Mild winter leads to early season insect problems

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

Alfalfa weevil

Mike Staton and Paul Wylie in southwest Michigan reported first instar alfalfa weevil larvae feeding last week; as the weather warms back up, expect an increase in activity. The easiest scouting method uses “tip injury.” Survey across the field, not just on an edge. Check tips of 100 stems for feeding. Treat if 40% of stems show damage AND the field won’t be cut for at least seven days. An alternative scouting method is the “stem and bucket” threshold from Ohio State University. Snap off 30 stems at random from a field and place in a bucket. Shake the stems to knock off late instar larvae (the big guys). Don’t worry about the little larvae. Count the total number of larvae in the bucket. Then randomly select 10 of the 30 stems you just picked, and measure the average stem height (I marked a ruler on the handle of my sweep net). The treatment threshold, in number of larvae per 30 stems, varies with average stem height in inches, as follows:

  • 12 inches - 30 to 50 larvae
  • 16 inches - 40 to 75 larvae
  • 20 inches - 45 to 85 larvae
  • 24 inches - 50 to 90 larvae

If a field is over threshold but will be cut within the next week, consider cutting early to kill weevils. This is the preferred control method because it preserves natural enemies and pollinators, and saves insecticide cost. Also, most insecticides cannot be used within 7-21 days of cutting, depending on the product. After cutting, remember to check for weevil larvae on the regrowth, which can delay or prevent green-up. The threshold after cutting is 6 to 8 larvae per square foot of regrowth.

If forage quality might be adversely affected, cutting may not be possible. If you decide to treat, remember to note the pre-harvest interval (PHI) of insecticide. The PHIs range from 0 to 28 days, depending on the product and rate per acre. Also remember that there are many beneficial insects in alfalfa that are killed by sprays and that most labels now include honeybee warning statements. Fields in bloom should never be sprayed because all of the insecticides registered for alfalfa are toxic to bees – these fields should be cut.

Springtails

Globular springtails are back in action damaging sugar beets emerging from the soil. There are reports of stand reduction and treatment of many acres in the Thumb. Springtails are tiny soil insects, greenish to purplish-gray in color, that jump when disturbed. They are common in soil, and normally are beneficial, eating decaying plant material, fungi or bacteria, breaking down residue and improving soil structure. On newly emerging beets, springtails feed aboveground on foliage, damaging the cotyledons as they emerge, and leaving a scraped or scarred appearance on leaves. Damage occurs when populations are very high (thousands per square foot), most often in fields with moist soil and high residue, especially corn residue. Economic damage likely occurs only when stand reduction is greater than 10%.

There is no defined threshold for this pest in beets and little research data on springtail control in crops using foliar sprays. Few insecticides list springtails on the label. However, insecticides registered on sugar beet to control other pests can be used against springtails – the site/crop is the legal issue, not the specific pest (this applies to the United States, not to Canada). The following products are being used on beets, and are reducing springtail damage. My senses is that a half rate – maybe even a 1/3 rate – is enough to take care of the problem.

Product name (full label rate)

  • Asana XL (5.8 to 9.6 oz/acre)**
  • Lannate 2.4LV (0.75 to 3 pints/acre)
  • Lannate 90SP (0.25 to 1 lb/acre)
  • Lorsban 4E (1.33 pints banded/acre) **
  • Mustang (2.4 to 4.3 oz/acre)

European skipper

European skipper caterpillar were reported in the Upper Peninsula this week feeding on timothy hay. This pest was first found in Ontario in the early 1900s, and has since spread across the northeastern United States and Midwest, including into all of Michigan. Eggs overwinter and hatch early in the spring. The caterpillars feed on timothy and other grasses, living in shelters made from leaves tied together with silk. They strip leaves, or when populations are high, damage the seed head as well. Caterpillars reach about 3/4-inch, then pupate; adults emerge in early July. The wings of adult skippers are burnt orange with black margins. They are often found feeding on flower nectar.

To control caterpillars in timothy, the only registered option is Bt, specifically Dipel. Read the label thoroughly. Small larvae are much more susceptible to Bt than larger larvae, and good coverage is essential.

For a bulletin on skipper management, see this publication from Ontario:
http://www.omafra.gov.on.ca/english/crops/pub811/5skppr.htm

For pictures of adults, see the Wisconsin or Michigan butterfly web sites:
http://www.wisconsinbutterflies.org/butterflies/species/226
http://www.npwrc.usgs.gov/resource/distr/lepid/bflyusa/mi/564.htm

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