Principal Investigators:

Summary and Conclusions:

Each county agent contacted growers in their areas prior to the beginning of the growing season to locate at least two fields for conduction of the research (Knox County had three fields, Wayne County four fields). The only requirements for the fields were that they fit the definition of conservation tillage and were being planted to either corn or soybeans.

Most of the supplies for the study were purchased by Hammond, including the beer, molluscicide applicators, hole cutters, plastic cups, flags, roofing shingles, etc. Deadline Granules were supplied by Bill Haddad, Valent Corp. Visits were made to each county during May to 1) deliver the supplies and 2) to visit each of the fields in the study. In situ samples were taken during the field visits to determine the slug population and stages that were present. Ten areas approximately 1 ft² in each field were searched for the presence of slugs.

In situ sampling suggested that few adult slugs were present in any of the counties in May unlike the previous year when numerous adult slugs could be found. Less than one slug per sampled area was collected in most fields. However, numerous slug eggs were usually found in all fields, suggesting a slug population was present. Eggs were either of the gray garden or dusky slug. Although few adults were seen, the predominant slug was the gray garden slug with fewer dusky and marsh slugs observed.

Slug Species and Life Cycle

Each agent began weekly sampling after all supplies were delivered. Attractant samples were taken using beer traps. A hole was cut into the ground using a hole cutter, and then a 16-oz plastic food cup was placed in the hole. The cup was filled 1/5 full with beer and then an aluminum foil-covered roofing shingle, 1 ft², was placed over the cup. These traps were sampled the following day when the slug species and numbers were recorded. The beer and cups were removed and the shingle placed back over the hole. The holes and areas underneath the shingles were again sampled the following week which constituted passive samples. Afterwards new holes were dug, cups and beer placed into the holes, and the entire process repeated. This sampling procedure continued through June, after which samples were taken on a monthly basis.

Populations of slugs in the fields varied greatly, in both the predominant species and the size of the populations. Overall, the gray garden slug was the most predominant species. The gray garden slug was found in all fields that were sampled, being the most dominant in 11 of 17 fields. Usually they were at least equal to other species in population size in the other fields; they were relatively low in numbers only in two fields. The next most common slug was the marsh slug, found in many fields. However, they were usually not found in high numbers compared with the gray garden slug. The next most sampled were dusky slugs, found in seven fields. However, numbers reached >60 dusky slugs per trap in one field in Knox county, while in a field in Richland county populations were >24 per trap. Although the dusky was not as common as the gray garden and marsh slugs, they had the ability to reach large populations. Banded slugs were rare, only being found in any numbers in a single field in Wayne County. (That field in Wayne county is separated by a distance of 2-3 miles from the other three fields). Few slugs were collected during the summer and fall months.

A special note should be made about that field in Knox County. We took observations on the extent of crop injury that was occurring. Soybean plant stand was reduced, although it was still considered acceptable according to Agronomic guidelines. However, little additional injury was being done to the soybeans that had emerged and had 2-3 trifoliates. This was interesting considering the very large numbers of dusky slugs that were present. At that time, few of the other slug species were being collected.

Slug Injury/Crop Damage Studies

Plots were established in another area of the field to study the slug injury/crop damage relationship. Individual plots measured 50 x 50 ft, with two treatments: an area with molluscicide applied at 20-40 lb/acre and a nontreated area. Experimental design was a randomized block, two treatments with eight replications, for a total of 16 plots. A heavy rate of Deadline granules was used to ensure complete mortality of the endemic slug population. These traps were established and treated prior to knowing if a significant slug population existed. Beer traps were taken within these plots. Our plan was to collect various data on slug populations and crop damage during the season.

Slug injury did not occur at levels sufficient to continue this portion of the experiment in most locations. The only exception was a soybean field in Wayne county, where there was a reduced plant stand in the nontreated plots. Plant stand in the treated plots was calculated to be 171,734 plants/acre, while in the nontreated plots, the stand was 120,214, a 30% reduction. Analysis indicated that this difference was significant. Little additional damage occurred to these soybeans. Sampling indicated a population of dusky slugs (although much less than that field in Knox County mentioned earlier), with fewer gray garden and marsh slugs. Although plant stand was reduced, harvest data indicated no difference in yield between the two treatments: 40.5 bu/acre in the treated and 39.6 bu/acre in the nontreated. It should be noted that Agronomic guidelines state that there should not be any difference in yield between these plant populations. A population of 170,000 per acre is considered quite large; agronomic recommendations call for populations of 100,000 to 140,000 plants at harvest.

Summary

DUSKY SLUGS appear to be able to cause significant stand reductions in soybeans, especially later planted fields. Soybeans had been completely destroyed two years ago in the one Knox field referred to earlier (recollection was that the plants never emerged). This year, the same field had a stand loss; however, the plants that did emerge did not have significant feeding injury. We also saw numerous soybean fields in Wayne county with poor stands but not a lot of feeding injury. When feeding was observed, we always saw a lot of grays and/or marsh slugs.

We need to ask ourselves: are dusky slugs a big stand reducer in soybeans, where the plants are being damage prior to emerging from seed furrows and underneath residues? Are dusky slugs NOT causing that much damage after the plants have expanded leaves? In some of the fields we visit, we often think its poor emergence causing a poor stand; we assume we will have better stands within a day or two. Could poor stands we thought due to "no-till" be from dusky slugs? Is there a problem in corn knowing that it is usually planted earlier? Can dusky slugs reduce corn stands like in soybeans? Can they cause that much defoliation? All corn fields with noticeable defoliation always seem to have gray garden slugs. If the problem is prior to emergence, will it require treatment at planting or immediately afterwards? Obviously it would not be worth it to treat after plant emergence because the damage is already done.

GRAY GARDEN SLUGS can significantly defoliate both corn and soybeans. These slugs can defoliate either crop completely down to the ground. We saw a soybean field in Dalton, OH, where the soybeans had emerged, but then were eaten down to stubs. A corn field near Wooster, OH, which had reached the 4-6 leaf stage, had plants destroyed with other plants greatly stunted. Are gray garden slugs mostly leaf feeders, unless they are so numerous they take stem and all? Do they do most of there feeding after plants emerge, or can they also cause significant injury prior to emergence as seen with dusky slugs?

MARSH SLUGS were not seen in as many fields as gray garden slugs. During the spring, they seem to be relatively larger in size than grays, that is, larger juveniles and adults compared with smaller juveniles of gray garden slugs. Are they causing that much damage, or are they just there? We need to find more fields with marsh slug problems to determine this.

BANDED SLUGS are not in many fields; they were only numerous in one location in Wayne County. These slugs always seemed more sluggish compared with other slugs. Are they doing much damage? Are they elsewhere in the state?

Recommendations:

We have started to gain a better understanding of slug populations in Ohio. Of importance, we now realize, and need to educate growers, that these slug species are quite different, not only in appearance, but also in their life history and damage potential.

We need to continue this sampling program for another year in the same locations to compare this year's findings with another year under differing environmental conditions. We especially need to examine overwintering; so far we have seen one year where mostly adults have overwintered and one year where its been mostly eggs.

The plant/damage studies should only be done in fields where slug injury is occurring. The suggestion will be to wait until slugs begin feeding, and then establish the plots. In this way, only fields with a good chance of providing useable information will be tested. The exception will be for late planted soybean fields with a known history of dusky slugs. The suggestion in these fields will be to establish the nontreated vs. treated immediately after planting only in fields being planted late and where dusky slugs have been collected.

Lastly, we need to continue in our attempts to document the slug problem in Ohio. We will be discussing how to do this at an upcoming Task Force meeting. Also, Hammond will be making a case for this information at various state-wide meetings during the winter.

For further information contact Ronald Hammond , Associate Professor, Dept. of Entomology, The Ohio State University or the Ohio IPM Office.

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