2020 UMN Cooperative Black Cutworm Trapping Network Report #3
Report #3 Week of April 11 - 17, 2020
For more information: Black Cutworm Reporting Network
April 24, 2020
After four weeks of operation, the 2020 black cutworm (BCW) trapping season has been markedly different than 2019. Very few moths have been captured to this point and considerable spring tillage and planting has occurred in much of Minnesota.
Again this week, there appear to be very few moths arriving into Minnesota. A single moth was captured on 4/12 by a McLeod County trap and a Dodge County trap had single moth on 4/14.
Obviously, we need an influx of a large number of female moths into the state to increase risk of crop damage. That has not yet happened this spring. It is reasonable to assume that black cutworm moths have not been impacted by the COVID-19 virus or its lockdowns. Rather, our low captures result from a lack of strong weather systems to help move black cutworm moths into the state.
Low-lying areas and winter annual or early spring emerging weeds are attractive to egg-laying black cutworm females. By removing weeds, spring tillage before the moths arrive, reduces risk of black cutworm damage. This spring’s field activity may help reduce the number of fields attractive for egg laying before black cutworm moths arrive.
In spring, migrating black cutworm moths can move long distances quickly. The moths can make it from South Texas to Minnesota within 2 days by hitching a ride on nocturnal low-level jet streams. These efficient transport systems are a common feature of the Great Plains in spring and summer and used not only by the black cutworm but other migrant Lepidoptera, aphids, leafhoppers and even rust spores. In North America, these low-level jets are powered by high elevation in the west and warm moist air in the Gulf of Mexico. Cool, dry, low pressure in the western plains interacts with moist high pressure systems in the eastern plains to create strong southerly flows that are especially strong at night and persist for more than 12 hours. The ideal weather pattern for spring migration into Minnesota involves a HIGH pressure center to our east with a strong LOW pressure center approaching from the west. This pattern produces strong, persistent southerly winds that can bring black cutworm moths northward.
The migrating moths fly upward from the overwintering areas at dusk. If weather systems cooperate, they are whisked off by surface winds and rising air in advance of thunderstorms into the lower-level jet stream. These winds are strongest at night, moving at 30 to 80 miles per hour, and can occur from about 330 – 3000 feet in altitude. The flight is mostly passive with moths carried along until they decide to “drop out”, encounter cold air, or rain out in thunderstorms. These migrating moths arrive in the north in excellent shape.
Two ingredients are necessary for black cutworm moths to arrive in Minnesota. First, the air parcels reaching Minnesota must have passed through the overwintering areas when migrating adults are present. Second, the track of the LOW pressure center is critical, if the LOW tracks too far south, migration is cut off south of Minnesota. If the LOW tracks through Minnesota or northern Iowa we have the potential for moths to drop out or precipitate out in Minnesota. These weather systems may stall with the frontal boundary cutting across Minnesota. In that case, if you’re south and east of the front, watch out! Several LOWS may ripple across the moist air pumping northward and compound the moth deposition in Minnesota. Moths often drop out on the edges of heavy rainfall.
Most evening migrating insects move at an altitude of 1700 feet or so. Wind trajectories can be used to estimate where a significant immigration event (8 or moths / 2 consecutive nights) might have originated. Migration south in the late summer and fall is assisted by southerly flows associated with cold fronts.
If you have not yet placed your traps, please do so as soon as possible. Also remember to report your captures to Travis (firstname.lastname@example.org) each Friday. We start analyzing the results Monday mornings. Because we are trying to determine where flight occurred, it is important to report both positive and negative captures each week. Finally, a quick email if a trap captures 8 or more moths over a 2-night period will be appreciated.
Until next week,
Bruce Potter and Travis Vollmer
This project is supported, in part, by the farm families of Minnesota and their corn check-off investment.