Thwarting the Stem Weevil
Anyone who’s ever had to spray for sunflower insects like the seed weevil, stem weevil, banded moth, sunflower head moth or sunflower beetle probably knows the significance of December 31, 2009. That’s the date on which the U.S. Environmental Protection Agency’s (EPA) cancellation of all remaining carbofuran (Furadan®) registrations took effect. The bottom line is this: Any crop receiving a Furadan application after January 1, 2010, cannot be legally sold.
Furadan’s manufacturer, FMC, did indicate in late October its intent to challenge the final EPA decision to deny an administrative hearing on the agency’s action to revoke all U.S. food tolerances for carbofuran insecticide. But there’s no assurance any such legal avenues will result in the reinstatement of carbofuran.
EPA’s May 2009 action to revoke carbofuran tolerances was the culmination of a regulatory process that began in 2006 when the agency published its risk assessments for carbofuran. In August of that year, EPA determined no uses were eligible for reregistration. Since then, FMC voluntarily canceled 22 carbofuran uses. However, EPA concluded “the elimination of these uses was not sufficient to allow the agency to make a finding that combined dietary exposures to carbofuran from food and water are safe.”
Among the agricultural groups, state departments of agriculture and university specialists who had advocated for the continued registration of carbofuran was the National Sunflower Association. For the sunflower sector, one of (liquid) Furadan 4F’s key strengths was its systemic activity on stalk insects like the stem weevil. Another plus was its being labeled for control of several key insects — i.e., banded moth, sunflower (head) moth, seed weevil, stem weevil, sunflower beetle and grasshoppers.
There are several other insecticides currently labeled for the same insects in sunflower. The list includes beta-cyfluthrin (Baythroid XL), chlorpyrifos (e.g., Lorsban 4E, Warhawk, Yuma 4E), chlorpyrifos + gamma-cyhalothrin (Cobalt), cyfluthrin (Tombstone), deltamethrin (Delta Gold), esfenvalerate (Adjourn, Asana XL), gamma-cyhalothrin (Proaxis), lambda-cyhalothrin (e.g., Silencer, Taiga Z, Warrior) and zeta-cypermethrin (Mustang Max EC). All these products also are labeled for cutworm control in sunflower. Like Furadan, most of them are restricted- use pesticides.
One insect for which Furadan’s cancellation has particular implications is the sunflower stem weevil. Early season applications of Furadan 4F went a long way toward protecting against stem weevil because of the insecticide’s extended systemic activity in the plant.
“It was a very nice material,” says J.P. Michaud, entomologist with Kansas State University at Hays. “In terms of its systemic activity, you could control not just stem weevil, but other stalk-boring insects as well — though not all of the Dectes (long-horned beetle).” Michaud says Furadan provided generally good control for stem weevil when applied as a foliar treatment around the V6-V8 stage of the sunflower.
That’s a mute point now, of course, given the cancellation of Furadan. So for 2010, the question for growers with stem weevil concerns still remains: When, how and with what should I treat for this insect in my sunflower fields?
The good news, says Michaud, is that “any registered organophosphate or phyrethroid is going to give you good control, in my experience.” The difference is, “none of them will have the systemic activity of Furadan. So keep in mind that you’re just going to be killing adults.”
The Kansas entomologist says he has had good results with foliar sprays of products like Warrior® if they go on at the right time, because the adult stem weevils “are very synchronous in their activity, not long-lived and not hard to kill.”
That said, Michaud indicates he is “actually wary of advising people to spray for stem weevil.” Why? “I realize there are losses [due to stem weevil] out there,” he replies. “But the standard IPM model doesn’t work so well when applied to stem weevil because we don’t really have a good economic threshold” for this insect. “There does not seem to be a strong relationship between the number of adults you can see and the number of larvae you end up with in the stalks.”
The reason for this weak correlation, he continues, is that “if the plants are very healthy, they can resist the development of a lot of larvae. So even if you have lots of adults, you won’t necessarily have a serious problem. But if the plants are stressed — either by being too dry (worse) or too wet — that seems to reduce the plants’ resistance to stem weevil.”
The other complication for stem weevil foliar treatments, Michaud adds, is that “we’ve never really established a threshold number of larvae and said, ‘OK, you start having significant yield loss with X number of larvae per stalk.’ Sunflower stalk size can vary significantly, depending on plant spacing and other factors.
“So when dealing with an insect whose numbers can be more than 100 per stalk, we really should be expressing the threshold as ‘number of larvae per unit of stalk diameter or volume.’ ”
Michaud suggests that on an “averaged-sized” oilseed sunflower plant, an economic threshold would be “more than 40 larvae per stalk.” Again, he emphasizes, no actual recommended threshold has yet been developed for stem weevil. “But if you have fewer than 40 per stalk, you’re probably not going to see any yield loss,” Michaud remarks.
That statement is contingent, though, on the absence of stalk-weakening diseases like Phoma. “If you have a lot of Phoma, [stem weevils or other stalk insects] exacerbate the disease infection. They initiate it,” Michaud points out. “But Phoma development also is contingent, to a large degree, upon the survival of insect larvae” because the disease proliferates by following the insect tunnels within the stalk.
“The real problem with Phoma and stem weevil together,” the KSU entomologist says, “is, if they occur early you’re going to have premature ripening; and if it’s later in the plant cycle, you’ll have lodging problems.”
While specific management recommendations for sunflower stem weevil are in short supply, there is good news from the breeding front: Host plant tolerance to this insect does exist and is being exploited. Screening and selection for tolerance to stem weevil and other sunflower insects has been going on for several years in a cooperative project among USDA-ARS entomologists and university researchers in Kansas, South Dakota and North Dakota.
In a 2008 trial at Colby, Kan., investigators found significant tolerance to stem weevil in several breeding lines. In one group of tested lines, for instance, per-stalk larval counts ranged from two to 40. Of the 83 lines tested, 13 had fewer than 10 stem weevil larvae per stalk.
Until commercial sunflower hybrids possessing good tolerance hit the market, however, growers must continue to rely upon rotation, planting date and/or a foliar insecticide. Michaud notes that stem weevil populations are most likely to manifest in rotations where sunflower is grown in tighter rotations. The insect does not travel well (as opposed to the banded sunflower moth, for example). “It can barely fly; and if it has a host plant, it doesn’t move,” the Kansas entomologist explains. “So if you have a history of stem weevil problems in a certain field, you may want to consider treatment at around the six- to eight-leaf stage” as an insurance policy.
A later planting date goes a long way toward avoiding stem weevil problems altogether. For High Plains sunflower fields planted after the second week in June, “I’d say ‘don’t worry about it,’ ” Michaud advises.“Where you want to be concerned is with early planted fields in areas with a history of intensive [sunflower] cultivation and weevil-induced lodging.”
— Don Lilleboe
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