Managing Slugs In Field Crops: Why Insecticides Fail & Molluscicides Work

SHREEYA ADHIKARI AND DR. RAUL T. VILLANUEVA

LEXINGTON, KENTUCKY

 Slugs are an increasing problematic pest in field crops. Slugs attack soybean and corn seedlings shortly after emergence. In soybean, they can cause severe defoliation, stand reduction, and when the apical meristem of a seedling is eaten, the plant is killed (Figure 1). In corn, the apical meristem is well hidden, so seedling death occurs less frequently compared with soybeans. However, slugs can still destroy or cause stunting of emerging corn seedlings by feeding on the growing tissue (Figure 2).

 Insects versus slugs: differences and implications for their control

Many growers inquire about the potential use of insecticides to manage slugs, especially during wet and cloudy springs when slug pressure is high. However, insecticides are generally ineffective against slugs. This is because slugs are not insects; they are mollusks, while common crop pests like cutworms and aphids are insects. This difference is critical when selecting effective control products.

Insects have a hard exoskeleton, jointed appendages, and a nervous system that responds to insecticides targeting specific neural pathways, such as sodium channels or acetylcholine receptors. Slugs, on the other hand, are soft-bodied organisms and use mucus for movement and moisture retention. They have very different physiological structures from insects, particularly in their nervous and respiratory systems.

Because of these biological differences, insecticides, such as pyrethroids or neonicotinoids, are generally ineffective against slugs. These compounds either fail to reach the target site within the slug’s body or the slug lacks the neural receptors that these insecticides are designed to disrupt.

However, molluscicides, such as metaldehyde, are designed specifically to target slug physiology. Metaldehyde causes slugs to overproduce mucus, leading to dehydration, loss of mobility, and eventually death. Conversely, metaldehyde has little to no effect on insects because insects do not rely on mucus production or similar water-regulating mechanisms. Their exoskeleton protects them from desiccation, and their metabolic pathways are not affected by metaldehyde in the same way. This mode of action is unique to mollusks, which is why metaldehyde is effective on slugs and snails but not insect pests (Slug Control | Pacific Northwest Pest Management Handbooks, n.d.).

Work in progress

To better understand slug management strategies, ongoing research in the entomology laboratory at the UK-Research and Education Center in Princeton is evaluating the efficacy of a new seed treatment (which may be effective against slugs), an insecticide tested overseas (which showed some negative effects on slugs), and molluscicides. These studies aim to provide valuable insights for farmers seeking alternative, sustainable, and affordable solutions for slug control.   ∆

SHREEYA ADHIKARI AND DR. RAUL T. VILLANUEVA: University of Kentucky