Terry Beck
Assistant Professor
Wayne County Extension Agent
The Ohio State University
Summary and Conclusions
Slug Identification:
There are four distinct species of slugs that
are found in Ohio fields. Gray garden slugs, Deroceras reticulatum, are medium
size, grayish colored slugs having breathing pores on the posterior portions of their
mantle. This breathing pore is surrounded by a pale area. When irritated, they exude a
white slime. These slugs were the most common and numerous in grower fields. Marsh
slugs, Deroceras laeve, were the next most numerous slug. Marsh slugs are smaller,
usually much darker slugs than gray garden slugs. They also have a breathing pore on
the posterior portion of the mantle. These slugs are difficult to differentiate from gray
garden slugs when both are small. Their slime is clear and watery. Dusky
slugs, Arion subfuscus, are large slugs, the largest slug found in grower's
fields. There are 3 distinct color phases commonly found in Ohio fields, a darkish
brown-black color slug with lateral bands that are difficult to see, a medium brownish
color slug with
distinct lateral bands, and light orangish color slug also with distinct lateral bands. All
three color forms can be found together. Their foot fringe is well developed with dark
vertical markings (often faint in the lighter colored forms). When irritated, these slugs
exude yellow to orange slime. Although not as numerous as gray garden slugs and
marsh slugs, they were readily found in most fields. Because their size and coloration
make them readily detectable, dusky slugs are often considered the most numerous in
certain fields. However, based on the fields sampled, dusky slugs were usually third in
population size. Banded slugs, Arion fasciatus, the fourth species found in Ohio fields,
is not as common as the others. This slug is gray in color with distinct black lateral
bands. It has a whitish, longitudinal ridge down its back. Its foot fringe and sole is white.
While gray garden slugs, marsh slugs and dusky slugs were found in all fields sampled,
banded slugs were only found in a few fields. The field where they were most common
has only recently been in field crops (within the past 15 years); prior to that it was a
swine feedlot.
Life History:
Various observations suggested the following
general life cycle for slugs. Numerous adult slugs were sampled in mid-April through
mid-May. These slugs would have had to overwinter as there was not enough time for
them to have developed to this stage. At this time, numerous egg masses were
observed, including many that were being laid. Whether some of the egg masses had
overwintered is
unknown, but as mentioned, we did observe many eggs being oviposited.
By late May, the number of adult slugs being sampled went down drastically. Our initial thought was that they were going farther into the soil since it was becoming dry and warm. However, we began sampling large numbers of newly hatched "baby" slugs. During the week of May 24, we often found large numbers around single traps (up to 40 baby slugs per trap) suggesting a recent egg hatching. These high numbers were a result of a nearby egg mass having hatched. By early June, these slugs had become juvenile slugs that were slightly larger in size but definitely much smaller than adults. Traps were very inefficient in sampling these juveniles. Field visits in early June in the evening allowed us to observed many of these juvenile slugs feeding on the corn and soybean. In some fields, numbers ranged from 5-7 slugs per individual corn plant and 1-2 per soybean plant (note: plant population of the soybeans is 2X-3X of the corn). The slugs being observed were gray garden slugs, dusky slugs, and some marsh slugs. Because of the difficult in distinguishing young gray garden slugs and marsh slugs, it was often impossible to correctly identify them. Of note, the dusky slugs, being larger and often an orange color, were very noticeable.
Sampling continued throughout the summer. With the exception of the banded slugs, slugs were always captured, with their body size increasing as the summer subsequently progressed. Although numbers were low in July, they went up in August, September and October. Banded slugs did not appear again until September. Depending upon the field, gray garden slugs, marsh slugs, and dusky slugs became quite numerous in the fall based on the trapping. Most of the trapped slugs were larger adults.
Slug Trap Evaluation:
Numerous traps were evaluated for their
efficiency in sampling slugs. After
examining various trap ideas, we came down to 6 traps that we examined in three fields
for 4 weeks. Average slug counts with different letters are significantly different at the
5% level.
| Total Adult Slugs per Plot | |||||
|---|---|---|---|---|---|
| May | June | Average | |||
| Trap Description | 18 | 24 | 2 | 7 | Slugs/Trap |
| 1. Covered shingle + yeast with 1 TBLS sugar | 24 | 12 | 9 | 1 | 0.92 a |
| 2. Covered shingle + bait | 30 | 14 | 9 | 3 | 1.17 a |
| 3. Covered shingle + hole w/ cup + beer | 20 | 17 | 4 | 7 | 1.00 a |
| 4. Covered shingle + hole | 7 | 5 | 0 | 1 | 0.27 b |
| 5. Covered shingle + cracked corn | 18 | 21 | 12 | 1 | 1.08 a |
| 6. Covered shingle + cracked corn + sugar | 13 | 21 | 12 | 5 | 1.04 a |
All traps with the exception of aluminum foil-covered shingle + hole were able to capture larger, adult slugs somewhat equally. When newly hatched baby slugs were initially present (around May 18 and 24), the beer traps captured significantly more (data not presented). Because the beer traps were easier to use than mixing up yeast or cracked corn mixtures, we recommend beer traps as the preferred trap for sampling slugs. However, based on these data and the discussion on slug life history, none of the traps were efficient at sampling juvenile slugs which cause most of the damage in early and mid-June.
We attempted to use the defined area trapping (DAT) method, but met with little success. Various problems associated with them limit their usefulness in IPM programs.
Deadline Efficacy: Because of the wetness of the Deadline Granules, their use at planting time through a drill (and insecticide bins) was limited. The material jammed in the hopper or the tube. Even a drier formulation had problems, although it could be used. Of more importance is whether Deadline Granules applied at planting has potential for reducing slug populations. Based on life history and other efficacy tests, if planting is done in early May, Deadline application at that time would not be appropriate because adults had already laid their eggs. If planting is done in late May - early June, there could be a potential benefit because the material would be applied to juvenile slugs.
Numerous studies were done applying Deadline Granules or other formulations as a broadcast application. In a study applying the Granules at different times following planting (0-5 weeks), the best slug control was achieved with material applied at least 1 week, preferable 2 weeks following the May 5 planting date. Injury ratings or slug counts with different letters are significantly different at the 5% level.
| Trt# | Date | Treatment | Ratings | Slugs/8 Corn Plants | 1 | Check | 2.8 ab | 6.0 a | 2 | May 5 | 0 week | 3.0 a | 6.5 a | 3 | May 13 | 1 week | 3.0 a | 2.5 bc | 4 | May 19 | 2 weeks | 1.8 bc | 3.0 ab | 5 | May 27 | 3 weeks | 1.0 c | 0.8 bc | 6 | June 2 | 4 weeks | 1.0 c | 0.0 c | 6 | June 9 | 5 weeks | 2.5 ab | 0.3 bc |
|---|
In a study applying various rates of Granules, all rates from 5 to 40 lb. per acres reduced damage and slugs. The best reduction in damage was with the 20 and 40 lb. rate.
| # Slugs/Trap | ||||
|---|---|---|---|---|
| Trt# | Treatment | Injury Ratings | June 2 | June 8 |
| 1 | Check | 3.5 a | 5.4 a | 1.0 a |
| 2 | 5 lb per acre | 1.8 b | 0.5 b | 0.0 b |
| 3 | 7.5 lb per acre | 1.8 b | 0.8 b | 0.0 b |
| 4 | 10 lb per acre | 1.8 b | 0.4 b | 0.0 b |
| 5 | 15 lb per acre | 1.5 bc | 0.0 b | 0.0 b |
| 6 | 20 lb per acre | 1.0 c | 0.3 b | 0.0 b |
| 7 | 40 lb per acre | 1.0 c | 0.4 b | 0.0 b |
In a study applying various formulations of Deadline, all were able to reduce the number of slugs, with the Granules being the best (at least numerically). Because this test was initiated after feeding had already occurred, injury ratings are high. However, all materials were able to limit further feeding.
| Trt# | Treatment | Rate | Injury Ratings | Slugs |
|---|---|---|---|---|
| 1 | Check | 3.5 a | 12.8 a | |
| 2 | New Material | 5 lb per acre | 2.5 b | 1.8 b |
| 3 | New Material | 7.5 lb per acre | 2.8 ab | 1.8 b |
| 4 | New Material | 10 lb per acre | 2.3 b | 1.3 b |
| 5 | Granules | 10 lb per acre | 2.3 b | 0.3 b |
| 6 | Bullets | 10 lb per acre | 2.3 b | 2.0 b |
In a final study with Deadline to a heavily infested field, both formulations were able to reduce the number of slugs, with Granules being the most effective. Injury ratings (not presented) also supported the idea that Granules were the best material.
| Slugs Per Corn Plant | |||||
|---|---|---|---|---|---|
| Trt# | Treatment | June 17 | June 18 | June 24 | June 30 |
| 1 | Check | 2.6 a | 2.3 a | 4.5 a | 6.1 a |
| 2 | Granules | 0.0 b | 0.1 c | 0.4 b | 0.5 c |
| 3 | New Material | 0.5 b | 0.7 b | 0.7 b | 1.6 b |
Many our of findings are a result of the large egg hatch that occurred around May
18-24 in all the
fields. Any of the early May applications of Deadline, while killing numerous adult slugs,
had no effect on the egg masses that were present at that time. Additionally, these
applications had no effect on later juveniles of the 1st generation. Applications made
near or after egg hatch were very effective in reducing slugs populations. Damage
reduction was related to the time the application was made. If the material was applied
prior to the juveniles doing any feeding, damage was averted. If the material was
applied later after some feeding had already occurred, continued damage was
reduced.
The 1994 Deadline Granules were quite effective in controlling slugs whenever, and
wherever, it was
applied. This included the test plots as described and other fields where Deadline
Granules was made
available to growers. Because of the formulation, a larger number of particles are
distributed compared with Deadline Bullets. Five (5) grams of each material had
approximately 32, 254 or 438 pieces for the Bullets, new material or Granules,
respectively. On a ft2 basis this corresponds to 0.7, 5.3 or 9.1 pieces. However, the
wetness of the material was a major problem. Unless the company can come up with a
drier formulation that will easily flow through drills, insecticide boxes, and spreaders, it
will not be accepted by growers. Although the Granules used this year worked, any new
formulations that are drier will need to be tested for their efficacy. Unless a drier
formulation can provide the results obtained this year, all this year's conclusions
concerning Deadline Granules have to be limited to a very wet, probably unacceptable,
material.
Planting Date:
The attempts to determine if varying the time of
planting could affect the amount of
damage from slugs proved unsuccessful. The areas where these experiments were
placed did not have slug
infestations sufficient to produce damage differentials. However, based on the
information on the history of slugs that was obtained, planting times should have a
major impact on slug damage severity.
The damage that we observed this spring occurred in early-mid June from juvenile
slugs of the 1st
generation. In the corn fields we sampled, the corn was in the 4-7 leaf stage by the time
these young slugs
began heavy feeding. The corn was too tall to be economically damage. A number of
soybean fields that were located in the surrounding areas, and were being
economically damaged, had been planted in late May - early June. These soybeans
were either emerging from the soil or were in the V1-2 leaf stages when the juvenile
slugs began their active feeding. Conversations with Dave Graham about previous
problems support this conclusion. It is later planted fields that tend to have problems in
June. Fields planted in early May often have sufficient growth to allow them to escape
serious problems from slugs. It should be pointed out that early planting does not limit
the presence of slugs, only the economic injury. Heavy feeding will occur, but due to
the overall size of the plant, economic damage might be prevented.
Recommendations:
The following are recommendations based on this year's data:
1) Some level of tillage will help to reduce the potential of slug damage by eliminating slug shelter and allowing for more vigorous plant growth.
2) Fields with a history of slugs where concern exists should be planted as early as possible to allow crops to get the most growth possible before slugs begin their heavy feeding.
3) Fields, especially those with a history, should be watched most carefully in late May and then during the first 2 weeks of June for the presence of slug activity. This should be determined (1) by examining plants for feeding and (2) by evening field visits an hour before sunset to determine the presence of slug populations.
4) Trapping, while being effective in early spring for the presence of adult slugs and in later summer for the presence of a slug population, is not effective in determining the relative size of a damaging population in late May and early June. The traps did not capture the large numbers of damaging juveniles very well. However, when traps are used, cups half filled with a heavy beer would be the best choice.
5) When crop damage becomes sufficient, a molluscicide should be applied. Deadline at a rate of 10-12 lb per acre is suggested. Deadline Granules would be the first choice followed by Bullets. At this time, economic thresholds are not available. However, a drier granule material is needed.
Future Needs:
Because this was the first year that we have conducted intensive sampling of slugs in conservation tillage fields (and then, only spring through fall), we need to continue with the sampling program at least into the spring of 1995. This intensive sampling should then be continued for a number of years to determine the life history of all four slug species. Currently, the life cycle of slugs in Ohio is based on limited information gathered during one spring, from other states, and from sites in foreign countries.
Because most of this information was from work done in Wayne County, it should be extended to other parts of Ohio where slugs are of a concern. In the future, similar work should be done in other midwestern states.
The predominant slug appears to be the gray garden slug, followed by the marsh
slug and dusky slug,
and then last by the banded slug. We should conduct specific sampling studies to
determine the geographic range and make-up of these four slugs throughout
Ohio.
Little information is known about the comparative damage potential of these four slugs. Gray garden slugs are known to be an economically important species; however, this might not be true for the other slugs. Studies should be initiated to determine feeding rates and damage potential of all the slugs.
We need to continued work on molluscicides, including a better formulation of Deadline Granules (drier and more consistent). New materials should continued to be evaluated.
Planting time studies should be continued. Specific studies should be established in areas of high slug populations to further explore the potential use of planting times as a preventive tactic against slugs. Damage studies should be done where juvenile slugs on placed onto corn and soybeans at different growth stages (as a result of different planting dates). During the next area-wide outbreak of slugs in the state, surveys should be conducted to attempt to correlate fields with economic-damage with planting times. We should not automatically assume that early planted fields, not experiencing noticeable defoliation, do not have large slug populations. This is perhaps the tactic with the greatest potential for preventive management of slug problems in conservation tillage fields (albeit that tillage would be the best preventive tactic).
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