|Crop Knowledge Master|
Veronicella leydigi (Simroth)
Victoria L. Tenbrink, Research Associate
Arnold H. Hara, Entomologist
Beaumont Research Center
Slugs are general feeders, consuming stems, leaves, flowers and roots of many plant species. Besides plants, they can live on decaying vegetable matter, fungi, molds, earthworms, and dead or slow-moving insects.
Veronicellid slugs are tropical (Deisler & Stange, 1984). The black slug was described from Queensland, Australia, and was introduced into Honolulu in the early 1900Ős (Hawaii Dept. of Agriculture, unpublished).
Slugs and snails feed by scraping off plant tissue. In the slugŐs mouth is a rough, tongue-like rasping organ called a radula (Hara & Iwata, 1987). Slugs feed on tender shoots, buds, and roots, as well as mature plant parts. Slug damage can be so serious as to cause plant death, especially death of seedlings and very young plants. Vegetable and ornamental crops can be ruined.
Eggs are oval, clear, and laid in groups of 10 to 200 (depending on species) under about 1 inch of moist soil. The eggs are about 1/8 in (3 mm) in diameter. Eggs hatch within 2 weeks to a month after they are deposited (Yates, 1988).
Newly hatched slugs remain in the area for several months. They are able to reproduce at 3 to 5 months, but may keep growing for 2 years (Yates, 1988).
The black slug has a soft, unsegmented body. The back is covered by a tough, leathery mantle skin with a yellow line down the middle. The head has 2 pair of tentacles. The upper pair have eyes and can be pulled back toward the head, while the shorter pair below have organs of smell (Williams, 1931). Like all of its slug relatives, the black slug is a hermaphrodite, having both male and female reproductive parts in the same animal (Crowell et al., 1986; Yates, 1988). Fertilization, however, is always by another slug (Yates, 1988).
Slugs can infest an area in very high numbers and yet not be seen. This is because they move around and feed at night. During the day, they hide under rocks, boards, decaying plant matter, and other places where it is moist and dark. Under moist conditions, the black slug may return to its hiding place very late in the morning. Having a leathery mantle, it is somewhat protected from drying by the sun. It lives on the ground and deposits its eggs in a protected place (Williams, 1931).
Sanitation is vital to slug control. Slugs escape hot, dry conditions by hiding between the soil surface and any material on the soil during the daytime. Removing boards, pots, and debris from growing areas will reduce slug cover and breeding areas Thinning dense foliage will allow sun to reach the soil (Yates, 1988). Weather affects slug populations. Hot dry weather or excessive flooding reduce populations, while cool, damp weather boosts populations (Crowell et al., 1986).
Slugs are known to be infested with tiny, internal parasites of the protozoan group (Crowell et al., 1986).
Domestic fowl and wild, ground-feeding birds eat slugs (Crowell et al., 1986). Toads and beetles also eat slugs (Yates, 1986).
In small plantings, trapping may be the best and safest method of control. Good sanitation involves removing hiding places, but one method of trapping is to put boards down to attract the slugs. The boards can be turned and the slugs hand picked and drowned in water (leave overnight, or add kerosene or detergent to the water for faster kill). Sunken traps containing beer will attract and kill slugs. Caution is advised when handling slugs and snails because they may carry disease. Waterproof gloves should always be worn when handling slugs or snails (Crowell et al. 1986; Yates 1988).
A promising, nonchemical postharvest disinfestation treatment for propagative material and cut flowers and foliage is hot water treatment. Water at 49ű C (120ű F) for 8 min is accepted by USDA-APHIS as a quarantine treatment for slug and snail-infested plant material (USDA 1993). Temperature and time must be closely monitored to avoid damaging the commodity. Dendrobium, anthurium, and some foliage are damaged by water at this time and temperature.
Baits are the preferred method of control. Baits consist of a base that is attractive to slugs plus a poison. Effectiveness on particular species varies with the bait formulation. Instructions should be followed as to time of day and proper weather for application. Generally, baits should be put out in the evening after a shower or irrigation. Slugs will be more likely to come out of hiding when the area is damp. Heavy rain after baiting, however, may ruin the bait. Also if rain occurs within 48 hours after baiting, it will counteract the dehydrating effects of metaldehyde, allowing slugs to recover. If the active ingredient is methiocarb, slugs are less likely to recover under wet conditions (Crowell et al., 1986).
Molluscide spray is the second means of chemical control. Slugs must be wetted with the spray or come in contact with the residue to be killed. The slime layer on slugs gives them some protection from contact poisons (Henderson & Parker, 1986).
Slugs and snails are of quarantine significance on exported plant material. Control must be 100% for quarantine security. Washing plant material in a mild, plant wash solution and then dipping in insecticidal soap at the label rate are recommended control measures. Caution is advised for anthurium, dendrobium, and cycad foliage (sago, circinalis, etc.) because strong soap solutions may cause damage. Liquid molluscides may be effective as dips. Read the label and check with the Department of Agriculture (Hara & Iwata, 1987; Tenbrink et al., 1991).
Arita, L. H. 1990. Control of the black slug, Veronicella leydigi (Simroth) and the Veronicellid slug, Veronicella cubensis (Pfeiffer) on chrysanthemums. Horticulture Digest, No 92. University of Hawaii College of Tropical Agriculture and Human Resources, Department of Horticulture: Honolulu.
Crowell, H. H., B. C. Simko, J. Capizzi. 1986. How to Control Slugs. Oregon State University Extension Service, FS 277.
Deisler, J. E. & L. A. Strange. 1984. The Veronicellid Slugs of Florida. Entomology Circular No. 261. Florida Department of Agriculture & Consumer Services, Division of Plant Industry: Gainesville.
Henderson, I. F. & K. A. Parker. Problems in developing chemical control of slugs. Aspects of Applied Biology 13: 341-347.
Hara, A. H. & R. Y. Iwata. 1987. Snails as Quarantine Pests of Ornamentals. Horticulture Digest No 84. University of Hawaii College of Tropical Agriculture and Human Resources, Department of Horticulture: Honolulu.
Tenbrink, V. L., J. D. Hansen & A. H. Hara.Phytotoxicity of SaferŐs Insecticidal Soap and Mavrik Aquaflow as a Postharvest Dip, 1990. Insecticide & Acaricide Tests 16: 262-263.
USDA, APHIS (United States Department of Agriculture, Animal and Plant Health Inspection Service). 1993. Treatment Schedules. Section 6.1, T200, p. 6.22.
Williams, F. X. 1931. Handbook of the Insects and Other Invertebrates of Hawaiian Sugar Cane Fields. Hawaiian Sugar PlantersŐ Association: Honolulu.
Yates, Julian. 1988. Slugs and Snails. Urban Pest Press. Volume 1, Number 3, December. University of Hawaii College of Tropical Agriculture and Human Resources: Honolulu.