Provisional Guidelines For Brown Marmorated Stink Bug Control In Almond
Scientific name: Halomorpha halys
DESCRIPTION OF THE PEST
Brown marmorated stink bug, Halyomorpha halys (Family: Pentatomidae) is an invasive pest that was accidentally introduced into the Eastern United States in the late 1990s. Although it has been trapped in the Los Angeles area and elsewhere in California since the mid-2000s, the first large, established population of this pest in California was reported in 2013 in Sacramento County. It has become a nuisance pest there during late fall and winter months when large adult aggregations have been known to enter houses and apartments to overwinter. During spring and summer, this insect infests backyard trees and community gardens. As of spring 2020, there are a more than 16 counties with established populations in residential areas. In 2017, a brown marmorated stink bug infestation was first reported from a commercial almond orchard near Modesto in Stanislaus County. Since then, this stink bug has been detected alive and collected from monitoring traps in several commercial almond and peach orchards in the northern San Joaquin Valley area.
Brown marmorated stink bug adults are approximately 3/4-inch long and with shades of brown “marmorated” (marbled) coloration on the upper body surface. They are typical shield-shaped stink bugs, almost as wide as they are long. Adults and nymphs, except the 1st instar nymph, have a two white bands on their antennae and legs as clear identifying characteristics. Other related stink bug species commonly found in California (e.g., consperse stink bug, brown stink bug, rough stink bug, spined soldier bug) and the leaffooted bug (Family Coreidae) lack these key features. To distinguish brown marmorated stink bug (BMSB) from other similar-looking insects that you might find in an almond orchard, see Table 1 below.
Brown marmorated stink bug is a polyphagous insect that feeds on over 170 host plants in the United States, including ornamental and landscape tree species. Some major host crops include apples, peaches, nectarines, pears, cherries, grapes, peppers, tomatoes, sweet corn, beans, and soybean. This stink bug has become an economically-important pest of hazelnuts in Oregon. Adults commonly overwinter in human-made structures such as houses, barns, shops, and even in dry and dead trees during the fall and early winter.
In spring, within two-weeks after emergence, overwintering adults mate and are ready to deposit eggs in host plants. Females lay a cluster of around 28 eggs on the underside of leaves. Eggs are light green when first laid and gradually become whitish near and after hatching. A single female can produce over 480 eggs in a lifetime.
It takes approximately 964 degree-days (DD) (base = 57.2oF) for eggs to develop into adults, and adults of the first generation live about 45 days. Brown marmorated stink bug has two overlapping generations per year. There are five nymphal or immature life stages known as instars. First instars have dark, reddish eyes and a yellow-reddish body with a black stripe. They do not feed and are typically clustered around the egg mass after hatching. Second through fifth instar nymphs are dark black to brown and actively walk searching for food; only the adult stage can fly.
Brown marmorated stink bug has caused severe crop loss in commercial tree fruits and vegetable crops throughout the United States. It has caused damage to almonds and peaches in California since 2017, with economically significant damage being reported in a few almond orchards in the San Joaquin Valley area.
Early season feeding (March–May) by brown marmorated stink bug can cause substantial injury to developing nuts, including aborted and dropped nuts. Late-season feeding (June-July), in contrast, can cause shriveled and ‘dimpled’ kernels at harvest. Gumming can occur with early- or late-season brown marmorated stink bug infestations. Repeated feeding on developing almonds leads to gummy nuts with multiple puncture spots. Damaged nuts show feeding signs both externally (multiple gumming sites, light brown speckles, and yellowing) as well as internally (pinholes, water-soaked lesions, corky tissue, and internal gumming. See Table 2 below for a summary of brown marmorated stink bug damage compared to other large bugs found in California almond orchards.
Although adults cause more severe damage than nymphs, both nymphs and adults feed on almond. They feed by inserting their piercing-sucking, needle-like mouthparts into the fruit and other vegetative tissue. A digestive enzyme is injected through their mouthparts upon feeding to liquefy the plant tissue for easy uptake of nutrients. On the fruit, feeding results in a characteristic necrotic lesion on the hull.
Some of these feeding damage symptoms match those produced by leaffooted bug and native stink bug feeding on almond. However the timing and duration of damage varies among these pests (see Table 3).
Compared to these other insect pests, brown marmorated stink bug is a significant threat to almond production for a greater portion of the growing season and numbers of bugs found infesting almond orchards tend to be much higher.
Based on field observations, some varieties such as Fritz, Monterey, Aldrich, Butte are more susceptible than Nonpareil. The presence of adults and the damage have been noticeably higher in border rows, especially near sides of the orchard next to an open field or by alternate hosts such as tree of heaven, Ailanthus altissima. Tree of heaven plants tend to be more abundant in or around residential areas, including neighboring orchards.
Brown marmorated stink bug requires management at the landscape-level since it invades orchards from nearby woodlots, residential host plants, and overwintering structures. Therefore, orchard edges are at the greatest risk for crop damage. Monitoring brown marmorated stink bug using pheromone-baited traps and visual searches is important. If this insect is detected on almond acreage early season, targeted pest management can begin as early as March and continue through June or July in some late almond varieties.
Several generalist arthropod predators may provide biological control services by attacking this pest and its eggs. A common egg predator in California urban settings is the Trachelas sp. spider. However, it only damages a few eggs with each visit. Other predators include assassin bugs, ground beetles, lady beetles, jumping spiders, praying mantis, earwigs, ground beetles, damsel bugs, and certain species of crickets and katydids. Although these predators were reported to attack native stink bugs as well as brown marmorated stink bug in various parts in the U.S., their impact in reducing exotic brown marmorated stink bug densities is minimal.
Several species of tiny, native wasps parasitize the eggs of native stink bugs. However, the level of egg parasitization of brown marmorated stink bug by these species is very low, so they do not significantly reduce this pest’s abundance at present. Therefore, these native parasitoids may not provide adequate control to minimize the economic loss in commercial almond orchards.
In 2018, a brown marmorated stink bug-specific egg parasitoid from Asia, Trissolcus japonicus, was detected in Los Angeles County. Surveys from China, part of this parasitoid’s native home range, indicate it can provide effective biological control for this pest. Lab and field studies in North America show promise for targeted biological control because this parasitoid has a limited ecological host range with a strong preference for brown marmorated stink bug eggs. However, this parasitoid’s actual impact on this stink bug in orchard systems throughout California has yet to be evaluated.
Pay close attention to potential overwintering sites and alternate hosts such as tree of heaven if present nearby and eliminate them if possible. They support brown marmorated stink bug populations throughout the season and can serve as a source of orchard infestations.
Organically Acceptable Methods
Biological and cultural controls can be used in an organic production system. Trials with products approved for use in organic production have been conducted in other states in various crops with varying degrees of success. These include azadirachtin plus pyrethrins (Azera), azadirachtin plus pyrethrins plus potassium salts of fatty acid (Azera+M-Pede), azadirachtin (Aza-Direct), spinosad (Entrust), pyrethrin (Pyganic), and pyrethrin plus kaolin clay (Pyganic+Surround). No efficacy information exists for these products in California almonds. Check with your organic certifier and read label instructions before using any pesticides.
Monitoring and Treatment Decisions
A trap baited with a brown marmorated stink bug aggregation pheromone and pheromone synergist, methyl decatrienoate, is effective in monitoring adults and nymphs season-long, even when low numbers are present. Use sticky panel traps (9x12-inch double-sided sticky) that can be affixed near the top of a wooden stake at about 4 feet above ground level. The lures should be suspended on top near to the sticky panel. Use a minimum of three sticky panel traps per orchard. The traps should be placed in a border-tree row adjacent to open fields, houses, or any other potential overwintering site from which the adult bugs may migrate into the orchard.
Trap captures may not be representative of the brown marmorated stink bug density in the orchard since the performance of lures can vary with the crop monitored. In almond orchards, the trap counts often underrepresents the orchard population and damage; adults and feeding activities in the tree have been seen in orchards without trap captures. Therefore, it is important to regularly scout for this pest and its damage in the orchard if it is believed to occur in the area. Visual observations of the insect (egg mass, nymphs, adults) and damaged fruits (deformed or exuded nuts) and beat tray sampling (shaking branches/twigs to dislodge insects) are early detection methods. Visual observations of the border trees for the presence of insects and fruit gumming are highly recommended.
No treatment threshold has been developed for this pest in almonds. At this time, the presence of any brown marmorated stink bugs in traps together with a confirmation by visual surveys should be of concern since almond nuts are highly susceptible to abortion and drop. Consider a spring spray for control. However, proper identification is critical before making management decisions, as there are many other insects that can be mistaken for brown marmorated stink bug. One look-alike, the rough stink bug, does not feed on almonds and is a beneficial predator that assists with biological control in almond orchards. Use Table 1 above to distinguish brown marmorated stink bug from other similar insects before applying any insecticides for this pest.
Because brown marmorated stink bug is a newly established pest in California, a management program for California crops is still being developed. Consult your local UC cooperative extension advisor or agricultural commissioner for more information on management plans for brown marmorated stink bug in your area.
Pyrethroid insecticides are considered effective against this pest. However, pyrethroids have short residual control. They are also a poor fit in an integrated pest management (IPM) program, especially early season, because of their negative impact on natural enemies. For example, the use of pyrethroids in the spring can flare up mite population in almonds.
If multiple sprays are needed, consider border sprays for follow-up applications because of the brown marmorated stink bug’s tendency to be found near orchard borders nearest potential overwintering hosts. If frequent sprays are needed, consider alternate-row-middle applications, which are used in other tree crops such as apple and peach in other states. Rotate insecticides among products with different modes of action classes and group numbers.
Always read, understand, and follow the label directions before using any pesticide. Below are some insecticide options based on lab assays using detached almond fruit as a substrate, and on studies conducted on orchard crops in other states.
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