Blake Wilson, Shirley, Mark G.
Blake Wilson, Julian Lucero and Mark Shirley
Invasive apple snails — sometimes called giant, golden or channeled apple snails — are native to South America but have been introduced into many regions. Several species of apple snails are invasive pests in many parts of Europe, Asia, and North and South America. But the species that has been introduced along the U.S. Gulf Coast is Pomacea maculata. The snails have been in Louisiana for more than 10 years and have been the focus of ecological research because of their consumption of vast quantities of subaquatic vegetation, eliminating habitat for fish and aquatic invertebrates. Apple snails are not picky eaters. They consume many types of living and decaying vegetation in addition to protein sources such as the eggs of frogs and other amphibians. Apple snails reproduce rapidly and are known for reaching high population densities in freshwater habitats including rivers, bayous, ponds and swamps. The first sign of apple snail invasion into new bodies of water is often the appearance of bright pink egg masses laid on structures and plants emerging from the water (Photo 1).
In 2018, rice and crawfish farmers started reporting high populations of apple snails in their ponds. This discovery was of concern because apple snails are considered major pests of rice in Spain, Southeast Asia, the Philippines, Central America and other regions, where they consume seedling rice. So far, their damage to U.S. rice has been minimal. The widespread adoption of drill-seeding rice (planting into a dry seed bed) and applying the permanent flood approximately five weeks after planting seems to have mitigated the impact of snails in Texas rice.
Drill-seeding is also prevalent in southwest Louisiana, which suggests the snails may be of minimal impact here. However, water management practices are more variable in Louisiana, with some acreage flooded continuously during rice production. Pest potential under these conditions may be greater. The snails can also interfere with rice production by burrowing into levees, requiring farmers to do additional maintenance. There is also potential for beneficial effects because some reports from Texas indicate the snails consume problematic aquatic weeds, such as ducksalad, while leaving the rice alone. Ongoing cage trials are examining what the snails are feeding on in rice fields and under what conditions they may develop into pests (Photo 2). While evidence collected to date suggests the snails may not be terribly damaging to rice in Louisiana, the situation appears to be more dire for the state’s crawfish producers.
Apple snails are reaching high populations in crawfish ponds in some areas and are affecting production. The omnivorous snails are attracted to the bait in crawfish traps. Smaller snails can enter traps and accumulate in large numbers, while larger snails block trap entrances, greatly reducing the daily crawfish capture. Farmers also have to sort through the capture to remove the snails. At some farms, apple snails are caught in such high numbers that finding a place to dispose of them is problematic (Photos 3 and 4). Crawfish producers in parts of Vermilion and Jefferson Davis parishes have found the situation so severe that fishing had to be stopped and ponds drained early in the crawfish season. Laboratory and field trials are examining potential controls that can eliminate the snails without adverse effects on crawfish growth and development.
Expansion into rice and crawfish ponds from natural bodies such as the Vermilion and Mermentau rivers was facilitated by the 2016 flood. Following detection of this expansion, LSU AgCenter scientists initiated an invasive species monitoring program across nine southwest Louisiana parishes. This program aims to identify the snails’ current distribution as well as determine the rates of expansion and modes of introduction into new ponds. Currently, apple snails are using the Vermilion River, Bayou Carlin and Delcambre Canal to extend populations north from Vermilion Bay into rice fields in eastern Vermilion, Lafayette and Acadia parishes. Other apple snail populations are moving from Lake Arthur and the Mermentau Basin into Jefferson Davis Parish and western Vermilion Parish (Figure 1).
Much of the region is not infested, but further range expansion is anticipated. Introduction onto new farms is likely to continue because many farmers use surface water connected to these major waterways as their primary irrigation source. While some introductions are unavoidable, farmers are being encouraged to prevent accidental transportation of the snails to new areas by checking equipment for egg masses before moving between ponds and by stocking ponds with crawfish only from sources known to be noninfested with apple snails. People are encouraged to contact their local AgCenter extension offices if they believe they have observed apple snails in new areas in southwest Louisiana.
While many adventurous Louisianians may be wondering how to best prepare the snails for dinner, people considering consumption of these mollusks should exercise caution. The snails are edible when thoroughly cooked and properly cleaned by removing all intestinal material. However, raw or undercooked snails can contain rat lungworm, a parasite that can cause potentially fatal eosinophilic meningitis. Other health risks are associated with the pink egg masses. The hot pink color serves as a warning to alert potential predators that the eggs are toxic. The eggs contain a protein neurotoxin called PcPV2, which has been shown to be lethal to mice and it can cause irritation of the skin and eyes of humans. Destruction of the eggs should be done using an implement to knock egg masses into the water, where they are prevented from hatching. Skin exposed to apple snail eggs should be washed immediately.
Blake Wilson is an assistant professor at the Sugar Research Station in St. Gabriel. Julian Lucero is a graduate student in the Department of Entomology, and Mark Shirley is a crawfish specialist and marine extension agent in the Southwest Region for the LSU AgCenter and Louisiana Sea Grant.
(This article appears in the winter 2020 issue of Louisiana Agriculture.)
Apple snail egg mass on a rice plant. Photo by Blake Wilson
Members of the Wilson Entomology Lab set up field cages to study impacts of snails in rice fields and potential chemical controls. Photo by Blake Wilson
A large apple snail in the entrance of a crawfish trap. Photo by Mark Shirley
Snails removed from a single pond during the 2019 crawfish season. Photo by Blake Wilson
Figure 1. Estimated distribution of apple snails in rice and crawfish production regions of southwest Louisiana as of 2019. Map based on monitoring of 47 rice and crawfish sites in addition to detections provided by the USGS.