Watch out for Slugs!

The cool and damp start to the season we’ve had not only impacts the germination and growth of young corn and soybean plants, it also can leave them vulnerable to pests like slugs. Fields with heavy residue from weeds or cover crops, especially no-till fields, are at higher risk of harboring slugs. This is due to the residue covering the soil surface holding in moisture and cooling its temperature, creating an ideal habitat for slugs. Research in PA has shown soil temperatures to be 1.3 to 4.3 degrees cooler when planting into high cover crop biomass (Reed et al., 2019).

Adult slugs typically lay egg masses in the fall in the soil. However, some may hatch and overwinter as adults laying eggs in the spring. Mild winters and high residue can provide extra protection, leading to increased populations the following spring. Juvenile slugs hatch in mid-spring, typically in May in northern regions, and begin feeding within a couple weeks. The slugs continue to feed on crop foliage and other residue throughout the summer and typically are mature enough to lay eggs by the fall.

Seedling defoliation from slugs can happen very quickly given the right conditions. Therefore, it is important to scout fields even before the crops begin to emerge. Look for slug eggs on the soil or residue that look like very small pearly orbs (Image 1). Juvenile slugs look just like adult slugs but are very small (Image 2).

Image 1 (left). Slug eggs. Image 2 (right) Juvenile slugs. Source:

As soybeans emerge, slugs may feed on the cotyledons causing irregular shaped holes (Image 3). As the plants grow, damage can be seen on the leaves (Images 4 and 5).

Image 3 (top image). Slug damage to soybean cotyledon.Source:

Image 4. Slug damage on young soybeans.  Source: 

Image 5. Slug damage on young corn. Source: Sara Ziegler, UVM Ext.

Because the growing point of a young corn plant remains under the soil surface and protected by the leaf whorl for some time, corn often is able to outgrow early slug damage. However, in soybeans, where the growing point is above ground and exposed to slugs, damage can be severe if it occurs at the right time and slug pressure is high. Scouting can help inform the grower as to relative slug pressure and can identify a serious problem early, allowing for replanting if necessary. Slugs are most active at dawn and dusk when moisture tends to be high and temperatures are cooler. Because the eggs and juveniles are so small, care should be taken to look very closely under residue and on and around seedlings. Unfortunately, there currently are no economic thresholds for slugs on either corn or soybeans to use as guidelines when scouting.

Management tactics for slugs are limited and may not fit within some cropping systems. However, there are some simple cultural practices that may help. Tillage can bury eggs and destroy slugs, however, obviously this is at odds with the goals of no-till cropping systems and will not be an acceptable control method. Within no-till systems, it may be possible to adjust equipment to provide better residue management (i.e. row cleaners) and can help dry and warm the soil allowing for faster crop growth. Additionally, ensuring that the furrow is closed can help as an open furrow can allow slugs to travel more easily from one plant to another. Spiked closing wheels can help close the furrow and disrupt slug travel patterns between plants. Another alternative may be shifting planting dates. Planting earlier when slugs have not yet hatched may allow for the crops to emerge and grow to a point where they can overcome the later heavier slug pressure. If planting prior to slug hatch is not possible, later planting dates may be considered when temperatures are warmer allowing for faster germination and growth to outpace the slugs. Although it seems counter intuitive, using untreated seed may help combat slug populations. One of the major natural enemies of slugs are ground beetles. These insects are sensitive to neonicotinoids found in corn and soybean seed treatments such as Cruiser (thiamethoxam) and Poncho (clothianidin). Using these seed treatments when there isn’t a pest problem in the field can lead to a secondary outbreak in slugs due to harming the natural enemy population. Preliminary research at Penn State by entomologist John Tooker, has also suggested that cover cropping may actually decrease slug populations. Having a cover crop of cereal rye or clover in the field provides the slugs an alternative and perhaps preferential food source over your crop. Additionally, having a cover crop provides habitat for natural enemies like the ground beetles that can help control the slug population.

If all else fails, there are two chemical slug control options: metaldehyde and iron phosphate. Metaldehyde causes the mucus-producing cells within the slug to burst, causing death. Products typically contain 3-4% metaldehyde and are mixed with a food-based carrier to bait the slugs into ingesting the chemical (Image 6).

Image 6. Slug baits containing iron phosphate or metaldehyde Source: Hollingsworth et al., 2013

Iron phosphate causes slugs to cease feeding causing death in a matter of days. Similar to metaldehyde products, iron phosphate is typically mixed with food-based bait carriers. Iron phosphate is less effective and more expensive than metaldehyde,Wawtch but is approved for use in organic systems. When using either product it is important to apply the material evenly to provide the best control. Continuing to scout following an application can help you determine if the control is working. These chemical controls also do not harm natural enemy populations as seed treatments can.

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