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Managing Stem Weevil In the High Plains

Saturday, March 1, 1997
filed under: Insects

Though most of the sunflower stem weevil’s notoriety has sprouted out of the Northern Plains, it should come as no surprise that this insect’s negative reputation among sunflower producers extends to the High Plains region as well. After all, the Mr. LeConte whose moniker is associated with the stem weevil’s scientific name — Cylindrocopturus adspersus (LeConte) — collected specimens of this insect in Texas way back in the 1870s.

The stem weevil also was found in Colorado sunflower grown for silage in the early 1920s. Fifty years later, Texas entomologists discovered cases of very high larval populations in sunflower stalks.

Now, during the mid-1990s, the infamous stem weevil has been causing economic damage in a number of commercial fields in the sunflower production areas of northeastern Colorado, western Kansas and certain locales in southern Nebraska. Weevil-influenced lodging of 20 or 30 percent of an infested field’s plants has not been uncommon; in extreme cases, that level has been as high as 70 or 80 percent.

The Northern Plains’ stem weevil experience during the past couple decades has stimulated considerable research on this sunflower pest, culminating in the development of current monitoring and treatment guidelines. Fortunately, much of that information is transferable to the High Plains, though there are some notable caveats due to the two regions’ differing environmental conditions.

For those High Plains producers who have incurred damage from the sunflower stem weevil — or are located in an area where the stem weevil has been active in recent years — the good news is that this insect can be controlled. But the control recipe calls for knowledge of its biology, vigilance in scouting, and timely application of insecticide treatments. Here’s a brief summary of what entomologists know and recommend.

— Life Cycle —

Having overwintered as mature larvae in sunflower stalks and root crowns, the pupated adults emerge in early to mid-April in the Southern Plains and in mid- to late June in the Northern Plains. Following emergence, they commence feeding on epidermal tissue of the sunflower stem and foliage. The adults remain in Northern fields until late August, whereas in the High Plains they’re typically gone by the end of July or first of August.

Mating takes place soon after adult emergence, with the females depositing each egg individually in the stem’s epidermal tissue. While the mating period does stretch over several weeks, the majority of oviposition typically occurs within a couple weeks after emergence.

Upon hatching, the early instar larvae feed on vascular tissue. Then, as they mature, they tunnel into the pith tissue. The feeding larvae descend down the stalk interior and eventually end up at the stalk base, just above the soil surface. There a chamber is constructed, where the weevils overwinter as fifth-instar larvae until pupation the following spring (in the High Plains) or early summer (Northern Plains). The sunflower stem weevil produces only one generation per year.

— Damage —

Larval populations of well over 100 per sunflower stalk have been documented. Fortunately, most concentrations in infested fields are substantially lower. However, even levels of 25 or 30 per stalk can result in significant weakening of stem tissue, with the plant then becoming much more prone to lodging — especially under windy conditions and/or drought stress. Breakage normally occurs at or slightly above the soil surface, since that is where the larvae have constructed their overwintering chambers.

The sunflower stem weevil also has been tied to the introduction of pathogens which lead to Phoma black stem (Northern Plains) and charcoal stem rot (High Plains). These pathogens and the resulting diseases can help bring on premature ripening in sunflower plants.

— Scouting —

Thorough, timely crop monitoring is the only way to accurately determine whether the stem weevil is present at economic levels. Because the larvae live and feed inside the sunflower plant, only by splitting open stalks can they be counted. That can be very difficult and time-consuming, so scouting for this insect is thus directed toward the adult.

When should growers or their hired scouts be on the lookout for adult stem weevils? The earlier-cited calendar dates provide a general clue as to probable timing of emergence. For the High Plains, research by Colorado State University entomologist Scott Armstrong has shown that growing degree day (GDD) information can help producers determine when to expect adult emergence. Using 43 degrees as the base temperature, Armstrong’s work suggests emergence will begin at approximately 300 GDD (± about 30). That level is normally reached sometime between May 7 to June 6, depending upon weather and one’s location within the High Plains.

“Degree days are used only as a guide to know when intensive scouting for stem weevil populations should occur,” Armstrong emphasizes. “When over 300 degree days have occurred in your growing area, fields should be closely scouted.”

Sampling sites should be in at least 75 to 100 feet from field margins. Using the standard “X” pattern, scouts should examine five plants per site, stopping at a minimum of five sites per field. The entire plant should be examined, with particular attention given to both sides of leaves, leaf axils, the lower stem, dried cotyledon leaves and to cracks in the soil around the base of the sunflower plant.

Scouting for the adult stem weevil can be tricky. If they sense movement, the adults tend to drop to the ground and play dead. Scouts must move slowly and deliberately to avoid that reaction.

Economic Thresholds &

— Foliar Insecticide Treatment —

North Dakota research has resulted in a recommended economic threshold of one adult per three plants, and this is the threshold currently used for the High Plains as well. (That infestation level — based on sampling in late June/early July in North Dakota — has been shown to result in up to 40 or more larvae per stalk by season’s end if no control measures are taken.)

It’s important to begin foliar insecticidal treatments prior to most of the egg laying. If that’s not done, succeeding larval populations will not be adequately reduced. In the Northern Plains, that timing typically is late June/early July. CSU’s Armstrong suggests using growing degree days as a yardstick for anticipating emergence and egg laying — and thus scouting and treatment — periods for the High Plains. Banding of insecticides can be an economical yet effective treatment method.

— Other Management Strategies —

Along with foliar insecticides, there is the option of an at-planting systemic treatment, Furadan 4F. Scott Armstrong suggests it be considered by High Plains growers who (1) have had significant stem weevil infestations the previous year and (2) are planting prior to June 1.

Preliminary CSU work has indicated that planting sunflower after June 1 “can significantly reduce economically threaten-ing stem weevil populations,” Armstrong adds. North Dakota research also has shown a dramatic decline in stem weevil larval numbers when planting dates were delayed from mid-May to early June.

Stalk strength obviously plays a critical role in determining a weevil-infested plant’s ability to avoid lodging. Later plantings can result in smaller-diameter stalks; however, the dropoff in numbers of weevil larvae appear to more than offset strength loss from the reduced stalk size.

Though research in the late ’70s and early ’80s suggested that post-harvest tillage of infested stalks was not an effective way to control stem weevil, more-recent research by USDA’s Larry Charlet indicates that adult emergence the following spring could be reduced — if the stalks are buried at least six inches below the soil surface. Also, larval survival can be reduced — if the stalks are sufficiently broken up to expose the larvae to the soil. Of course, these methods are not complementary with the reduced-tillage systems presently employed by many sunflower producers. — Don Lilleboe
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