Monitor crops for pests
By Assefa Gebre-Amlak
Colorado State University Extension Specialist
It is time of the year that we should keep an eye on a number of pests including second-generation European corn borer, western bean cutworm, western corn rootworm adults, spider mites, sunflower head moth, banded sunflower moth, sunflower seed weevils and grasshoppers.
Western bean cutworm: Our pheromone trap-based survey shows that western bean cutworm moth numbers continue to increase in Akron, Eckley and Haxtun areas whereas other locations, Burlington, Kirk, Wauneta and Yuma showed decreased moth populations in traps.
Fields should be scouted closely, as once the larvae move into the ear, good control will be difficult to obtain. Chemical control should prove economical if eight percent or more of the plants have egg masses or small larvae in the tassels, and the crop is at least 95 percent tasseled. If tasseling is much less than this, the percentage of infested plants should be raised as fewer larvae are likely to reach the ears. Many of the insecticides registered for western bean cutworm control have been associated with spider mite outbreaks, so fields should be monitored for mites after a treatment is made.
Sunflower insects: Sunflowers should be scouted for sunflower head and banded sunflower moths along with other insect attacking sunflower heads the next three weeks in Colorado. We are seeing banded sunflower moth in all pheromone traps in different locations but sunflower head moths have not found in pheromone traps yet.
Insecticide applications made at early bloom (R5.1) to prevent moths from laying eggs. Scouting in the early morning or early evening will provide the most accurate counts, since moths are most active at these times. When scouting, sample sites should be 75 to 100 feet from the edge of the field. Use an X-pattern, counting moths on 20 heads per sampling site for a total of 100 heads. One moth per two plants is the currently accepted economic threshold level for sunflower head moth.
The two sunflower seed weevils, namely red and gray seed weevil have been seen in sunflowers.
Both seed weevils are small weevils found in sunflower heads, although the gray sunflower weevil is larger. Females usually lay a single egg directly into the developing seed, and the larva completes its development within the seed. Some seeds may be totally consumed; however, most seeds are only partially fed upon. Egg laying begins at the outer edge of the head and progresses inward, following seed development.
Insecticide applications are made to prevent adults from laying their eggs. Treat red sunflower weevil on oilseed sunflower when about 30 percent of the plants have reached the R5.1 stage. The economic threshold ranges from five to 15 weevils per head, depending on plant population and market conditions.
Confection sunflower should be treated to avoid quality penalties if less than 10 to 15 percent of the plants have reached R5.1 and one or more red sunflower seed weevil can be found per head. Gray sunflower seed weevil is thought to be economically insignificant under most conditions.
Western corn rootworm beetle: The beetles often feed on corn silks and severe silk pruning may result in yield reduction due to poor pollination.
Control of rootworm adults is intended either to protect silks during pollination or to prevent egg laying and damage to roots in next year crop. Adults rarely become numerous enough to interfere with pollination. Control may be justifiable if there are more than 10 beetles per ear zone during the wet silk stage. If treatments are intended to prevent egg laying then treatment is recommended when beetle counts exceed 18,000 beetles per acre (three beetles in four plants at 24,000 plants per acre). This threshold also can be used for determining the need for a soil insecticide in the following year if adult control is not used.
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