Wednesday, May 25, 2022

Apple snails invade rice and crawfish systems

⋅ By Julian M. Lucero and Blake E. Wilson ⋅
Louisiana State University AgCenter

The apple snail, Pomacea maculata, is a global invasive rice pest. Within the past decade, the apple snail has established itself in Louisiana but has only recently begun infesting rice farms in the southwestern region.

Pictured is an apple snail egg mass on a rice plant.
Apple snails have a fast reproductive rate.

Adult snails have large, brown-green or gold shells and lay large, pink egg masses. Their fast reproductive rate and voracious appetite allow the snails to reach high population densities in natural bodies of water, as well as in rice and crawfish ponds. The snails are so disruptive to crawfish trapping that some ponds have been drained early and fishing terminated.

Initial observation suggested impacts to rice production in Louisiana would be minimal, but more recent evidence suggests water-seeded rice, approximately 15% of the state’s acreage, can be devastated if high snail populations are present at planting. Because of the potential for apple snails to become pests, it is important to determine the locality of apple snails in Louisiana and study their expansion into rice and crawfish production systems.

Rice and crawfish ponds surrounding natural waterways — including the Mermentau River, Vermilion River and Bayou Lacassine in Iberia, Vermilion, Jefferson Davis, Lafayette, Cameron and Acadia parishes — are known to harbor apple snails. These ponds have been monitored for the presence of apple snails since 2018.

Spatial analysis with ArcGIS mapping software is being used to map current data. This data is then used to extrapolate future expansion. To date, expansion into new farms has only been detected at 11 sites across the study area, a relatively low rate of expansion for invasive species.

A large apple snail was found in the entrance of a crawfish trap. Adult snails have brown-green or gold shells.

Irrigation with well water rather than surface water is thought to be the primary factor preventing introduction into new ponds. Severe weather events, such as floods and hurricanes, can create opportunities for the snails to expand into new areas. Some of the newly invaded farms were discovered following Hurricane Laura in 2020.

Fortunately, for now, it appears that establishment of the snails across the rice and crawfish production region will be a relatively gradual process. Once the snails become established, getting them under control poses another challenge.

Exploring chemical controls

Toxicity assays investigating potential chemical controls are being conducted on the LSU Baton Rouge campus as well as at the crawfish laboratory at the Rice Research Station South Farm in Crowley, Louisiana. The aim of these studies is to identify chemicals that are highly toxic to snails, but safe for crawfish.

Insecticides are frequently highly toxic to crawfish because insects and crustaceans are both arthropods and closely related. Apple snails, however, are mollusks and can be targeted with different chemicals (known as molluscicides), which may be less toxic to crawfish.

One chemical that has been identified as a potential solution is copper sulfate. Laboratory assays reveal that rates that cause high mortality of snails did not have any effect on adult crawfish. However, research into nonlethal effects on crawfish, such as reduced growth or feeding, is needed before the product can be used commercially.

Other research is examining chemical control of apple snail eggs. Preliminary results suggest solutions of inexpensive and widely available crop oil can greatly reduce egg hatch rates. However, heavily infested ponds may see thousands of new egg masses laid nightly, so spraying for eggs on a large scale may not be practical.

Report snail sightings

As rice and crawfish producers continue to struggle to manage this invasive pest, LSU AgCenter researchers are looking for solutions. It is unlikely any silver bullet that can stop the snails in their tracks will be identified, but preliminary findings suggest some relief may be coming.

Slowing the spread of these snails is critical to mitigating their impact. Farmers and boaters are encouraged to check equipment for apple snails and snail eggs before moving between locations.

It is important to contact the local LSU AgCenter Extension office to report expansion of apple snails into new rice and crawfish ponds to assist with this research. 

Julian M. Lucero is a graduate research assistant in the Department of Entomology. Blake E. Wilson is an assistant professor and field crops entomologist at the Sugar Research Station, St. Gabriel, Louisiana. This article appeared in the winter 2022 issue of Louisiana Agriculture.

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