Artichoke slug, snail bait trials point to high metaldehyde concentrations

New research data comparing the molluscicide products used by California's perennial artichoke growers to control slugs and snails has shown that baits containing the highest active ingredient of metaldehyde, and offering the greatest number of baiting point sources, can reduce the number of severely damaged, or unmarketable buds to less than 1 percent.

Mohammad Bari, entomologist with the Artichoke Research Association in Salinas, Calif., conducted replicated trials in two locations last summer to compare the efficacy of three 15-pound applications of Durham 7.5% metaldehyde with three 20-pound applications of Orcal 3.25% metaldehyde. These treatments were also evaluated under different irrigation schedules, with treatments applied at 7, 14 and 20 days post-irrigation.

When Bari grouped the averages regardless of irrigation timing, he found that three applications of Durham 7.5%G resulted in zero to less than 1 percent unmarketable buds, while three applications of Orcal resulted in 7 percent to 11 percent unmarketable buds.

The average per grouped treatment shows that three applications of Durham 7.5% produced an average of 64 percent undamaged primary buds, about 35 percent buds with moderate damage, and 1 percent or fewer buds with severe feeding damage. By comparison, plots treated with Orcal 3.25% produced about 29 percent undamaged buds, 61 percent to 68 percent buds with moderate damage, and from 7 percent to 11 percent buds with severe damage from slug feeding.

Bari's trials also included two 15-pound applications of Durham 7.5% followed by one 20-pound application of Deadline 4.0% Bullet. This combination of treatments resulted in an average of 62 percent to 73 percent undamaged buds, 27 percent to 36 percent buds with moderate damage, and just over 1 percent buds with severe feeding damage.

“The performance of Durham is superior to that of other baits for two primary reasons,” Bari explains. “First, Durham contains 7.5 percent metaldehyde, while Orcal contains less than half that concentration. And, pound for pound, the Durham formulation provides five times more baiting or feeding points than Orcal, which typically offers about 25 point sources per square foot.”

He also determined that irrigation timing will impact the performance of each molluscicide treatment. He achieved the best results with applications at 7 and 14 days following irrigation, with a measurable decline in slug and snail control at the 20 days post-irrigation treatment.

“We recommend that growers let their fields dry for at least a week after irrigation before they make a metaldehyde treatment,” Bari adds. “The baits last longer under drier conditions, especially Orcal, which dissipates more quickly than Durham under moist conditions.”

Bari also evaluated an application of 15 pounds of Durham 7.5%, mechanically applied through a Gandy applicator. Those trials demonstrated that about 45 percent of the primary buds suffered zero feeding damage, 50 percent displayed moderate damage, and about 4 percent exhibited severe damage.

“Since slug and snail baits are often applied by hand, my objective was to see if mechanical application could reduce the growers' labor costs,” Bari explains.

“While a mechanical application of Durham produced better results than a hand application of Orcal, the damage ratings were a little bit higher than those from a hand application of Durham. I believe that wind conditions might have blown some of the granules away from the intended site. We hope to improve those results by reducing the distance from the outlets to the ground, and perhaps by increasing the rate from 15 to 20 pounds.”

Bari recommends that growers of California's summer and fall-harvested artichokes treat their crop with a proven molluscicide as soon as they find two slugs per shoot.

Left unchecked, he says, slug and snail feeding could cost a grower as much as 25 percent of his late summer and early fall crop.

“We calculate that about half of the primary artichoke buds produced would be unmarketable without the use of slug and snail baits,” he concludes. “Assuming that primary buds represent a third of total production, that would account for lost production of 42 cartons out of an average annual yield of 500 cartons per acre. With the grower's cost of production about $10 per carton, that could represent as much as $420 per acre in out-of-pocket losses alone.”

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