PREVENTATIVE TREATMENTS FOR THE CONTROL OF BLISTER WORM Polydora websteri IN THE EASTERN OYSTER Crassostrea virginica

The polychaete Polydora websteri, or "blister worm", burrows into the shells of several commercially important shellfish species, including those of the eastern oyster Crassostrea virginica. Although oysters can deposit new shell material over P. websteri burrows, the build up of mud and detritus inside of the worm burrow creates unsightly blisters. In addition, mud and detritus can leak from burrows during shucking affecting the flavor of oysters served on the half shell. Infestations of blister worm can impact the marketability of oysters and the reputation of a farm. Various types of chemical, fresh water, and brine baths have been used to rid oysters of adult worms. Few of these approaches have reliably eliminated infestations and they can be expensive, labor intensive or render oysters unsalable.

On the other hand, P. websteri has a biphasic life history that includes a dispersive planktonic stage. Larval blister worms initially settle and form a mud tube on the external surface of the oyster shell and then use chemical means to burrow into the shell. Recently settled worms, thus, do not have the protection of the burrow and are more vulnerable to harsh conditions than are adult worms. Our research explores whether periodic air-drying and pressure washing can reduce the settlement success of larval P. websteri and block blister worm infestations before they occur. In an initial experiment conducted at a single oyster farm in Maine from June to October, 2014, oysters were held in surface cages and air-dried for 4 h or 24 h every other week. Another set of oysters were air-dried and washed every other week. We sampled a subset of oysters in each treatment bimonthly and examined their shells for the presence of newly constructed worm burrows. Our results indicate that air-drying for as little as 4 h results in a significant decrease in both the number of oysters with new blisters and the number of new worm burrows per oyster relative to untreated oysters. Air-drying for 24 h resulted in an even larger decrease in the incidence of new burrows. Washing oysters also resulted in a further decrease in worm burrows for oysters dried for 4 h but had no effect on the incidence of worm burrows for oysters air-dried for 24 h.   

We repeated our experiment during the summer of 2015, and applied our methods at two additional Maine oyster farms impacted by blister worm using the same methods employed in 2014. Preliminary results suggest that the treatments are as effective at the two new farms as they were at the original farm in 2014. We will also report on whether the air-drying and washing treatments have any effect on the growth and survival of oysters, themselves.