Increased mollusk activity in corn and soybeans observed in Spring 2021

Dr. Raul Villanueva Extension Entomologist
University of Kentucky

Current Situation in Field Crops

Rainy and cloudy days with soggy and cool nights are favorable for mollusk activity. These conditions describe the past 3 to 4 week’s weather conditions in Kentucky, and the damages these organisms are causing to soybean seedlings. During the first week of May I heard reports from two County Extension Agents (Lyon and Daviess counties); as well as a report from an entomologist from the North Central Region on damages they have been observing in commercial soybean fields.

Key Features of Slugs and Snails

Slugs and snails are mollusks that 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. Slugs (Figure 1) do not have the protective outer shell that snails have (Figure 2). Both types of organisms have two pairs of retractable tentacles in their head; optical tentacles with eyes located in the upper part and sensory tentacles (tasting and smelling) in the lower position.

As described above, slugs and snails prefer cool, moist habitats and can be found under leftover organic matter left from the previous season, soil cracks or rocks in field crops. In gardens or urban areas slugs and snails are found in logs, under pots or any well shaded and moist corner. 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.

Damage to Soybeans

Soybeans plants (V2 or older) are resilient and can support a great amount of foliage losses. They will recover if there is partial feeding on leaves or if the stems were not completely damaged. However, earlier stage (VE, VC, or V1) will not recover if the two cotyledons and apical bud are completely eaten. These damages may result in reduced planting density or produce empty spots in rows (Figure 3).

Chemical Treatments

Two products are registered for control of slugs and snails in corn and soybeans in Kentucky, Dealline™ M-PS Mini-Pellets (metaldehyde) and Sluggo™ (iron phosphate); whereas for wheat only Sluggo™ is registered. The rates of these two products are provided in Table 1. The price of these products are somewhat prohibitive growers need to rethink based on price of the crop and also in the possibility of replanting.

At this time insecticides cannot be used for control of slugs. Some carbamates might be effective, but many of them have already been phased-out. For example, Sevin® (carbaryl) has been effective when formulated as a bait but is ineffective when applied as a spray; furthermore, it is not registered for this purpose. In addition, these insecticides can reduce populations of some species of carabid beetles considered predators of slugs.

More information on Slugs of Kentucky

There is an online publication titled “A Field Guide to the Slugs of Kentucky” that provides useful information on the life cycle and habitat of slugs. This publication provides a 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.