Abstract:
Sampling during the past years has suggested that the size of the spring slug
population appears correlated with the slug numbers the previous fall. We developed the
hypothesis that fall slug counts might be an indicator of spring populations. This proposal
addressed the hypothesis that fall sampling can be a predictor of spring slug populations and
damage. Results indicate that there is a strong possibility that fall population sampling can indicate
which fields have a good potential for problems in the spring and which fields do not. All fields with
a moderate population in the spring were preceded by significant slug numbers the previous fall.
Fields having few slugs in the fall did not have many slugs in the spring. In only a few fields were
moderate fall slug populations not followed by many slugs in the spring. Thus, growers could
obtain information based on fall sampling that will enable them to either 1) more closely monitor
fields if moderate to high slug populations are present in the fall, or 2) assume that if numbers of
slugs are reduced, that the potential for a slug problems in the spring are reduced and use their
time more efficiently in the spring.
Summary:
We had sampled numerous corn and soybean fields in the fall of 1997. Adult gray
garden slug counts ranged from <1.0 slugs per trap (in 3 fields to be considered fields with low fall
populations) to 1.6 to 7.6 slugs per trap in 7 fields (to be considered moderate to high fall
populations). We beer trapped sampled during the spring of 1998. Holes were cut in the soil with
a cutter, and then a plastic container was placed into the holes. These containers then were filled
1/3 full with a heavily flavored beer. An aluminum-covered, 1 ft2 roofing shingle was placed over
the holes. The following morning, the number and species of slugs underneath the shingles were
recorded. Ten such traps will be placed in each field. Sampling occurred on a regular basis.
Because little slug damage occurred, in situ counts of slugs directly on the plants were not taken
as planned. No data on slug injury, stand counts, and other relevant agronomic parameters were
taken as crop injury was not sufficient to justify the effort. Sampling continued into the fall of 1998.
Table 1 presents slug counts per trap from various fields for the 4 slug species. Although numerous samples were taken in the fall of 1997, only data from the single date having the largest collection of slugs from the fall of 1997 are presented. Numerous fields had a significant population of gray garden slugs, most had marsh slugs, while few fields had many banded or dusky slugs Beginning with the late April and early May samples in 1998, many fields had significant populations of slugs. Gray garden slugs were numerous in the City Garden and Hutton fields, both fields having had many gray garden slugs in the fall. Marsh slugs were numerous in most fields, with the same fields also having generally high populations in the fall. Banded and dusky slugs were relatively low in the fall of 1997 and again in the spring of 1998. The only fields which did not match up were the South Well-North field and the Wharton field which had moderate to high populations of the gray garden slugs in the fall, but not in the spring. Statistical analyses were done on the data from 1997 and from early spring (either from the 29 April or 6 May sample); data from all 7 fields were included in the analyses. Linear correlation analyses were conducted on the number of slugs in the fall with the number collected in the spring for each species.
Gray Garden Slugs - When all 7 fields were included in the analysis, linear correlation indicated
a poor fit (r = 0.46, P = 0.30). Examining the data suggested that the South Well-North field did
not fit well with the other data points; there was a large slug population in the fall in this field that
was followed by a very small population in the spring. When data from the South Well-North field
were removed from the analysis, the correlation became significant (r = 0.84, P = 0.03).
Marsh Slugs - When all 7 fields were included in the analysis, linear correlation again indicate
a very poor fit (r = 0.13, P = 0.77). While in general most fields had numbers that greatly increased
from fall to spring, there were 2 fields that did not follow this scenario. The population decreased
slightly in the Wharton field and remained somewhat similar in the South Well-North field. When
these 2 fields were removed from the analyses, the correlation improved dramatically (r = 0.90, P
= 0.04).
Banded and Dusky Slugs - There were too few slugs to obtain meaningful analyses for either
slugs. No banded slugs were collected in the fall samples. There were few dusky slugs collected
in the fall of 1997, which only allowed us to determine their presence within a field. Linear
correlations were not possible.
Although good correlations were not obtained when all fields were included, strong correlations were obtained when a single field, or 2 fields, were removed from the analyses. In those fields, numbers of slugs were high in the fall samples but declined in the spring. Although those scenarios might cause a grower unnecessary concern, they would not cause a grower to miss a slug population that is large in the spring. All fields with relatively moderate to high populations of slugs in the spring had been preceded by a similar size population of slugs the previous fall. Although the predictive capability of fall sampling might not be able to predict a specific population density, it should indicate to a grower the need for spring monitoring if fall populations are high, and lack of concern if fall populations are non-existent.
Following the 1st week of May samples, we had sent out notices to growers to be aware of potential slug problems. However, this information proved to be somewhat faulty because following hot, dry weather conditions in May, slug populations and their activity generally disappeared. However, following significant rainfall that began on 5 June, slug populations again became active and we saw many more slugs by the 9 and 17 June samples. By this time, most crops were actively growing and plants were large enough to withstand any injury. However, we did receive reports of isolated slug problems throughout the state following these rainfall events. It is our belief that if the weather conditions in May were more conducive to slugs, we would have had serious slugs problems in some areas.
Of note, we have been collecting similar data over the past 3-4 years in a number of fields. From these fields, we were able to examine (20 field situations over that time period. For the gray garden slug, the main slug pest, correlations of fall to spring slug populations were highly correlated, ranging from r = 0.63 to r = 0.92 (P = 0.005 and 0.0001, respectively). In the latter case, analysis was conducted after removing a single field that had an extremely high population of slugs in the fall (>24 per trap) but few in the spring ((5 slugs per trap). All other fields were included in the analysis, including all the fields from 1997-1998 that were included in this report.
Fall Sampling in 1998
From the initial fall sampling of 1998, it appears that many slugs have survived the past summer.
Gray garden slugs are already being observed in numerous fields in moderate numbers, and dusky
slugs are being seen at higher populations than in previous years. Sampling will continue into
November. We will be making predictions as to potential slug problems next spring later this
winter.
Extension Program Implementation:
As with any pest management program, the ability to
predict the problem, or lack thereof, can be an extremely useful tool in integrated pest management
(IPM). Although we do not assume that fall sampling will lead to predictions of specific population
levels within a field, we do feel that it will give growers an edge in their battle against slugs. It will
provide them with the knowledge that slug populations and the potential for economic injury are
either low, or that populations and damage potential are great. In the latter situations, more
intensive monitoring would then be required. Although situations do exist where a large fall slug
population are not followed by significant slug numbers in the spring, the only result would only be
extra time spent in sampling, not a loss of the crop and/or reduction in yield.
Whether further efforts by a grower entails only a concerted monitoring program towards slugs, conducting some tillage in suspect fields to reduce the level of infestation and injury, altering planting dates, or taking another action to reduce the potential for economic losses, remains to be seen. However, the knowledge of potentially damaging populations will give the growers something that they did not have in the past.