Current Situation in Field Crops
Rainy and cloudy days with soggy and cool nights are favorable for mollusk activity. These conditions describe the current weather in Kentucky, which has been conducive to the damage these organisms are causing to soybean seedlings. Last week, I heard reports from two county Extension agents (Lyon and Daviess Counties) on damages they have been observing in commercial soybean fields.
Key Features of Slugs and Snail
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 do not have the protective outer shell that snails have. 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 snail prefer cool, moist habitats and can be founds under leftover organic matter from the previous season, in soil cracks, or under rocks in field crops. In gardens or urban areas slugs and snail are found in logs, under pots, or in any well shaded and moist corner.
During the day, slugs move deep into the ground. At dusk, they become very active, from sunset until 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 foliage losses; they will recover if there is partial feeding and leaves or stems were not completely damaged. However, earlier stage (VE, VC, or V1) will not recover if the two cotyledons and the apical bud are completely eaten; this type of damage may result in reduced planting density or produce empty spots in rows.
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). For wheat, only Sluggo™ is registered. The rates of these two products are provided in Table 1. The prices of these products are somewhat prohibitive, so growers need to also consider the price of the crop and also 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.
Table 1. Rates of Dealline™ M-PS Mini-Pellets (metaldehyde) and Sluggo™ (iron phosphate) for different stages of growth for corn, soybeans and wheat.
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 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.
By Raul T. Villanueva, Entomology Extension Specialist