Stand Reduction: Eleven fields were used in this study. Within 48 hours after planting, plots were established with three replications of three treatments. The treatments were 1) no control, i.e., natural slug populations, 2) recommended control, i.e., 10 lb. per acre rate of Deadline Minipellets applied, and 3) no slugs, i.e., an area where there were no slugs, achieved with two applications of Deadline MPs applied at 20 lb. per acre at two week intervals. Individual plot size was 75 X 75 ft. Beer-trap samples were taken in all plots 1-2 weeks following planting to determine level of control. After sufficient time for complete crop emergence, stand counts were taken. Our intention was to take yield samples from those plots suffering significant damage; however, none of the fields experienced damage.
Plant stands (plants per 25 ft) are given in Table 1. Statistical analyses indicated no differences among treatments in any of the fields. The probability of a significant F value ranged from 0.20 to 0.92. Most fields had excellent stands, ranging from 163,000 plants per acre to >200,000 plants per acre. There were two exceptions. Field 7 had about 134,000 plants per acre (which is still considered to be very good), while Field 10 was at 85,000 plants per acre requiring replanting. This field was being scouted by an IPM consultant who determined that poor planting operations resulted in poor seed depth leading to low germination (few slugs were collected in this field). Field 11 which was adjacent to Field 10, planted at the same time with the same equipment, also had a poor stand and was replanted. No stand counts were taken in this latter field. Field 9 had been flooded and stands were very poor and variable; no stands are reported.
Slug populations were very low this past spring in these fields. Table 2 lists the total numbers of slugs collected in all the fields except for Field 5. As seen, total counts on all sampling dates <= 6 slugs, except for Field 10 where a total of 13 slugs were collected on two dates. However, those 13 slugs were evenly distributed among the treatments; no trends were observed.
The only field having a moderate population of slugs leading to differences among treatments was Field #5 (Table 3). All the slugs were marsh slugs in this field. Five samples were taken in this field, from 15 May to 12 June. Significant differences were obtained on three of the five dates, with both treatments significantly reducing the numbers of slugs. No differences were obtained on the last date, 12 June. However, this was over a month following the first application, indicating that long-term control was achieved. Although numbers of slugs were adequate to get excellent efficacy data, they were not sufficient to cause stand reductions (Table 1). (It is questionable if marsh slugs have a high potential for causing significant stand reductions compared with gray garden slugs and dusky slugs). It should be noted that the rest of this field was treated with the molluscicide on 2 May following planting because of the large population of marsh slugs. Sampling of those treated areas also indicated complete control.
Subsequent observation of emerged plants indicated that little injury occurred. Numbers of marsh slugs in the check were not sufficient to cause noticeable feeding injury.
Plant Stand Establishment: Soybeans were planted on 7 June at various planting rates using a no-till drill into the corn residue. The intent was to replant the lower-rate plantings in mid- June after the plants fully emerge to represent replanting of fields with poor emergence. The treatments were the following seeding rates (seeds planted per acre): 220,000, 160,000, 120,000, 80,000, and 40,000 for the single plantings, and 80,000 + 80,000, 40,000 + 160,000, 0 + 220,000 for the dual, replanted plots. Following emergence, it became evident that the intended plant populations were not achieved. All plots had much higher populations than intended, with the lowest rate being >150,000 plants per acre and the highest >250,000. It was determined that the cause was inexperience with the soybean drill being used. We decided to defer this study.
Extension Program Implementation
Populations were not sufficient to determine whether slugs can cause stand loss, and thus,
whether Deadline MPs can prevent such losses. However, data collected from Field 5 showed
the excellent efficacy of Deadline MPs in controlling slugs.
It appears there is a cyclic nature to slug populations. Over the past few years we have seen two springs with moderate to high slug populations (1994 and 1995) followed by springs with extremely low numbers (1996) and then slightly higher numbers (1997). 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.
Sampling in the fall of 1997 suggests that slug numbers are definitely on the rise. We are collecting many adult gray garden, marsh, and dusky slugs. We also have been observing a substantial amount of mating and egg laying. Our hypothesis is that the spring of 1998 will bring much concern to growers. Whether populations will be as high as 1994 and 1995 remains to be seen. However, there appears to the possibility that fall sampling might be indicative of the potential of an economic slug population the following spring.
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