NDSU Crop and Pest Report

Entomology


ISSUE 3  May 16, 2002

 

CANOLA PLANTING CONDITIONS SUGGEST CLOSE MONITORING FOR FLEA BEETLE

Monitoring flea beetle feeding pressure on seedling canola will be important in the next few weeks. Flea beetles are just starting to move from overwintering sites. Planting and emergence of canola has been slow in the greater acreage areas of the state. Current weather conditions for planting were similar in 1999. Those conditions contributed to problems with the level of protection provided by the seed treatment insecticide.

In 1999, many canola fields where insecticide seed treatment was used, still required foliar treatments. Several factors contributed to control concerns. The first was the large populations of flea beetles migrating and concentrating on only a small percent of the total acreage that had emerged. Second, fields had been seeded early under good conditions, then it turned rainy and cold. Germination/emergence was delayed for two to three weeks in many cases. Third, it was reasonable to conclude some loss of the insecticide on the seed coat occurred in the wet soils. Even small losses could contribute to lower concentrations in plant tissue when cotyledons need the protection. Finally, rate of application and seed size probably influenced the amount of insecticide available in the slow emerged seed.

So watch canola fields closely to determine if protection is adequate or whether follow-up treatment may be required. Seed treatment insecticides that are working should result in dying beetles around plants. Remember, the beetles must feed to receive a toxic dose. Beetles that are affected will be sluggish, compared to rapidly moving, healthy beetles.

For making post emerge insecticide treatment decisions, the Economic Threshold (E.T.) for flea beetle on canola is 25% leaf damage (pitting) during the seedling stage. Remember, scout several areas of the field. If fields are above the E.T., a foliar treatment of Capture is recommended. After plants reach the two true leaf stage, canola plants can normally sustain themselves and outgrow most flea beetle damage. Risk of loss is greater under dry conditions compared to conditions where soil moisture is not limiting.

 

DEGREE DAY ACCUMULATIONS SLOW

Degree day accumulations have been slow to accumulate due to the cold weather the past three weeks. Wheat midge programs have relied on degree day accumulations to provide insight into management activities.

The high risk planting window used to predict when wheat will be heading when midge are active is from 200 DD to 600 DD. In the past four years, 200 DD has been reached in late April for southern counties. This year, we reached 200 DD in the southern most ND counties from May 11 to 13. An overview of the DD accumulations through May 13 are summarized in the following figure.

Degree Day accumulations map

Updates on degree day and risk window for midge will be available in the next few weeks. This slow accumulation of degree days will affect the development of midge and other insects in the region, such as European corn borer.

 

GRASSHOPPER OUTLOOK 2002 - WATCH SOUTH CENTRAL ND

Grasshopper surveys during last years IPM Small Grains survey found low numbers of nymphs during June and July in most of the state. Exceptions were several central counties, from Wells to McIntosh, where some nymphs numbered in the threatening level of 50 or more nymphs per square yard. Additional surveys will get underway for 2002 in a few weeks. With the cold weather, expect grasshopper emergence to be slow. It has been suggested in the past that when common lilac bloom, it is time to begin checking fields for grasshopper nymphs and get an indication of critical hatching sites and the numbers present.

Grasshopper outlook map

Grasshopper treatment guidelines are:

Nymphs (Yong hoppers) Adults
per square yard per square yard
Rating Margin Field Margin Field
Light 25-35 15-23 10-20 3-7
Threatening 50-75 30-45 21-40 8-14
Severe 100-150 60-90 41-80 15-28
Very Severe 200+ 120 80+ 28+

Whenever grasshopper populations reach the threatening level, feeding damage to crops should be anticipated. Directing control efforts at nymphs in hatching sites is recommended to minimize the total area requiring insecticide treatment, permits lower insecticide rates for effective control of small nymphs, and minimizes the potential for future crop damage.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu


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