Slugs are a well-known problem in production strawberry fields. A very common slug found in the Monterey Bay area is the little grey slug, Deroceras reticulatum. This slug measures 1-2 cm in length, and has a mottled grey color.
They can be found on the plant at night and in the early morning, and under the plastic or other mulch during the day. Slugs seek moisture, making the humid environment under the mulch of strawberries attractive to them, especially during the frequently wet early season.
Slugs feed on the leaves of strawberries and occasionally the fruit, and the rasping feeding of slugs leaves ragged holes in the leaves and fruit. It is important for growers and pest control professionals to distinguish this damage from the clearly margined cuts of lepidopterous larvae, since the control measures for each are quite different.
Certain recommendations for slug control, such as the construction of copper barriers around the planted area and use of beer baits, are appropriate for home gardeners but not economically feasible on a commercial scale.
Growers can seek to eliminate hiding places such as rocks, weeds, logs and boards to reduce the numbers of slugs. To avoid slug problems, growers can strive to plant away from areas such as orchards, forested and brushy areas with this sort of debris.
Recently, there has been some discussion about the use of food grade caffeine for control of slugs. Work done in ornamental plants in Hawaii indicates that at high concentrations, caffeine becomes a lethal neurotoxin to slugs. Solutions containing a concentration of 1 percent to 2 percent pure caffeine (compared to 0.05 percent concentration of caffeine for an ordinary cup of coffee) killed or at least repelled slugs infesting target plants.
However, food grade caffeine may not be used in a way inconsistent with its labeling, in this case as a pesticide. In discussions with the California Department of Pesticide Regulation, it was determined that the normal registration and toxicology process would have to be pursued to permit use of caffeine as a molluscicide to control slugs in commercial production fields. In addition, caffeine can be harmful to humans at high concentrations, putting the possibility of its registration for use in strawberries into doubt.
Three materials are currently registered as molluscicides for use in strawberries for the control of slugs are: Deadline M-Ps (Metaldehyde), Deadline Bullets (Metaldehyde) and Sluggo (Iron phosphate).
Growers should be aware that both formulations of Deadline have a number of restrictions which can limit their use in strawberries. There is 12-hours re-entry restriction after application, and care must be taken not to contaminate the fruit with the bait.
Sluggo has not been used extensively yet in strawberries. Made from iron phosphate and a wheat derivative which serves as bait, Sluggo is non-toxic to humans and other mammals, and has no re-entry, preharvest or application restrictions. Also, Sluggo is currently under consideration by the USDA for registration in organic strawberries.
A spring 2005 trial was conducted consisting of 20 pounds per acre each of each treatment of Deadline Bullets, Deadline M-Ps and Sluggo to augment work done previously with these baits two years ago. The test was placed prior to a heavy rainstorm to test the baits' rain fastness and efficacy in these wet conditions.
Each treatment was composed of four replicate plots of 20 feet long by 52 inches wide, and was placed in a field of Camarosa strawberries experiencing slug infestation adjacent to a brushy area with lots of debris. The slug species found on this ranch was the little grey slug, Deroceras reticulatum.
The trial application was followed by 1.69 inches of rain on that day, and another 0.12 inches on the following day.
Prior to application, a pre-count of slugs was made to determine overall population. Later counts were done seven days, 14 days, 20 and 28 days after application.
Numbers of slugs counted per treated plot declined sharply seven days after application of molluscicides, significantly so in the case of Deadline M-Ps and Deadline Bullets. The two Deadline formulations were effective in controlling slugs through the course of the study.
Overall, the number of slugs declined over the duration of the study, perhaps because the weather became less favorable for slugs over this time period.
Despite the heavy rain, Deadline M-Ps and Sluggo pellets lasted longer in the study than did the Deadline Bullets. The blue colored M-Ps and Sluggo pellets were still to be found whole 28 days after application and after several rainstorms.