‘Atlas’ bad news for some pests
While the aftermath of Winter Storm Atlas is still felt by ranchers, growers of field and forage crops in storm-hit areas of western South Dakota might see an unexpected positive outcome for the coming season when it comes to insect pressure said Anitha Chirumamilla, SDSU Extension Entomology Field Specialist.
“The timing of the storm and the amount of precipitation might have a negative impact on field insect populations leading to low insect pressure on crops,” Chirumamilla said.
She explains that the early storm didn’t give insects an opportunity to go into diapause – the winter survival technique of many ‘cold hardy’ insects.
“Diapause is a physiological mechanism equivalent to “dormancy” used in plants and animals. Insects in diapause are in a sleeping mode with no food, little or no movement or development, and minimal metabolism just to keep them going,” Chirumamilla said.
“Diapause allows insects to survive through the winter and have enough energy reserves to start development when it warms up in the spring. “
However, as she explained, to have a successful diapause, insects have to do a lot of planning ahead of time.
“They need to eat as much as possible and as quickly as possible to pack on fat reserves and keep an eye on daily temperatures and hours of light while they are busy munching on plant tissue or sucking plant sap. By the time it gets cooler and days get shorter, insects should have built enough body reserves and reached appropriate stage of development to start making their travel to winter lodging (overwintering) sites, which could be deep in the soil, seed, stubble, plant debris, and even our houses,” she said.
Chirumamilla points to two unusual climatic events that occurred this fall which might have fatal effects on insects: 1) early snow likely killed many insects as their bodies were not physiologically ready to tolerate the cold stress; and 2) the immediate wet cycle that lasted for more than two weeks probably killed a large portion of insects that were already making their move to dormant sites but did not reach their destination.
Considering the cropping pattern in West River South Dakota, Chirumamilla said the major insects of economic concern such as alfalfa weevil, grasshoppers, blister beetles, banded sunflower moth, red sunflower seed weevil, wheat stem saw fly, wheat midge, and corn rootworm, that overwinter in soil or stubbles will be hit hard.
“With the exception of grasshoppers, the rest of the above insects overwinter as mature larvae in or at considerable depths of soil. The mature and immature larvae that are still feeding will succumb to the early snow while the mature larvae that dropped to the ground will likely be killed by drowning or disease,” she said.
On the other hand, Chirumamilla said because grasshoppers overwinter as eggs deposited in soil and glued together as egg pods, the heavy snow combined with prolonged wet conditions might lead to drowning or exposure of egg pods to diseases.
“All the above factors suggest that we may be seeing lower than usual insect pressure on crops in the coming season. However, beneficial insects like pollinators, predators and parasitoids will also be equally affected by the winter storm and it will be no surprise if we see few of them around as well,” she said.
While Atlas may have affected insects that overwinter locally, the migratory insects that cause seasonal damage will be free of the impact.
“For example, insects like sunflower head moth, and potato leaf hopper (alfalfa) do not have the ability to overwinter in South Dakota and migrate every year from southern states,” Chirumamilla said.
She encourages growers to spend time scouting for migrating insects as they are at an advantage because of lack of competition from local insects as well as natural enemies.
“Also, it is important to keep in mind that there is always a certain percentage of insects that survive the odds and contribute to the future buildup of the populations. It might take a couple of years for the pests to bounce back with full force, but it is always wise to keep scouting,” she said.
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Outtagrass Cattle Co. cartoon by Jan Swan Wood for the June 19, 2021, edition of Tri-State Livestock News