Apple snails pose threat to Louisiana’s rice industry

Linda Benedict, Reed, Donald P.  |  1/8/2011 3:48:51 AM

Don Reed

Plants and animals introduced into Louisiana cause a wide range of ecological problems. Notable examples include water hyacinth, Chinese tallow tree, nutria, English sparrows and European starlings. Unfortunately, another animal can be added to the list that could pose problems for us in the future, the apple snail.

Apple snails are tropical or subtropical freshwater snails that occur naturally in Africa, South and Central America, and Southern Asia. They can reach sizes up to 6 inches in diameter and are well-adapted to regions with periods of drought alternated with periods of excessive rainfall. They possess gills similar to fish on one side of their body and lungs on the opposite side. This lung/gill combination greatly aids their mobility when searching for food.

Most species of apple snails lay brightly colored egg masses above the normal water line. This strategy prevents egg predation from fish and other aquatic invertebrates.

Apple snails reached the United States by way of the aquarium pet trade. Their attractive appearance and size have made them a popular addition to many home aquariums. In other parts of the world, apple snails have been introduced in an attempt to start an escargot industry. It was believed that this type of food culture could provide protein for local populations who were lacking this essential component in their diets. These introductions primarily made in Taiwan quickly spread to Indonesia, Thailand, Cambodia, Hong King, southern China, Japan and the Philippines. In none of these areas has the food value of apple snails been realized. Introductions into Hawaii in 1989 have also failed to serve as a food source.

Apple snails are not selective in their eating habits and will consume almost any type of vegetation. They do, however, tend to prefer soft succulent plants. When food supplies are lacking in their aquatic environment, apple snails take advantage of their amphibian life style and leave the water in search of food. Apple snails can live from three months to three years, depending on the species of apple snail, food availability and water temperatures. Species with shorter life cycles will reproduce throughout the entire year, while those with longer life cycles reproduce mainly in the spring and early summer.

When depositing their eggs, female apple snails climb from the water, usually during the night or early morning. They deposit their eggs on a stem of emergent aquatic vegetation, tree trunk, rock or other structure. They lay a new egg approximately every 30 seconds until an egg mass of anywhere from 100 to 1,000 brightly colored orange, pink or green eggs appears. The time from egg deposition to hatching varies from two to four weeks, depending on temperature and species.

Apple snails have been found in significant-enough numbers in Louisiana to cause concern. The discoveries so far have been along the ditches and streams of the north shore of Lake Pontchartrain. Releases from aquariums are believed to be the source of these invaders. The biggest concern at this point is the effect that large numbers could have on the rice industry. In many countries where they now occur, significant damages are reported on rice fields. They feed on young succulent rice plants. The LSU AgCenter has tested several insecticides labeled for use on rice fields in Louisiana, but none has been found effective in controlling apple snails. At present, the best control measure is to knock the brightly colored egg masses back into the water any time they are spotted.

Don Reed, Professor, Bob R. Jones Idlewild Research Station, Clinton, La.

(This article was published in the fall 2010 issue of Louisiana Agriculture Magazine.)

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