Slug Species and Life Cycles
The study in 1996 was a continuation of the project that began last year. The fields sampled in 1995 were used again unless circumstances prevented their use. This occurred in a few fields; in those situations, other fields in the same general vicinity with a history of slug problems were sampled.
Supplies were purchased in Wooster and distributed to all county agents in early spring. Numerous visits by Hammond's crew were made to all fields during April and May to take in situ samples. More in situ samples were taken this spring compared with the previous year. Samples were taken by searching 20 areas per field, approximately 1 ft2 each, for the presence of slugs.
Weekly beer-trap sampling began after all supplies were delivered. Holes were cut into the soil using a 4" hole cutter, with a 16-oz plastic food cup inserted into the hole. The cup was partially filled with beer and a 1 ft2 aluminum foil-covered roofing shingle was placed over the cup. Traps were sampled the following day when species and number of slugs were recorded. The beer and cups were removed and the shingle placed back over the hole. Slugs underneath the shingles were counted the following week which constituted passive sampling. The entire process was repeated. This process continued through June, after which samples were taken on a monthly basis.
The spring of 1996 was extremely wet and cool, resulting in late planting in most fields. In situ sampling in early spring indicated that adult slugs and eggs were present in very low numbers. In most fields, juvenile and adult slugs were almost nonexistent, with only a few eggs being seen. The few slugs sampled were mostly marsh slugs. This compared with early spring visits in 1995 when numerous eggs were present. The larger number of eggs suggested significant populations in many of the fields (relatively few adults had been seen); subsequently large slug populations were observed that year. Compared to last year when slug populations were often large, few slugs were collected by the beer traps or the passive samples in 1996. Of note, very few gray garden slugs or dusky slugs were collected. The most numerous slug collected was the marsh slug, and then, only in a few fields. One field in Wayne county had an average of 11 slugs per trap on a single date; however, other samples in that field always were low. This single field is a low lying field that is tends to be wetter during the spring compared to surrounding fields.
Sampling in other fields that were not part of this study also hinted at the absence of slugs. Additionally, telephone calls were made to people associated with the slug problem in Ohio (e.g., crop consultants, other extension agents, and industry representatives). No one was aware of significant problems with slugs this year.
It should be noted that these same fields had been sampled in the fall of 1995. At that time, there were less than 0.5 slugs per trap of any species, with most traps never having slugs. Whether these low numbers were indicative of low slug densities or little slug activity was difficult to ascertain. This had compared with fall sampling in 1994 (done only in Wayne County) where very large slug densities were observed. Slug numbers in Wayne County in the fall months of 1994 ranged from 8 to 20 gray garden slugs, 2 to 5 dusky slugs, and 5 to 13 marsh slugs per single trap. The following spring (reported in last year's report), slug populations were quite high and there was significant damage in numerous fields. The point is that large slug populations in the fall preceded large populations and economic damage the following spring, while a nonexistent fall population resulted in very low spring populations.
Slug Injury/Crop Damage Studies
In 1996, our plan was to conduct the damage studies only in fields having a large slug population prior to planting, or with significant slug injury being observed. Because of the extremely low slug densities this spring, damage studies were not conducted in any of the fields. An attempt was made to conduct damage studies in Wayne County in fields not part of these counties studies, but our attempts also proved fruitless because of the low slug densities.
Of note, we have been observant of soybean stands in no-till fields the past few years. We believe that stands are often much smaller than anticipated. To illustrate with an example, one grower in Wayne county plants around 210,000 seeds per acre to achieve 170,000 plants per acre. This often results in 120,000 plants per acre. We think that much of this stand reduction might be caused by slugs. During 1995, we saw improved stands in an experiment where we applied a heavy rate of molluscicide at planting time. We intended to explore this idea this year, having treated numerous soybean fields with molluscicide at planting. However, the low slug densities prevented the collection of meaningful data. However, our observations this year revealed no significant stand reductions in any field that we examined. Rather, we saw some of the best plant stands we have seen. This improvement in stands might have been caused by the lack of slugs, and warrants further investigation.
Recommendations
Although few slugs were collected this year, we have continued to gain a better understanding of slug populations in Ohio. It appears there is a cyclic nature to slug populations, albeit it will take many years of observations to confirm. Over the past few years we have seen two springs with moderate to high slug populations (1994 and 1995) followed by a spring with extremely low numbers (1996). Although the exact cause for the low populations this year is unknown, it probably relates to the drought that occurred in the summer of 1995. What we are aware of is that the significant slug problem in the spring of 1995 had followed a fall with large numbers of slugs, while very low slug densities in 1996 had followed a fall with an extremely low populations.
We are continuing to take in situ samples and beer traps in the study fields, and also in surrounding areas. Sampling suggests that slug numbers are on the rise, albeit not to the densities observed in 1994. We are collecting many adult gray garden slugs and numerous marsh slugs. We also have been observing a substantial amount of mating and egg laying. We need to examine the possibility that fall sampling might be indicative of the potential of an economic slug population the following spring. This might lead to predictive capabilities for growers. Additionally, we should examine whether fall molluscicide treatments could control adult slugs before they mate and begin egg laying.
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