June 11, 2003
Vol. 5 No. 3
By Mike Doerr, WSU-TFREC, Wenatchee, WA
Stink bug behavior, or misbehavior, in orchards
Christian Krupke, PhD student of Dr. Vince Jones (WSU-TFREC, Wenatchee), has spent the last 3 years working diligently to understand stink bug behavior in apple orchards and their surrounding habitat. A standard approach to insecticide based pest management has been difficult with stink bugs because the control options are limited and they spend much of the year living on host plants other than apples. To confound the understanding of stink bug behavior, there is no reliable lure/trap protocol that accurately measures densities or adult movement. Krupke has focused his research program to accomplishing the following objectives:
- Determine the suitability of orchard cover crop plants as hosts that will mature stink bugs.
- Determine if control programs directed at orchard cover crops would be a practical management strategy for stink bugs without disrupting integrated mite management.
- Evaluate systems of monitoring stink bugs in orchards (border or internal) that predict arrival of immigrants in late summer and/or occurrence of new adults in the orchard ground cover.
- Implement a border management program with combinations of aggregation pheromone, attractive plants and feeding stimulants.
- Evaluate new candidate pesticides as controls for stink bugs.
Stink bug management in orchards: Laboratory bioassays suggested that Danitol could be a viable stink bug control insecticide. Krupke evaluated the efficacy of Danitol for control of stink bugs in replicated orchard blocks. Applications of Danitol in 2001 were encouraging from a management standpoint. Compared to the Phosphamidon or untreated plots Danitol reduced damage by over 80% on average. The reduction in fruit damage was achieved with only a single Danitol application, and the timing of this spray could likely have been improved (i.e., earlier in season). It is also important to note that the sprays were applied at, or shortly after, dusk - the period of highest stink bug activity.
In 2002 stink bug damage timing was investigated in detail, and it was found that the onset of damage occurred at the end of July and continued until harvest. These data demonstrate that there is not a discrete period of stink bug injury that growers could target for spray applications. This is of interest in light of our other work showing that Danitol is extremely disruptive after even 1 application, meaning that in-orchard prophylactic treatments may not be a viable option. Krupke will continue to evaluate the long-term effects of applying Danitol only to borders to see if problems with spider mites can be mitigated in any way.
Bait-and-spray trials: Experiments conducted in 2000 revealed that stink bugs could be concentrated on orchard border plants (mullein) in large numbers using synthetic pheromone. Beginning in mid-May (onset of mating/egg-laying for E. conspersus), aggregation pheromone lures were placed at 20-foot intervals on host plants (mullein). These baited plants were clearly flagged and remained constant throughout the season. Flagged plants were sprayed to drip using a handgun at weekly intervals throughout the entire egg-laying period (ending in early July).
Large numbers of bugs were attracted to treated plants during this study, and it was not uncommon to see 80 or even 100 bugs on a single mullein plant. Dead bugs were frequently noted beneath treated plants following insecticide treatments. Extensive sampling was conducted during the season following sprays and fruit injury reductions of 50% were noted.
Orchard cover crops as hosts: Euschistus conspersus were reared from the egg to adult stage in a greenhouse on 5 broadleaf weeds commonly found in orchards (common mallow, mullein, dandelion, lamb's quarter, and white clover), as well as orchard grass. The reaching the adult stage, sex ratio, and weight of males and females were used as criteria to compare development on different hosts. Results of rearing experiments conducted with a variety of host plants indicate that stink bugs are able to develop from egg to adult on common mallow, mullein and white clover only. These plants could be managed with effective broadleaf weed control. Since previous experiments have shown that stink bugs are unable to develop upon apple, this may represent an ideal way to restrict stink bug populations to areas outside orchard borders. However, Krupke does note that the evidence is limited to support the idea that stink bugs are reproducing and building within orchards. Samples taken from orchard ground cover yielded very few stink bug nymphs compared with border samples and damage counts conducted in the orchard once again revealed a trend of decreasing damage away from border rows.
Orchard ground cover treatments: Two treatments were applied to the orchard ground cover in an attempt to reduce the in-orchard populations of stink bugs. Danitol, a broadleaf herbicide, 2,4-D or nothing (control) was applied to ground cover in an attempt to either kill stink bugs or eliminate possible host plants. Treatments were applied to replicated plots. Pre-treatment counts showed very few stink bugs in the orchard and certainly less than in the border areas in late June. No stink bugs were found in the herbicide and Danitol treatments the rest of the summer except a low level in the herbicide treatment in mid-August. Combined with results of earlier experiments that indicated stink bugs were unable to develop on apple, these data suggest that effective control of broadleaf weeds in orchard may remove any potential hosts for stink bug nymphal development, but management efforts may likely be more efficient if confined to orchard borders.
Aggregation pheromone studies: Prototype lures designed to have a release rate of the Euschistus aggregation pheromone approximate to that of the polyethylene cap-lure developed by WSU were supplied by three companies. In all cases, lures were placed on mullein (Verbascum thapsus) plants located on the borders of orchards in areas known to support high stink bug populations. It became apparent that all lures showed diminished attractiveness after three weeks of field-aging. WSU will continue to work with monitoring companies to develop an optimized lure. Krupke feels it would not be prudent to mention lure names at this time as companies are still refining their release devices to meet WSU specifications.
Summary: Research on stink bugs has shed a great deal of light on the biology and behavior of the most important species, Euschistus conspersus. The primary challenge is to develop a control strategy that will protect fruit without disrupting biological control of spider mites and other insects. The use of mullein and the aggregation pheromone form a basis for monitoring stink bugs as they develop and move to orchard borders. The aggregation pheromone does not attract stink bugs from long distance so can be used to assess relative densities in a small area like an orchard border. The research has shown that the day-time counts on mullein underestimate local densities but they are representative of that population. Host plant management in orchard cover crops could be a way to minimize stink bugs reproducing and developing in orchard, however, in-orchard stink bugs densities does not seem to be a major contributor to overall fruit injury. The development of lures by commercial companies could be a great help in developing a more reliable monitoring system. The use of traps is still a barrier to an easy monitoring system for stink bugs.
How soft is 'soft', effect of pesticides on Typhlodromus occidentalis
Dr. Elizabeth Beers-Peryea (WSU-TFEC) is dedicated to understanding the potential disruptive properties of novel insecticide chemistries. Many new insecticides do not appear to be as disruptive to biological control as broad-spectrum organophosphates, carbamates or synthetic pyrethroids. The belief that these products are softer on biological control agents is based on results that suggest the chemicals are less toxic when sprayed directly on the biologicial control agent and/or the products are shorter lived in the environment than the older, conventional insecticides. However, these studies do not address the sublethal effects on beneficial insects, specifically undesirable effects on the reproductive potential of predators and parasitoids. Dr. Beers-Peryea is studying the effect of pesticides on fecunditiy and survivorship of predatory mites (Typhlodromus occidentalis) after exposure to various insecticides in laboratory bioassays.
Methods: Bioassays were conducted using a cohort of female T. occidentalis, with assurances made that the female mites were of similar age. The test substrate was a lima bean leaf disk treated with insecticide using a Potter spray tower. Females were present on the disks at the time of application, thus the method of exposure was both contact and through residues on the leaf disk. The disks were evaluated daily through the 7-d test, recording both the number of live females and the number of eggs produced. Eggs were removed each day as they were counted but females were left undisturbed. During this time, they were fed with mixed stages of twospotted spider mites and pollen.
Results: In the untreated checks, the average (range) of total eggs produced was 80 (16-153); female-days, 75 (25-116), and fecundity (eggs/female-day), 1.1 (0.3 2.4). Similarly, the percentage of live females after 7 days (checks only) varied considerably (12-81%). This inherent variability is more than is desirable, however standardizing the source of females, age of females, and their rearing conditions for at least one generation should have minimized the variability among treatments in the same assay.
The two specific miticides, Acramite (bifenazate) and Secure (etoxazole) had no effect on fecundity or residency of T. occidentalis at the high field rate. Conversely, the two chloronicotinyls tested (Actara [thiamethoxam] and Provado [imidacloprid]) both showed one or more effects on T. occidentalis in one or more tests. Two of the four tests using Actara showed an effect, viz., reduction (66%) in total egg production or residency (29%). The effect of Provado was more pronounced; in one test there was a 99% reduction in overall egg production at the field rate, with corresponding reductions in residency and fecundity. There was also a 24% reduction in residency after exposure to 10% of the field rate, indicating that the effect might continue well beyond the time of application.
The insect growth regulators Intrepid (methoxyfenozide) and Esteem (pyriproxifen) were tested against three populations each, and there was a detectable effect in one of the variables in one of the three tests. For Esteem, one field-collected population experienced a 52% reduction in total egg production, and a 35% reduction in female residency. In the case of Intrepid, there was a 29% reduction in total egg production, but this effect was only found in the lab colony.
Perhaps the most surprising result of this experiment was the single bioassay using Success, a fermentation-product insecticide with primarily lepidoptera activity. This material is thought to be neutral both to phytophagous and predatory mites, although other members of the fermentation group have significant mite activity (e.g., abamectin and milbemectin). One field- collected population of T. occidentalis experienced a 44% reduction in total egg production, a 23% reduction in residency, and a 28% reduction in fecundity of the surviving females. This preliminary result merits further investigation with other T. occidentalis populations.
Summary: The two specific miticides, Acramite and Secure, showed no effect on typhs. The two chloronicotinyls tested (Actara and Provado), showed some suppression of either fecundity or residency, with the Provado having the stronger effect. Both fecundity and residency were reduced at the field rate (300 mg AI/liter), with a net effect of shutting down egg production; survival was reduced at the 10% field rate concentration (30 mg AI/liter). It should be noted that "residency" per se was calculated as live females on the disk; however, very few were found dead in any bioassay, most of the attrition from the initial number was due to runoff. Provado appeared to be particularly repellent to T. occidentalis. Both of the IGRs (Intrepid and Esteem) show some slight deleterious effect on typhs, although in the case of Intrepid, this occurred in only one test. The suppression of fecundity and residency of typhs by Success was the biggest surprise; this test needs to be repeated.
Laboratory bioassays are an excellent tool for researchers to screen effects of pesticides applications against a variety of insects. However, interpretation of these data should be done carefully. An individual insect's response to a treatment in the laboratory should not be viewed as a model of how a population will respond under field conditions. Many other factors are working in the environment such as absorption and translaminar flow, UV break-down of residues, immigration, emigration, environmental perturbations, etc. Hopefully the work of Dr. Beers-Peryea and others (to be presented in upcoming newsletters) will help us all to understand what really is happening in the orchard when a pesticide application is made.
Mike Doerr, Coordinator Areawide II
Phone: 509-663-8181 x248