With spring and almost summer-like weather prevailing in Wisconsin since mid-May, insect activity has accelerated, creating a threat to some field, fruit and vegetable crops as they start their year's production cycle.
Tracking the timetable and location of those pests is the purpose of the weekly Wisconsin Pest Bulletin, which is issued on Thursday afternoons by the Wisconsin Department of Agriculture, Trade and Consumer Protection with the cooperation of Extension Service agents and private sector crop specialists around the state.
In its edition for the third week of May, the WPB pinpointed the probable dates for damage to seedling corn plants from blackworm cutworm larvae. It is giving special emphasis to that concern because of the high number of migrating moths that have been caught in 34 traps in the southern part of Wisconsin —161 during the reporting week and 813 for the season.
Based on temperature degree day accumulations, the WPB predicted evidence of damage from black cutworm larvae by May 29 in Rock and Grant counties and by June 6 as far north as Brown County and in the central part of the state. Poorly-drained and low-lying fields, especially those with weeds, nearby natural vegetation of other types and no or minimal tillage are the most vulnerable.
Because of much lower numbers of overwintered pupae, the concern is not as great about European corn borers for this year, the WPB reported. It expects moths began emerging in the past several days in the Beloit and Platteville areas. It also advised black light trappers everywhere to begin checking their traps within the next week in order to document the population of moths in their locality.
As the most advanced fields of alfalfa began to reach the maturity ideal for harvesting for haylage or dry hay, the WPB continued to track the year's population of alfalfa weevil, which has been the most damaging insect in the crop for the past several years in the state. The latest weekly update noted that the catches of alfalfa weevil adults had increased to an average of 9 per 100 net sweeps from 5 in the previous week.
Catches ranged from 1 to 13 per 100 net sweeps in 10 percent of the fields that were checked; the WPB did not consider those to be high populations. It indicated that surveying for larvae populations would continue and that determination of defoliation levels would start May 27. The year's first larvae were found May 19 in Richland County.
Migrations of potato leafhoppers were documented in southern counties. As far north as Westby in Vernon County, adults were found in 11 of the 39 alfalfa fields that were surveyed. Pea aphids were caught at rates of up to 225 per 100 sweeps in alfalfa fields.
With virtually no soybeans available yet for their dining, bean leaf beetles were found in only 2 of 129 alfalfa fields; the first detection was May 21 in Vernon County. The first colonization of soybean aphids is anticipated during the first half of June.
Although catches of tarnished plant bugs averaged 9 per 100 net sweeps in alfalfa fields, the WPB observed that the most immediate concern with the pest is with strawberry plants. It indicated that the economic loss threshold is a population of 4 or more per plant and that a single insecticide treatment is likely to control the pest.
The emergence of codling moths in Green and Racine County apple orchards last week prompted the WPB to call on owners in southern and central areas to begin daily monitoring for the flight of moths. It explained those flights are most likely between 5 and 10 p.m. when the temperature is above 62 degrees and the wind speed in below 3 miles per hour.
A catch of 215 spotted tentiform leafminers at Montello in Marquette County accounted for just under one-half of the week's total. Those numbers suggested the spring flight has been completed and larvae should be appearing soon. The WPB reminded apple orchard owners to use pheromone traps to catch the oblique banded leafminers that come from the larvae now emerging from under the tree bark.
The warmer weather also stimulated activity by the plum curculio beetle. The WPB advised apple growers to look for oviposition scars and feeding injury and noted that those infestations usually become evident by 10-14 days after the apple trees' petal fall.
Growers of cabbage should check for imported cabbageworm eggs when they transplant to fields, the WPB stated; a 30-percent infestation is the starting point for economic losses from the pest.
Seedcorn maggots are a potential threat to beans, corn and curcurbits, whose emergence is delayed by cold or wet soils, the WPB pointed out. It suggested planting onion sets before onion maggots emerge for their flight.
Colorado potato beetle adults were likely to emerge from hibernation by early June and begin to lay eggs in clusters of 15 to 30 on the underside of plant leaves.
In the monitoring of activities at nurseries and greenhouses during the week, the WPB reported that rose mosaic virus was found on plants originating in Tennessee, iris borers were found in greenhouse plants and aphids were detected on flowers and herbs at retailers in southeast Wisconsin.