Kentucky Wheat: Slugs – A Management Challenge for Farmers and Researchers

Slugs - Photo: Scott Stewart, University of Tennessee

Key Features of Slugs

Slugs are mollusks and do not have legs.  In order to travel, they secrete a mucus from a gland located at the anterior part of their bodies. This mucus helps slugs and snails slide over surfaces, leaving a “slime trail” that indicates their presence. Unlike snails, slugs do not have a protective outer shell.

Slugs have two pairs of retractable tentacles in their head; optical tentacles with eyes are located in the upper position and sensory tentacles (tasting and smelling) in the lower position.

Slugs prefer cool, moist habitats and can be founds under logs, rocks, or under pots. During the day, slugs move deep into the ground. At dusk, they become very active from sunset to sunrise.  During this time, slugs rasp leaf surfaces, which may result in scars or holes in foliage or death of small seedlings.

Continuous rains, wet soils, and cloudy days of early spring 2018 are favoring population outbreaks of slugs. This condition increases when planting is conducted on no-till fields.

Damage to Wheat

Wheat seeds become attractive to slugs at planting within hours after sowing. Slugs feed on embryos, which kills seeds and results in fields with lower plant densities.

There are some reports of slug damage in wheat in Europe. In England, slugs have been observed grazing on emerged leaves, but this damage is less likely to affect yields.

Current Situation in Field Crops

The current rainy spring in Kentucky is delaying agricultural activities, such as planting and applications of fertilizers and pesticides (insecticides, fungicides, or herbicides). In addition, it has been observed that wheat fields across many counties (i.e. Hardin, Henderson, and Caldwell) are infested with high numbers of slugs.

This is a worrisome issue for farmers because slugs may damage plants by feeding on leaves or on developing grains. Previous research studies on slug numbers in order to determine threshold levels in wheat, which could help in management, are scarce or not published here in the USA.

Research in Kentucky

At this time, at the University of Kentucky’s Princeton Research and Education Center, researchers are tallying slug numbers in wheat in a project funded by the Kentucky Soybean Growers Board.

This study, conducted in irrigated and non-irrigated fields, will continue with soybean (double crop system) after wheat is harvested.  We are using pitfall traps and white-painted roof shingles with hotdog baits to tally slug numbers.

In the pit fall traps, we found an average of 10 slugs per trap collected in a 7-day period. In the hotdog-baited white-painted roof shingle traps, we found an average of 6 slugs (Figures 1 to 3) per trap after 12 hours.

These numbers alone do not provide too much information – we do not know if these numbers are high, low, or average, because previous historical information is nonexistent or unpublished, making it difficult to do a comparison or make a science-based recommendation.

Figure 1. White painted roof shingle used to trap slugs (Photo: Raul Villanueva, UK)

Figure 2. Slugs found on hotdog-baited traps. (Photo: Rocio Davila, UK)

Figure 3.  Slugs adhering to the unpainted side of a roof shingle. (Photo: Zenaida Viloria, UK)

Anectodotal evidence of slug in wheat is limited, but does exist. For instance, in 2016, Dr. Carl Bradley, UK Extension Plant Pathologist located in Princeton, KY, observed slugs in one research plot. In this plot, he used continuous mist irrigation to increase humidity in the field, thus increasing the presence of Fusarium head scab in the plot.

Dr. Bradley did not observe slug damage in wheat, although in that case, the wheat was between the boot stage (Feekes 10) to Kernels with doughy or mealy consistency (Feekes 11.1).

Right now, the wheat research fields described in this article are at flag leaf growth stage (Feekes 9). Our visual inspection of the wheat plots did not show any feeding damage by slugs. It is possible that slugs may be feeding in some other plants and skipping wheat.

Also, it is known that slugs prefer soybeans and corn rather than wheat. Thus, during the seedling stage for corn and soybean, farmers should be scouting for slugs if rains continue to prevent replanting and reduce yields.

Management

Cultural Practices

In Kentucky, most farmers are committed to no-till or reduced-till practices. However, under the current situation, a light or minimal tillage may help control slugs in wheat if necessary. There is information on the use of fertilizer as a means of control.

Urea (30%) mixed with an equal amount of water, and applied with 20 gallons of water per acre is recommended by Douglas and Tooker (2012). This tactic is typically repeated a few nights in a row to reach as many slugs as possible and to maximize its effectiveness.

Chemical Treatments

I have received inquiries about the use of insecticides. Some carbamates might be effective, but many of them have already been phased-out. For example, Sevin has been effective when formulated as a bait, but is ineffective when applied as a spray; furthermore it is not registered for this purpose.

The only molluscicides registered for wheat in Kentucky is iron phosphate (Sluggo). The recommended rate for Sluggo is 10 to 44 pounds per acre when slugs or snails are detected. The cheapest price I found online was $138.95 for a 40-pound bag.

Hence, for 10 acres of wheat using the minimum and maximum rates recommended by the manufacturer, a grower might need to expend $347.37 to $1528.45 for a single application of Sluggo. Based on these figures, I think the price of this product is prohibitive for use in wheat.

Registered molluscicides to control slugs and snails in corn and soybeans area metaldehyde bait (e.g., DEADLINE M-PS mini pellets) and iron phosphate (Sluggo). See Table 1 for registered products to control slugs and their respective rates for field crops.

Table 1. Rates of Dealline M-PS Mini-Pellets (metaldehyde) and Sluggo (iron phospahte) for different stages of growth for corn and soybeans.

Products
Crop
Growth Stage
Max. single applic./ Acre
Total No. of applic./season
RAI
PHI
Type of application
Deadline Field Corn Up to V8 25 lbs/Acre 3 7 days 0 days Broadcast or ground directed
Field Corn V8 to VT 25 lbs/Acre
Soybean Up to V4 10 lbs/Acre
Soybean V4 to R1 10 lbs/Acre
Sluggo Field Corn Seedling or later stages 20 to 44 lbs/Acre n/a n/a n/a Broadcast or ground directed
Soybean
Wheat Seedling 10 to 44 lbs/Acre

Information on Slugs of Kentucky

There is an online publication titled A Field Guide to the Slugs of Kentucky that provides useful information on life cycle and habitat of slugs. This publication provides detailed description of invasive species, given that they cause more damages on vegetable, ornamental and field crops.

Among the invasive species described are the Arion hortensis, Lehmannia valentiana, Arion intermedius, Limax flavus, Arion subfuscus, Limax maximus, Deroceras laeve, Milax gagates, and Deroceras reticulatum. For information on the biology, ecology, and species description, the above publication should be consulted.




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