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Not Your Average Bug

Wednesday, April 1, 2009
filed under: Insects

It seems oxymoronic to use words like “normal” or “average” when talking about any insect — and probably none more so than one called the “long-horned sunflower stem girdler.” Better known as the long-horned beetle (Dectes texanus), this insect is particularly unusual for several reasons:

First, it is cannibalistic. Though several larvae may hatch within a single sunflower stalk, usually only one will survive to overwinter in that stalk.

Second, the long-horned beetle doesn’t inflict its particular form of damage until late in the growing season — well after seed fill and even plant physiological maturity have occurred.

Third, its level of damage is very dependent on environmental factors — and, under some circumstances, even a high Dectes population may cause no yield losses.

Finally, because of these unusual factors, there are no established economic thresholds — or registered insecticides — for management of this insect in sunflower.

J.P. Michaud, entomologist with Kansas State University at Hays, points out that the long-horned beetle does not cause seed yield loss or affect oil quality as do other sunflower insects. It does, however, precipitate preharvest lodging — which, in turn, obviously results in crop loss.

A primary reason why there are no foliar insecticides labeled for Dectes control is the timing and length of this insect’s emergence period. In the High Plains, for instance, adults start appearing in mid-June to early July, with emergence continuing through August. After mating, eggs are laid (singly) in leaf petioles. Each female lays about 50 eggs, each in a somewhat laborious process.

Because of that extended emergence period, “no one chemical treatment — no matter how good it is — will provide protection for the entire period of oviposition” against Dectes, Michaud notes. A seed treatment protectant might be much more feasible; but to date nothing is available. (A promising seed treatment for Dectes in soybeans has been developed, but it’s not yet registered. Nor has the product been tested to determine whether it likewise would be effective in sunflower.)

After egg hatching, Dectes larvae tunnel and feed in the petioles and stem pith, eventually moving to the base of the plant to overwinter. During late summer/early fall, the mature larvae (again, just one per stalk) girdle the inside of the stalk or root crown, then move below the girdle and pack frass into the tunnels they’ve created. This girdling activity is what weakens the stalk and results in plant lodging. The worst case the KSU entomologist has seen was a sunflower field in which more than 50% of the plants were lodged due to long-horned beetle girdling.

“But remember,” Michaud reiterates. “There’s no impact on yield by the actual larval tunneling. It all pivots around that damaging behavior of the larva when it decides to go to bed for the winter.”

So what’s the best defense against Dectes? Strong, thick stalks. “Based on measurements we’ve made of stalks and girdles, it would appear the insect is physically capable of making a girdle of only one inch in diameter,” Michaud relates. “So if the stalk itself is just an inch in diameter, it’s ‘gone.’

“This is why, in confection sunflower where they’re concerned with seed size and have stalks with good girth, they can have Dectes and never even notice them. They’re getting girdled, but the stalks are too big for the insects to cut off.”

Not surprisingly, most problems with Dectes occur in dryland, oil-type sunflower fields. “Oil-type” because plant populations typically are significantly higher than in confection fields; “dryland” because under dry conditions stalk size tends to be smaller. “It all goes back to soil moisture and environmental conditions around the time of crop maturity and thereafter,” Michaud observes.

Michaud’s recipe for a worst-case Dectes scenario runs as follows: “You have a field where the grower has used a fairly high seeding rate, so the plant population is higher than it should be. Then comes a growing season where rainfall is significantly below normal, so those plants get spindly. And then, you have hot, dry weather post-maturity — which leads to rapid desiccation of those slender stalks. Follow that with anything that would delay the harvest — such as a strong wind storm.” Total it up and, if Dectes is present, lodging is virtually guaranteed.

So a low to moderate plant population is, logically, a primary ingredient in one’s defense against the long-horned beetle. But just as important — if not more so, says Michaud — is a uniform, consistently spaced plant stand. Having doubles or triples, even in a lower-population field, is going to result in plants with smaller, weaker stalks, he points out.

Another benefit of larger stalks is they become girdled later in the season. So even if Dectes is present, the grower may have already harvested the field by the time the insect’s impact is felt. “You can have 100% infestation and zero yield loss,” Michaud advises, if conditions allow the field’s harvest before lodging occurs.

Delayed stalk desiccation is key to managing the long-horned beetle, according to Michaud, since it lengthens the “safe window” wherein a field can be harvested prior to the onset of Dectes-induced plant lodging. That’s one reason why he’s very interested in “stay-green” hybrids. “If they’re agronomically acceptable in terms of harvestability, they could be a wonderful solution to the [Dectes] problem,” he says. “If the stalks remain moist while the seeds dry down to harvestable levels, those larvae will continue feeding (rather than girdling). As long as the insect feeds until after harvest, it’s not going to cut off the plant.”

— Don Lilleboe

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