Floods aid expansion

Giant invasive snail threatens the rice-crawfish rotation in southwest Louisiana.

By Dustin Harrell

channeled apple snail

One of many channeled apple snails found in and around a crawfish farm in Acadia Parish, Louisiana — photos by Dr. Dustin Harrell, LSU AgCenter

The channeled apple snail, a native of South America, is an invasive pest believed to have been introduced into the United States via the aquarium trade. It has been observed for several years in and around rice fields in the production region east and south of Houston, Texas. But now it has been found by at least one southwest Louisiana producer involved with rice-crawfish rotations.

The snails begin small but can grow to the size of a tennis ball. The snails feed on vegetation. However, significant rice stand reductions due to the pest have not been reported. The drill-seeded, delayed-flood rice production system seems to keep the snails out of a rice field when it is most susceptible to feeding and stand reduction.

But the snails are a threat to rice production because they tend to gather in and restrict water flow from irrigation pipes. They also lay large pink egg masses that can further restrict water flow, especially when they attach to the wire mesh some farmers use to keep trash out of the drains.

The snails love to burrow, and the edges of water boxes and overflows seem to be some of their favorite places. This compromises the integrity of rice levees and causes frequent levee blowouts. Farmers with the pest are spending more time walking levees and repairing damage caused by the snails.

The pest is not only a nuisance in rice, but it also poses an economic threat due to the time and constant repairs to the rice paddy infrastructure.

Snail challenges crawfish farmer

channeled apple snail eggs

Channeled apple snails deposit bright pink eggs on grasses or other structures.

The channeled apple snail and their bright pink egg masses have also been seen in Louisiana. Many sightings have been in the southwest rice-production region in and around the Mermentau River Basin. The September 2016 floods caused much of the Mermentau River Basin and connecting bayous to overflow.

This facilitated the widespread and fast movement of the channeled apple snail in the region. Rice and crawfish farmers using surface water from connecting bayous to irrigate are susceptible to introducing the snail into their operations.

This is what happened to an Acadia Parish farmer in Louisiana who irrigates his rice and crawfish fields using water from Bayou Queue de Tortue (Turtle-tail Bayou). He reports that the snail population exploded after the 2016 floods and has been a serious problem for him, especially in his crawfish fields.

The snails are voracious feeders, eating a considerable amount of the vegetation intended for the crawfish. The snails also plagued his crawfish enterprise by entering traps in alarming numbers.

channeled apple snails in a crawfish trap

Larger snails are big enough to block entry holes in crawfish traps as demonstrated in this photo.

Fishermen had to spend additional time separating snails from crawfish during harvest. He reports the fields were fished three days per week and his fishermen would fill six to 12 crates per day with snails. In addition, the larger snails would actually plug the inlets on a crawfish trap, making it impossible to catch the crustacean. The fishing became so bad because of the snail that the farmer pulled his traps on 220 acres before the last week of January.

This represents a significant economic loss to his crawfish enterprise. For example, average crawfish production on his fields were historically around 500 pounds per acre. Assuming that he lost approximately 400 pounds of that production this year and assuming an average seasonal price of $1.50, his losses this year due to the channeled apple snail were about $132,000.

Finding chemical control will be challenging

channeled apple snails

Channeled apple snails were separated from the crawfish during harvest. Six to twelve crates were filled each day, and fields were harvested three times per week. the fisherman pulled his traps in late January because the snails were just too much.

Currently, there are no pesticides labeled for removal of the invasive channeled apple snail in crawfish ponds. Molluscicides do exist that will kill the snails, but finding one that will not impact crawfish or other aquatic life will be challenging. It is often joked that the snails could be collected and sold as escargot.

However, since the snail is an invasive pest, it is illegal to collect, sell or transport them. One thing is certain, though — the channeled apple snail poses significant risk to the rice-crawfish rotation in southwest Louisiana.

Dr. Dustin Harrell is Extension rice specialist and research coordinator at the LSU AgCenter H. Rouse Caffey Rice Research Station in Crowley, Louisiana. He may be reached at DHarrell@agcenter.lsu.edu.