Plant and Stink Bugs of Almond Orchard
April is the time that we need to pay close attention to “true bugs” infestation in almonds. In entomological terms, true bugs or simply ‘bugs’ are the insects within a group (technically, sub-order) called ‘Heteroptera.’ All true bugs have a slender beak-shaped or “straw” like mouthparts (in technical terms – piercing and sucking type of mouthparts, see Fig. 1) to poke through the seeds or fruits and uptake the sap or juice. Although several small and large bugs may be present in almond orchards, in this article, we discuss economically significant large bugs – leaffooted bug, a few native stink bugs, and a new invasive stink bug.
Leaf-footed bug (LFB) adults are relatively large insects, brown, and about 1-inch long, with a narrow white zig-zag band across the back and have a leaf-like structure on the hind legs (Fig. 3G). LFB may not be a big issue in every almond orchard every year, but they can cause a severe economic loss when they occur. LFB adults overwinter outside but migrate to the orchard in the spring (April-May) as the temperature warms up. LFB feeding on young almonds causes nut abortion, and ultimately nut drop. Late season feeding (i.e., mid-May and after) causes feeding injury manifested by a clear gummings on the hull, and minimal nut drop. The majority of these nuts have kernel damage (gummy, dark spots; Fig 2) at harvest. Unfortunately, no specific trap/lure is available LFB monitoring. The best option is to conduct regular scouting in the orchard to look for egg masses, live bugs, and damaged or dropped nuts. Visually scanning of the nuts, especially on the sunny side of the trees on edges, helps to detect live adults. Almond varieties with softer shells such as Fritz, Sonora, Aldrich, Livingston, Monterey, and Peerless are more susceptible than Nonpareil. Although LFB has two generations per year in San Joaquin Valley, the overwintering adults moving into the orchard in the spring are the ones causing the most economic damage.
Native stink bugs (Green, Uhler, Red-shouldered, and consperse) have a shield-shaped body (Fig 3 A-B, D-E). Green and Uhler stink bugs both are green, are about the same size (¾-inch). The main distinction is that Uhler stink bugs are more rounded and have yellow spots on the back and head (Fig. 3B), compared to the green stink bug, which is solid green with yellow lining with dark spots on the entire body margin (Fig. 3A). Red-shouldered and consperse stink bugs are relatively smaller in size (1/2-inch). The red-shouldered has either red/pink band across the shoulder and has the ‘red feet’ (Fig 3D), while consperse stink bugs lack the shoulder band, and have yellowish legs with scattered dark spots on the leg (Fig 3E). Native stink bugs often overwinter as adults outside potentially in weeds and move into the almond orchard in summer (June-July). Some species (e.g., green stink bug) may overwinter within the orchard. Don’t confuse these pest stink bugs with the predatory one - rough stink bug (Fig. 3F), which can be found commonly in tree fruit and nut orchards in California. The rough stink bug is a faded gray or “dull” color compared to pests (consperse or BMSB) and has a toothlike projection on the lateral side of the head (Fig. 3F).
Native stink bug feeding can produce clear gumming on the hull (Fig 4), with potentially some kernel damage, but minimal nut drop. Also, this late-season feeding may cause external gumming to the hull, but most of that iinfestation can end up not impacting the kernel. Similar to LFB, visual monitoring of gummy fruits (Fig. 4A), barrel-shaped egg masses (Fig 4B), and live bugs are the methods to monitor stink bug activity in the orchard. Stink bugs are sporadic pests, and the damage level is not justifiable to spray insecticide.
Brown marmorated stink bug (BMSB) is a new invasive stink bug species and has established more than 40 states since its first detection in Pennsylvania in the early 2000s. BMSB is a fairly big (size ~ ¾ inch) and marble brown stink bug species. The most important characteristic to identify BMSB is having two white bands on its antennae and legs (Fig 3C). BMSB has over 170 host plants. In Central Valley, this stink bug was first established in the Sacramento area in 2013 but now found pretty much all counties from Glenn to Fresno, mostly residential areas. To date, damage in commercial orchards is limited to Stanislaus and Merced counties. BMSB feeding may begin as early as mid-March when overwintering adults migrate into the orchard and can feed on almonds throughout the season. They overwinter near farmland in homes, shops, woodpiles, etc. Early-season feeding (from the fruit set and before shell hardening) tends to be very serious as adult feeding at this time can cause substantial nut abortion and drop.
BMSB feeding signs in almonds include gummy nuts in a cluster with multiple feeding spots within the fruit (Fig 5). See various signs of BMB feeding in Fig. 5. Some of the signs of BMSB feeding resemble the LFB and other stink bugs, but the severity and timing of damage are different (see Table). BMSB damage occurs as early as mid-March and seems to continue for a few weeks to months. In the northern San Joaquin Valley, we observed BMSB infestation in multiple almond orchards in the past three years, with a substantial nut drop in April and sometimes until mid-May. The feeding signs found on the hull after the shell hardening (mid-May to August) is identical with the LFB bug and native stink bug damage; however, the degree of kernel damage by the late-season BMSB feeding (Fig. 6) tends to be higher than other bugs. Also, once established, BMSB abundance in the area tends to be steady, and that poses a higher risk. In our experience, Fritz, Monterey, Aldrich, and Butte are more susceptible than Nonpareil. Also, damage incidence is much higher in the border rows (Fig 7), mostly in a side of the orchard next to the open field or alternate hosts such as Tree of heaven, Ailanthus altissima. These trees are abundant in many residential areas, near to the highways, and in orchards that are closer to residential areas.
BMSB monitoring. Commercial traps and lures that attract both nymphs and adults of BMSB are commercially available to use. We recommend to use sticky panel traps (9 x12-inch double-sided sticky) that can be affixed to the top of a 5-ft long wooden stake, 1-ft of which is pounded into the ground. The lures can be suspended on top near to the sticky panel (see Fig 8). BMSB lures and sticky traps can be purchased from Trece Pheromone company. These traps should be placed in the border-tree row facing the open field and other potential overwintering sites. We recommend a minimum of three sticky panel traps in an orchard. Besides, it is vital to do a visual sampling of the orchard for stink bug life stages (Fig. 9) and damaged nuts. The visual observation and beat-tray sampling should also be focused on trees on the edges of the orchard.
Managing LFB and stink bugs in the orchard. It is crucial to conduct visual and trap-based monitoring (if traps are available) to detect bug activity in the orchard. There are several predators and parasites (e.g., birds assassin bugs, spiders, and parasitic wasps) out there to keep the bug population below a certain level. A known egg parasite, Gryon spp., can keep the leaffooted bug population under control in later part of the season. However, as egg parasites, they cannot control the adult leaffooted bugs in the spring. Seeing a few native stink bugs in the orchard in June may not need any kind of treatment as not all levels of pest pressure warrants treatment. There are currently no treatment thresholds for leaffooted and stink bugs in almonds. Spring infestations, either by the leaffooted bug or by BMSB, are more critical as the nuts are much vulnerable to abortion and drop. For all true bugs, pyrethroids (bifenthrin, lambda-cyhalothrin, esfenvalerate), and neonicotinoids (clothianidin) are considered effective. Keep in mind, though, that the use of these broad-spectrum insecticides in the early part of the season likely impacts the natural enemies of mites and other pests. Please follow the UCIPM Pest Management Guidelines for the stink bug and leaffooted bug for specific insecticide, rates, and potential effectiveness and impact on the natural enemies. Typically, the one-time spray should be enough for the leaffooted bug control in the spring, if scouting warrants a treatment. For BMSB, due to the overlapping generations and extended period of the adult presence in the orchard from spring through summer, multiple applications may be required. In that case, it is essential to rotate among the active ingredients. Our lab-based bioassays showed that bifenthrin, lambda-cyhalothrin, lambda-cyhalothrin+chlortraniliprole based insecticides are effective against BMSB in almonds. For organic productions, based on the trials conducted in other states in various crops, here are some insecticide products to use against BMSB: Azera (Azadirachtin + pyrethrin), Azera + M-Pede (Potassium salts of fatty acid), Aza-Direct (Azadirachtin), Entrust (Spinosad), Pyganic (Pyrethrin), Encap (Pyrethrum), Pyganic + Surround (Kaolin). Please follow label instructions before using any pesticides.