Development and Implementation of Integrated Pest Management of Burrowing Shrimp on Washington State Commercial Oyster Beds

Development and Implementation of Integrated Pest Management of Burrowing Shrimp on Washington State Commercial Oyster Beds

Summary

Since the 1940s, and for reasons not entirely clear, two indigenous species of burrowing shrimp have come to severely impact oyster production, and the mudflat community in general, by greatly increasing their previous range of distribution. Both ghost (Neotrypaea californiensis) and mud shrimp (Upogebia pugettensis) reside inside burrows that can extend up to a meter beneath the mudflat surface (Dumbauld 1994) and abrogate habitat from other benthic organisms. Ghost shrimp are predominately filter feeders (MacGinitie 1930) competing for plankton resources important to bivalves and other estuarine fauna. Bioturbation by mud shrimp disrupts the structure of the mud-flat substrate by suspending fine sediments, and at high densities causes surface dwelling organisms to literally sink in the mud (Peterson 1977, Brenchley 1981, Bird 1982, Murphy 1985, Posey et al. 1991, Dumbauld 1994, Tamaki 1994). Burrowing shrimp are tenacious perennial pests that can quickly return to areas where they have been completely eliminated (WDF/WDOE 1992, Brooks 1995, Simenstad and Fresh 1995). The detrimental effects of high burrowing shrimp densities to the rest of the mudflat community have been demonstrated by the return of higher levels of diversity and key indicator species upon the suppression of burrowing shrimp. Bioturbation by burrowing shrimp has been documented to have a negative effect on seagrass communities (Suchanek 1983). For the Pacific oyster, bioturbation associated with burrowing shrimp may interfere with suspension feeding (Rhoads and Young 1970) and surface-deposit feeding (Tamaki 1988), bury newly settled larvae (Swinbanks and Luternauer 1987) and initiate small scale emigrations (Tamaki 1988).
It is estimated that burrowing shrimp have eliminated over 3,000 acres from commercial oyster production (i.e.~25% of the historically farmed acreage) (Burrowing Shrimp Control Committee [BSCC] 1992). This acreage might be reclaimed if burrowing shrimp could be suppressed to low densities, allowing the return of fine surface sediments and associated microbial, macroinvertebrate, and vegetative communities.
Since the 1960s, aerial applications of carbaryl on selected and legally limited acreage of commercial oyster beds have effectively suppressed burrowing shrimp. Carbaryl is applied on acreage of high shrimp density based on a “rule of thumb” threshold of 10 burrows per m2. Carbaryl is applied to each bed usually only once during the 3 – 5 years of oyster development, usually by helicopter during one or two extreme low tides in July or August when migratory salmon are not present. Beds cannot be treated within one year of treatment.
Beginning in the early 1990s, the carbaryl-based plan to manage burrowing shrimp has been subject to increasing regulation. In January 2001, The Willapa Bay/Grays Harbor Oyster Growers Association (WGHOGA), Washington State Department of Ecology (WDOE), and other state agencies signed a memorandum of agreement (MOA) to transition the industry towards IPM. Some of the tasks specified in the MOA included: a) development and application of accurate…and cost-effective techniques to monitor …[burrowing] shrimp, b) quantification of the relationship between the density of burrowing shrimp and damage to oyster yield, c) development of objective decision making criteria to determine when and where to deploy control tactics [e.g. economic injury level and economic threshold models], d) seek alternative physical, biological or chemical control methods…, and e) development and implementation of an Integrated Pest Management (IPM) plan for burrowing shrimp control.
An NPDES permit issued in May 2002 also specified several IPM-related requirements, all of which were met by 2005.
In 2003, the WGHOGA agreed to settle a legal challenge to the NPDES permit by the Toxics Coalition and another ad-hoc Coalition. One provision of the Settlement Agreement was that the amount of carbaryl used on oyster beds would be successively reduced by 10% each year for three years followed by a total termination of carbaryl use by 2012. In summary, the project readily matches the goals of the Western Region SARE program. The first two state goals are especially germane. The primary goal of the project is to provide the means toward sustainable oyster production over the long term in the context of better stewardship of the entire estuary. All alternatives to the carbaryl-based management plan for burrowing shrimp will be assessed in terms of their effects on not just burrowing shrimp, but to all members of the mudflat community, including eelgrass. Effects on transient species like migratory salmon and birds will be noted. Nevertheless, the project’s immediate effects will be to allow oyster producers to stay in business. If the regulatory mandates towards IPM development described above are not met, most oyster producers in Willapa Bay and Gray Harbor will be unable to remain in business and the local economy will be severely crippled.

Objectives/Performance Targets

This project is meant to supplement and complement activities already in place. We have assembled a research team with expertise in agricultural engineering, mechanical engineering, mud flat ecology, shellfish culture, and IPM development to meet the following objectives:
● Determine the relationship between burrowing shrimp density and oyster yield (e.g., the damage/density relationship) to improve monitoring techniques and develop economically based action thresholds.
● Evaluate the efficacy of alternatives to carbaryl-based tactics to suppress burrowing shrimp, such as sub-surface applications of registration-exempt compounds, or the mechanical crushing or shallow rototilling of shrimp burrows, within a tier of experimental designs that progress from small tightly controlled arenas, through larger microcosm studies, to field plot trials.
● Using grower interviews and surveys, appraise the financial costs of burrowing shrimp damage and potential alternative management tactics to derive economically based action thresholds.
● Write an IPM plan for burrowing shrimp; implement and deliver it to oyster producers using workshops, newsletters and demonstration trials; to the scientific community using conferences and articles and to the public using a pre-existing website, and adopt the plan into an Environmental Code of Practice for the West Coast shellfish industry.

Accomplishments/Milestones

In 2005, we made substantial progress toward the development of an integrated management plan for burrowing shimp. Using data from previous years’ damage/density experiments, we developed a decision tree for control action that was based on not only burrow density, but also crop status and shrimp recruitment history. We demonstrated that synthetic pyrethroids and organic materials, applied subsurface using shank or spikewheel technology, significantly suppressed burrowing shrimp relative to untreated plots. Implementation / evaluation processes continued through workshops, newsletters, and websites.
In the coming year, the overall research initiative will be greatly expanded through the acquisition of other funds, especially a Washington State proviso for research of alternative materials. WSARE funds will continue to support the testing of alternative pesticides using barge-based apparati and in larger plots. Workshops and newsletters will also continue under WSARE, which currently provides the only funds for extension.

Impacts and Contributions/Outcomes

This project has contributed to the survival of the oyster industry in SW Washington. Workshops were well attended with wide-ranging discussions related to research directions. Growers now speak favorabley of IPM and the potential of integrating a variety of tactics. Unfortunately, grower impact has been difficult to guage, due to the failure of two grower surveys. Program evaluation will continue through a series of interviews.

Collaborators: