PRINCETON, Ky. — Slugs, a pest once found in homeowner’s gardens and greenhouses, have quickly become a problem in crop fields. Slugs belong to the gastropod family, which includes slugs and snails.
Slugs have a unique biology because they have both male and female reproductive parts, making them a hermaphrodite. If slugs do not find a mate, they are capable of self-fertilization so that eggs are guaranteed to be laid. Slugs usually lay eggs in batches of around 10-50 eggs (Figure 1).
Problem
Slugs can cause significant damage to field crops in their early stages. Slug outbreaks and feeding can reduce plant population densities and damage plants beyond the point of recovery. In soybeans, slugs usually feed on plants in the VE and VC development stages. If this occurs. and the apical meristems are injured, the plant cannot recover (Figure 2).
Slugs prefer weather that is cool, moist and, cloudy; under these conditions they will be active and feed all day. Otherwise, you are most likely to find them feeding from dusk to the early morning hours. It is important for farmers to know when to scout for slugs in their crops so they can take preventative actions to avoid a slug outbreak that would cause them to have to replant. In the spring of 2021, it was reported that there were soybean farmers that had to replant at least 4 times due to slugs feeding on seedlings in Central and Western Kentucky.
Management
There are many different approaches to controlling slug outbreaks; there is not a one-size-fits-all solution, and treatment plans can vary from farmer to farmer. There are biological, chemical, and physical methods to control or reduce population of slugs.
Natural enemies
Nematodes are being used to control mollusks in Europe. However, this approach has not been used in the United States yet. Carabid beetles are also known to be predacious of slugs. Josey Tolley (unpublished coauthor of this report), has been evaluating carabid species (ground beetles) feeding on slugs at the Research and Education Center at Princeton, KY during this season on field and laboratory studies.
Molluscicides
A typical method for controlling slugs in vegetables or produce of high value is through applying molluscicides. They suppress the slug population by drawing slugs to the area of application and then killing them after the molluscicide has been ingested. Since slugs are becoming more of a problem in field crops, some farmers have started using molluscicides in their fields. In Hardin County, a farmer has applied molluscicides to his soybean field at costs averaging $20/acre to avoid replanting due to damage from slugs. In his case, this farmer said that this is a feasible practice.
Fertilizers
It is commonly believed that potash is effective in repelling or reducing slugs. Some farmers use this approach, but there is not much evidence that supports this claim. Potash is an alkaline potassium compound, or a salt. It is believed that it could burn or suffocate slugs. Below, we are going to describe studies conducted about this topic.
Studies Using Potash and Molluscicides
Results
In this article, we are providing preliminary results regarding the use of potash to reduce slugs. The research for this project was conducted in the spring and summer of 2021 at the Research and Education Center in Princeton, KY. Both field and laboratory studies are included in this report.
In the field study, potash was applied at 2 rates of 100 and 200 lbs/A and plots were replicated 5 times. Tallies of slugs were conducted in these plots after application of potash in 4-ft row lengths and compared with an untreated plot. Six days after the application of potash, the number of slugs found in the 200 lbs/A plot were reduced to less than 1 slug and remained at that level for the rest of the study. In the control and 100 lbs/A plots, the number of slugs were above 1 during most dates of the study. (Figure 3) The greater number of slugs in the control and 100 lbs/A plot could be a result of the 200 lbs/A plot of potash effectively repelling the slugs to an area where a lesser or no rate of potash was applied.
In the laboratory, we studied the mortality of slugs caused by different rates of potash representing 50, 200, and 300 lbs of potash/A, as well as 2 molluscicides (metaldehyde and iron phosphate (Sluggo®)) using plastic containers filled with soil. Potash and molluscicides were sprinkled on the soil. In each container we released a known number of slugs to evaluate slug mortalities and oviposition. In this study, we found that the molluscicide treatment (10 lb/A of metaldehyde) had the greatest percentage of slug mortalities (60%) followed by the recommended rate of iron phosphate (44 lbs/A) (55%), and the 200 lbs/A potash treatment caused 47% mortality (Figure 4). Interestingly, other rates of potash were used, and the mortality effects varied. For example, in the 300 lbs/A potash application there was a mortality rate of only 31% (Figure 4). In addition, in this study we found that the number of eggs oviposited was not affected by any of the potash rates, whereas molluscicides affected oviposition by reducing the number of eggs or totally blocking reproduction.
Conclusions
Potash has some effect in repelling slugs, but the exact efficiency of its application or the recommended rate to use is still undetermined. Based on this project, the 200 lbs/A application of potash seemed to be useful in repelling slugs in the field study and it did a fair job of killing slugs in the laboratory study. However, when comparing molluscicides to potash, molluscicides had a higher rate of mortality in the laboratory study.
— Josey Tolley, Student Intern in Entomology (Murray State University), Raul T. Villanueva, Entomology Extension Specialist, and Edwin Ritchey, Soil Extension Specialist at the Research and Education Center, Princeton, KY
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