or egg masses. Some of these bugs are difficult to spot in the orchard, so finding the infested nuts is the best way to detect them. If gummy nuts are present but no bugs are visible, inspect the gumming fruit by cutting a cross-section of the damaged area to check for feeding puncture marks (Fig. 1). This helps distinguish pest damage from physiological causes of fruit gumming. Leaffooted bugs are strong flyers and their feeding damage can be scattered, while stink bugs are not as mobile and the damage from their feeding tends to be clustered.

Insecticide spray decisions against these pests are based on orchard damage history and current season bug activity. Since the spray target is adult control and large insect adults are hard to kill, broad-spectrum insecticides such as pyrethroids (Brigade, Warrior, Assana, etc.) provide a good, long-lasting control. These pesticides are also damaging to mite and scale predators, so the decision to spray should be weighed carefully with the possible need for a miticide or scale spray later in the season. Spray decisions are based on the pest:

Leafooted bugs: Typically, if the bug(s), their eggs, or damaged nuts are found in the orchard in March-May, a spray should be considered to reduce further damage.

Stink bugs: In orchards where pyrethroid use is common, just one application every three years is usually sufficient to prevent economic damage. Where pyrethroids are not commonly used and bug damage is found, it may be necessary to use a broad-spectrum pesticide (pyrethroid) to limit the bug damage.  

Additional active ingredients are also available for rotational purposes; visit the UC IPM guideline pages on Leaffooted Bug and Stink Bugs for more information.