Hot, dry weather accommodates population buildup on some crop pests
The prevalence of hot and dry weather through the end of June across the major agricultural areas of the state increased the threat from several insect pests which thrive in such conditions, according to the weekly Wisconsin Pest Bulletin (WPB) covering the fourth week of the month.
Potato leafhoppers were being caught in net sweeps in many areas, including at rates in Fond du Lac and Green Lake counties that were twice those considered to cause economic damage to alfalfa standing at 12 inches. Catches above the economic threshold of two per net sweep were recorded in 18 of the 42 alfalfa fields surveyed during the week, the WPB reported.
Potatoes are also vulnerable to leafhoppers, the WPB noted. It recommended an insecticide treatment, especially on Russet Burbank potatoes, if the nets yield more than 1.5 adult leafhopper per sweep.
Up to four to six grasshoppers were being caught per net sweep in Adams, Columbia, Juneau and Monroe county alfalfa fields. The WPB predicted that they would quickly move into corn and soybeans once the alfalfa is cut.
Soybean and corn growers in the very dry southern and east central counties were urged to monitor their fields every four to five days for evidence of the two-spotted spider mite, whose population can explode rapidly under current conditions, the WPB explained. It noted that symptoms of damage from the mite were already detected in several areas.
Thanks to some chemical treatments and the natural timetable, the siege by the variegated cutworm has run its course in most areas, the WPB indicated. It added, however, that the emergence of moths could lead to another outbreak in August.
Populations of the yellow striped armyworm were still being detected in Dodge, Columbia, Sauk and Vernon counties, but the chewing on two to 14 percent of corn leaves is well below the 50 percent that is considered to be the point at which an insecticide treatment is needed, the WPB observed.
Among the pest insects that Wisconsin farmers are more accustomed to, Western bean cutworm adults were emerging and being caught in traps in southern and central counties. The highest catch during the reporting period was 92 moths at a trap near Wautoma in Waushara County.
The WPB noted that the cutworm's egg laying was well underway and that larvae were already being found on corn in Sauk and Columbia counties before the end of June. Economic tolerance levels are five percent infestation in field corn and four percent in sweet corn when tasseling reaches 90-95 percent.
Corn earworm moths were also on the move with 35-41 being caught in separate traps at Ripon. Growers were advised to monitor their corn once tasseling begins.
Emergence of corn rootworm beetles is expected to intensify in the next two weeks. The WPB urged growers to advise their seed company representative if the beetles inflict root damage to any of their corn hybrids with the Cry3Bb1 protein gene.
The summer flight of European corn borer moths has begun, but populations continue to be quite low. Field surveys for first generation larvae are showing infestation in 16 percent of corn fields with infestation of 24 percent of the plants.
One upside of the hot and dry weather is a low population of aphids in soybeans. Weather conditions in the past few weeks were not favorable to reproduction of the soybean aphid.
Japanese beetles, however, are not affected by such weather. Adults are becoming noticeable in home gardens and yards in many areas with raspberries, roses, linden trees, grapes and other cultivars being vulnerable for up to six more weeks, the WPB indicated.
Soybeans are also subject to damage from Japanese beetles. Low populations have been found in soybean fields in Dane, Monroe, Richland and Vernon counties. The WPB advises inspecting entire fields for infestations and applying an insecticide if defoliation reaches 30 percent before bloom or 20 percent between bloom and pod fill.
Colorado potato beetles are well along in the growth of their larvae. The WPB says a treatment is warranted if leaf defoliation hits 5-10 percent at flowering or 30 percent during the tuber growth stage.
Squash vine borer egg laying was at its height during the last week of June. Once the eggs are laid, control measures are in order, the WPB stated.
Catches of 962-1,600 spotted tentiform leafminer moths at traps near Rio, Hillpoint and Beldenville are likely to correlate to heavy egg laying in apple orchards, the WPB observed. The economic tolerance for this pest is one leafminer per tree leaf.
Seven apple maggot flies were caught on a baited red trap near Plymouth in Sheboygan County. Based on those flights, the WPB expects emergence beween July 7-21.