Description of the Pest
Walnut twig beetles are very small, about 1/16 inch (1.5 mm), reddish-brown bark beetles that bore through the outer bark and into the phloem of the branches and main stems of walnut trees. This species is characterized by four to six concentric ridges on the upper surface of the pronotum (the shield-like cover behind and over the head).
In California walnut trees, the only other related insect that might be confused with walnut twig beetle is the fruit tree pinhole borer, Xyleborinus saxeseni, which is an ambrosia beetle. Although X. saxeseni resembles walnut twig beetle in size and shape, there are clear morphological differences visible under a dissecting microscope or hand lens (see Figure 21 in Detecting and Identifying Walnut Twig Beetle: Monitoring Guidelines for the Invasive Vector of Thousand Cankers Disease of Walnut for more details). X. saxeseni bores into the xylem of walnut trees and colonizes trees that are typically in a later stage of decline than those favored by walnut twig beetle.
Walnut twig beetle larvae are white, C-shaped, and found in the phloem. Egg galleries created by adults are generally horizontal (across the grain) and the larval galleries tend to be vertical (along the grain). Male beetles bore into the bark first, creating a push-pin-sized hole in the tiny cracks and corky furrows of the otherwise smooth bark surface of English walnut. Several females then join the male and create the gallery system.
Males produce an aggregation pheromone, which attracts more males and females to the colonization site and results in a mass attack. The insect is present throughout California walnut orchards and in black walnut trees growing in windbreaks and surrounding riparian areas. It colonizes standing trees, fallen branches, and prostrate trees. Beetles do not bore through bark below the soil line to start their galleries.
The beetle is completely dependent on walnut, butternut, or wingnut trees as hosts. Although walnut twig beetle appears to prefer certain species of Juglans, all cultivars of English walnut seem to be susceptible to colonization by the beetle. Exhaustive and specific host suitability tests of English walnut cultivars have not been carried out.
Walnut twig beetles colonize branches of all sizes, but generally not those smaller than 0.5 to 0.75 inch in diameter; they will successfully colonize the trunk of a tree when the trees has begun to decline. Even in its presumed native host (Juglans major) and range (Arizona and New Mexico), it also colonizes the larger branches and trunk of trees, rather than solely infesting "twigs."
In California, adult walnut twig beetle is active in flight nearly year round. There are likely two to three overlapping generations per year. It has been caught in pheromone-baited flight traps during every month, although at extremely low levels in December and January. Flight appears to be triggered when ambient air temperatures exceed 65ºF (18–19ºC). Other physical factors such as wind speed, light intensity, and relative humidity also appear to play a role in mediating flight. The beetle prefers to fly when temperature is moderate and wind speed, light intensity, and relative humidity are low; it can be particularly active at dusk. Maximum flight occurs in May and June and then again in September and October. Adult flight activity declines drastically in November. Walnut twig beetles overwinter primarily as larvae and adults beneath the bark.
In addition to the physical damage from adult and larval feeding, the walnut twig beetle also carries spores of a pathogenic fungus, Geosmithia morbida. Numerous spores have been observed on the surface of the beetle's wing covers. As beetles penetrate the bark, the spores are transferred to the phloem. The infection of the phloem results in necrotic lesions (cankers) that encircle the gallery system. Because of the aggregation behavior of walnut twig beetle and the multiple inoculations, many cankers form and the ensuing disease has been called thousand cankers disease.
The numerous regions of necrotic (dead) tissue from gallery construction and fungal infection may overlap over time and girdle individual branches and main scaffolds, leading to branch flagging and crown decline. Presumably the flow of carbohydrates is disrupted as a result of the phloem necrosis.
The pattern of decline generally begins in the crown and moves downward as beetles colonize larger and larger portions of the trees. However, in some instances (e.g., in trees previously impacted at the base by crown gall or other diseases), walnut twig beetle first colonizes the Paradox or black walnut rootstock or the lower portion of the main trunk of the scion. As the crown is gradually killed in black walnut trees, epicormic branches (water shoots) may sprout from the trunk and larger branches. This has rarely been observed in English walnut trees.
Colonization of walnut trees by walnut twig beetle is a progressive process that can take many years. Trees that are over- or underwatered or that have various root diseases or mistletoe infections may be more susceptible to colonization and may experience an accelerated decline. Ultimately, it appears that colonization by walnut twig beetle and infection by G. morbida initiates a decline that will kill the tree. Death caused by the disease has not been quantified in California orchards because in most instances, growers remove walnut twig beetle and thousand-canker-disease-infested walnut trees from orchards before the trees succumb. Decline and mortality are more obvious in landscape black walnut trees, which are often left unattended for many years along California's rural roads and highways.
Capturing and identifying the tiny beetle is the key to early detection of the disease in new areas. If walnut twig beetle is detected in traps, survey nearby walnut trees to assess the extent of the beetle infestation and symptoms of thousand cankers disease. The outer bark can be peeled away to reveal walnut twig beetle galleries in the phloem, a key feature for diagnosing the thousand cankers disease complex.
Two parasitic wasps, a Neocalosoter and a Plastonoxus species, are prevalent in walnut orchards and help control this pest. Predaceous beetles such as Narthecius simulator, Parandrita cephalotes, and Temnochila chlorodia, and snakeflies (Agulla) may also contribute to biological control of walnut twig beetle.
Currently, orchard sanitation is the primary strategy for walnut twig beetle management within orchards. Remove infested wood piles and prunings during the winter months in advance of the primary period of walnut twig beetle emergence and flight activity in the spring (April–June). When possible, burn infested material to reduce the build-up of beetle populations.
Once a tree becomes infested, no control is available, so it is critically important that growers, practicing arborists, and landscapers be aware of this beetle and the accompanying disease and not move any infested wood.
The tiny beetles can survive undetected beneath the bark in dry walnut wood; they may emerge later and transmit the disease to other walnut trees. It can take up to 7 months for the walnut twig beetle to leave firewood-sized pieces of English walnut wood. It may take even longer to disinfest larger pieces of wood with thicker phloem and rough and convoluted (folded in on itself) bark.
Organically Acceptable Methods
Use cultural control in an organically certified crop.
Monitoring and Treatment Decisions
The purpose of trapping (see videos for the set up and maintenance of traps) is to detect the spread of walnut twig beetles or delimit (determine the boundaries of) a known presence of walnut twig beetle where it has been recently discovered. The trap and guidelines were developed in Northern California native black walnut and English walnut orchard ecosystems with a relatively high presence of walnut twig beetle.
A small multiple-funnel trap is baited with the male-produced aggregation pheromone lure. The baited trap captures both sexes of the walnut twig beetle while attracting few other insect species; it includes only low numbers of most other bark or ambrosia beetles, making detection of walnut twig beetle easier. Baited traps have been used primarily to detect the presence of walnut twig beetles; little information is available on how to use traps to assess the population density of the beetle.
Ideally, deploy pheromone-baited walnut twig beetle traps from March through November when ambient air temperatures exceed 65ºF (18–19ºC). Depending on available resources, more targeted detection protocols may include:
- Trapping for about six weeks from late August through mid-October or late April through mid-June
- Trapping for three weeks during May and June and three weeks during September and October
A primary consideration when selecting locations for traps and choosing a density of traps in the landscape is whether the objective is to detect a new presence or delimit a known population. Use a higher density of traps to assess the extent of an introduced population. If the goal is to detect a new population of walnut twig beetle over a large land area (e.g., an entire county), only a much lower density of traps would be economically feasible. Whatever the overall goal, traps must be placed near walnut trees [within 9.8 to 16.4 feet (3–5 meters) of the trunk].
Currently, no insecticides have been shown to reduce mortality rates of trees infested with walnut twig beetle; do not use chemical control at this time. Research is proceeding with systemic insecticides injected into the base of trees, but this is unlikely to be practical in orchards.
Take down infested trees and grind or burn (where allowed) on-site. Do not move or ship freshly-cut walnut branches, logs, or burls from infested areas; do not move even for woodworking purposes, because the beetles are very small and difficult to detect. Infested wood should not be moved off-site either.
Seasoning wood on-site for 2 to 3 years should allow walnut twig beetle and other woodborers time to leave the wood. However, it is still best to have all wood inspected by a knowledgeable entomologist or cooperative extension advisor prior to movement of the material from the site, even when properly seasoned and debarked. The disinfestation of burl and lower stem pieces destined for woodworking can be accelerated by steam heating the wood to a minimum outer sapwood temperature of 56°C maintained for 40 minutes. This wood should be shipped promptly, however, as walnut twig beetles may re-colonize steam-treated wood if the phloem is still fresh.
For a brief review of how to install and service of walnut twig beetle traps, see Quick Guide: Installing and Maintaining Walnut Twig Beetle Pheromone-baited Traps . For detailed guidelines on using pheromone-baited traps to detect and monitor walnut twig beetle, see Detecting and Identifying Walnut Twig Beetle: Monitoring Guidelines for the Invasive Vector of Thousand Cankers Disease of Walnut .