August 1, 2002
Vol. 4 No. 7
Stink Bug Management: a research update
By Christian Krupke, WSU-TFREC, Wenatchee, WA
History and description of the pest
Stink bugs have been increasing as a pome fruit pest for approximately ten years in many fruit-growing areas of north-central Washington. There are several species of stink bugs commonly found in and around orchards, but the key pest species appears to be the consperse stink bug. The consperse stink bug is approximately 3/8" long as an adult, drab brown in color, with alternating off-white and black colored squares along its lateral margin on the back half of its body. There are several other stink bug species capable of causing fruit damage; however, the consperse stink bug is responsible for the vast majority of reported damage and all research findings detailed below are confined to this species.
Diagnosis of a stink bug problem: Stink bug vs. Bitter pit
Stink bug damage superficially appears very similar to the nutrient deficiency known as bitter pit, and correct diagnosis is key in making subsequent management decisions. The first thing to note is the time of year when damage is noted. Stink bug damage is first apparent in mid to late-July, generally increasing in severity right up until harvest. Bitter pit symptoms often appear earlier in season. Both stink bug damage and bitter pit appear as darkened, sunken areas on the fruit surface. However, stink bug damage is most frequently confined to the shoulder area (the bugs feed while hiding in the stem bowl) and the top one-third of the apple, while bitter pit is more apparent on the lower half of the fruit, especially near the calyx. The next step in damage diagnosis is cutting the damaged section to view it in cross-section. This is accomplished by cutting directly through the center of the damaged section, then making a second cut perpendicular and bisecting the first so that a wedge or triangle that can be removed. This will allow viewing of the damaged area in cross-section, exposing the differences in damage symptoms. Stink bug damage is generally conical in shape, with the light brown damaged area widening toward the skin of the apple. In contrast, bitter pit damage is generally circular in shape, and often may not extend all the way to the skin. Bitter pit is dark brown or almost black in mature fruit, whereas stink bug damage ranges from off-white to a medium brown color. Stink bug damage is usually greater on the borders of orchard adjacent to native habitats and is found on trees with both heavy and light fruit set. Bitter pit can usually be found throughout an orchard and is most common on trees with low fruit set. Finally, if stink bug is suspected as the culprit, catching the bugs in the act is the next step. This is most easily accomplished at or shortly after dusk, or early in the morning. Bugs will be found feeding while hiding in the stem bowl or in the sheltered areas between fruit and leaves.
A successful system of monitoring for presence/absence or relative numbers of stink bug is essential if we are to manage this pest successfully. We have worked with an aggregation pheromone for the past three seasons, and have found this pheromone attracts large numbers of both male and female adult bugs when placed on host plants, especially mullein. It is attractive to adults both during the initial mating period (April-June) and in the late summer when 'new', non-reproductive (but fruit-feeding) adults are present (late July-early September). While we are able to attract large numbers of bugs to baited mullein plants, the most difficult step in monitoring has been developing a trap to actually retain bugs, and this remains an ongoing research effort. An important discovery is that the vast majority of aggregation activity occurs at dusk, so any observations of aggregated bugs or insecticide treatments (more on this below) should be conducted at dusk or slightly thereafter. Although bugs can also be seen during daylight hours, we have found that these numbers represent only 10 to 50% of the nighttime totals. Commercial lures has been developed and field-tested for stink bug monitoring and at least one should be available in 2003.
Stink bug management: In-orchard control options
Chemical control of stink bugs in the orchard has been difficult because stink bug invasions occur late in the growing season, limiting the number of insecticides that can be used due to pre-harvest interval restrictions. In addition, there is a lack of solid information on what time of year the bugs are actually present in the orchard. To ensure proper timing insecticide treatments should not be applied until the first signs of damage are seen and confirmed as being caused by stink bug, as outlined in the section above. The first damage will usually be found on orchard borders in mid to late July in the highest pressure areas, usually adjacent to native habitats. Although Carzol has been recommended as an in-orchard treatment, only one post-bloom application of this material is permitted and then only when specific written recommendations from a certified pest control advisor are obtained. We have conducted insecticide screening tests with a variety of products, ranging from established products to some of the newer, 'soft' chemistries. A product that has looked promising is Danitol, a synthetic pyrethroid. However, this product has significant non-target effects, and in particular has been shown to cause flare-up of pest mites due to its negative impact upon beneficial mite species. We are currently conducting trials to determine the severity of these non-target impacts, in order to evaluate the possible role of Danitol as part of a management plan for stink bug. One possible alternative to whole-orchard treatments is treatment of the border rows only, or alternating treated/untreated rows in order to preserve a reservoir of predatory mites.
We have conducted a number of experiments to investigate the possibility of using the highly-attractive pheromone in combination with host plants, in an aggregate-and-kill strategy. In these tests mullein plants were baited with lures with stink bug pheromone at intervals of 20 feet along orchard borders beginning in mid-April (the onset of the mating period). These baited plants were sprayed using a backpack-sprayer to apply an insecticide (Carzol) early in the morning at weekly intervals throughout the mating period, or until early July at which point all baiting and spraying activities ceased. Although these early season bugs did not represent a direct threat to the orchard (they die after mating), they do represent the parents of the bugs that will invade orchards in late summer, and we wished to reduce this breeding population as much and as early as possible to impact mating/egg-laying. This treatment was evaluated by comparing stink bug fruit damage in baited and sprayed blocks vs. unbaited and unsprayed checks in a total of four orchards. The results were encouraging; stink bug damage was reduced by over half in the border rows of the treated areas (8.2% damage vs. 3.7% damage). This presents a highly-selective, low-input alternative to the spraying of orchards, but it is important to note that this treatment is not yet refined to the point where it is applicable to all situations. Our current system is dependent upon mullein plants as a 'trap crop'; a lure placed on inanimate objects or non-host plants will initially attract - but not retain - stink bugs. Not all growers with stink bug problems have mullein growing on borders, so we hope to find a synthetic lure or other attractants that may act as an "artificial plant" in retaining stink bugs so that insecticides can be applied.