Degree day accumulations mark progression of insect threats

Little or no corn has reached the 18-inch heights at which European corn borer moths like to lay their eggs. Because of this, many of the moths have been depositing those eggs instead on snap beans, lima beans, peppers and potatoes, according to the mid-May edition of the weekly Wisconsin Pest Bulletin.

The publication explained that this is happening in southern and central areas where degree day accumulations had reached at least 450 on a base scale of 50 degrees.

Corn rootworm hatching was also expected as the degree day accumulations - which accelerated in the days following the deadline for the weekly report - moved into a range of 684 to 767 on a base scale of 52 degrees. The Pest Bulletin cited reports of rootworm hatching in east central Illinois and western Indiana - the earliest hatching in 35 years.

Surveys on May 14-15 documented economic levels of damage to corn in Dane and Rock counties from chewing and cutting by black cutworm larvae.

Other noticeable black cutworm activity was noted in Sauk and Walworth counties.

The field surveyors tabulated damage from the cutworm larvae on 3 to 13 percent of the corn plants, with an average of 4 percent.

The Pest Bulletin suggested that an insecticide treatment is warranted if at least 3 percent of the plants are damaged and warned corn growers around the state that their crop is vulnerable to damage until nearly mid-June or until it has passed its V-5 growth stage.

Migration of stalk borer larvae from grassy areas into corn fields was likely by the latter part of May, the Pest Bulletin indicated. Crop scouts are advised to check the outside 4 to 6 rows of corn for holes in the leaves or wilted whorls and to consider control measures at infestation rates topping 5 to 10 percent.

The Pest Bulletin pointed out that the stalk borer larvae start feeding once the degree day total reaches 1,400 on a base of 41 degrees. As of May 16, those accumulations had reached 1,064 at La Crosse, 1,047 at Madison, and 843 at Wausau.



other pests

With the harvest of the first cutting of alfalfa having proceeded rapidly in many areas by mid-May, concerns about crop damage from alfalfa weevils was alleviated. This redirects the major concern about alfalfa pests to the potato leafhopper, whose nymph counts were escalating in southern and west central counties, the Pest Bulletin reported.

Consistent scouting for potato leafhoppers in regrowth alfalfa was recommended. The Pest Bulletin cited the potential for rapid building of leafhopper populations, especially during hot and dry weather.

Pea aphid numbers were also higher, as indicated by catches of 6 to 16 per net sweep, and plant bug nymphs were found in 21 of the 39 alfalfa fields checked in southern and central counties. The Pest Bulletin attributed a portion of the pea aphid population to the loss of its natural enemies in the wake of the spraying of some fields to control the alfalfa weevil.

Moderate to high populations of the aster leafhopper were detected in alfalfa fields this spring. The Pest Bulletin explained that they are likely to migrate to lettuce, celery and onions, which should be treated if catches of the leafhopper approach 25 to 100 per 100 sweeps, respectively, for those vegetables.

The Pest Bulletin continues to monitor the potential for true armyworm outbreaks, especially since another 246 migrating moths were caught in black light traps during the latest reporting week.

So far, low numbers of armyworm larvae have been noted in Monroe and Sauk counties, but the potential for outbreaks continues into early June, especially in minimum tillage fields with grassy weeds or with crops following sod or an early forage grain harvest.

Weed growth has cropped up as a concern in the Pest Bulletin because of the competition to corn for nitrogen and subsequent yield losses. It cited the potential of 20 percent losses from velvetleaf competition if there are at least three plants per foot of corn row.

Velvetleaf plants had reached 3-inch heights in Rock, Dane, and Walworth counties.

Growth of winter annual weeds such as field pennycress, shepherd's purse, chickweed, yellow rocket and speedwell was early and vigorous this spring, the Pest Bulletin emphasized. It urged a herbicide burndown before the planting of soybeans, especially in no-till fields.

An unusual phenomenon that appears to concentrated in the northern half of Wisconsin is the hatching of variegated cutworm eggs that were deposited on the exterior of residences, the Pest Bulletin observed. The pest is a threat to garden and some crop plants.

At 10 locations in southwestern Wisconsin, meanwhile, catches of 1 to 10 migrant cabbage looper moths in traps are creating concerns for infestations on cabbage, broccoli and cauliflower. The yellowish-white hemispherical egg masses are laid on the underside of the plant leaves.



codling moths

The Pest Bulletin informed fruit growers that huge flights of codling moths were observed from Racine to Polk counties, suggesting a bio-fix in the next 5 to 17 days. The year's second flight of the redbanded leafroller was expected this week.

Orchard owners whose outside trees lost their fruit set due to the April freezes were advised to check the inside parts of their orchards for the plum curculio.

Based on the finding of flatheaded appletree borers at Rio in Columbia County, apple orchard owners were also asked to check for possible infestations on trees transplanted within the past two or three years, on those with bark damage, and on those suffering from environmental stress and to apply an insecticide once the borer adults emerge.

Along roadsides across the state, evidence of the presence and pupation of the Eastern tent caterpillar is apparent in the defoliation of some trees, the Pest Bulletin reported. It noted that adults should begin appearing in black light traps by this weekend in areas where the degree day accumulation has reached at least 725 on a base of 50 degrees.