Mild winter, dry conditions could cause insect pressure
By Jennifer Carrico
A mild winter is a welcome sight for many people in Iowa, but it might not be such a great thing once the crops have emerged and the insects have reappeared. Or will it?
Iowa State University Extension Entomologist Erin Hodgson says that because Des Moines and many parts of the state of Iowa haven't experienced a temperature of zero degrees, it could increase the chance of insect survival. Lack of snowfall in the state as of mid-February has led to moisture concerns as well.
"Perhaps it is true, but there are many factors that influence successful overwintering and are worth strong consideration," said Hodgson.
Those factors include: insects that overwinter aboveground like the adult bean leaf beetle may be more likely to survive with fewer cold days, but lack of snow cover can expose insects to those days with below-freezing temperatures and could increase mortality compared to a year with insulating snow.
Another factor is insects that overwinter belowground like the Japanese beetle grubs will not likely be affected by a mild winter because soil temperatures are more constant.
"However, there could be more survivors than normal if the frost layer is shallow," she said.
Hodgson said all insects develop based on temperature. A warm winter day might cause some insects to become active when they would normally be dormant. This activity would use up stored fats they need to survive until the spring.
"Without access to food, these active insects could starve to death before food becomes available," she said.
The last factor Hodgson suggested is that most insects adapt to cold winters by slowly preparing in the fall and staying dormant until the spring. Large temperature swings can be detrimental to insects; the body can be injured or death can occur. She said some insect mortality is expected due to cold intolerance when temperatures regularly fluctuate from 0 to 50 degrees.
While some of these factors are thought to help kill the insect population, Hodgson said they can also apply to beneficial insects--the ones that kill the bad insects.
"Ultimately it might not matter too much if more pests survive in a mild winter, because more beneficial insects will likely survive and help regulate spring populations," she said. "The uncertainty of insect survival in the winter can make predicting pest populations very difficult."
Hodgson said she always encourages farmers to scout all their fields on a regular basis to gauge insect activity.
"Estimating pest density over time will help determine if populations are approaching a treatable level. The most important time to fully protect yield is from flowering through seed set," she added.
Insecticidal seed treatments can help suppress overwintering bean leaf beetle populations on emerging soybeans and those treatments were selected last fall, therefore Hodgson said the treatments should help just as they do every year.
Iowa State University Agronomy Professor Roger Elmore says farmers in most parts of Iowa are concerned that they will have persistently dry conditions at planting time.
"As of Jan. 30, the modeled volumetric root-zone soil water in the northwestern half of the state was one-third or less," he said.
While there aren't specific research experiments for each year regarding actual yield data for dry planting conditions, he also pointed out that Iowa rarely has dry planting conditions.
Modeling tools that simulate dry conditions at planting show without making changes in seed or plant populations, with drier soils at planting, probabilities of reduced yield still vary depending on many other conditions. Persistently dry conditions throughout the growing season would result in lower yields.
"We all know that many things can happen between now and planting. If soil conditions do not improve by planting, yields will be reduced at many Iowa locations," said Elmore. "Meanwhile, let's hope for complete recharge of our soil before planting."
Iowa State University Professor of Plant Pathology and Microbiology Greg Tylka said during a dry year, Iowa farmers need to be cautious of the soybean cyst nematode, which is one of Iowa's most serious and persistent soybean pests.
"The nematode has the potential to cause devastating yield losses," he said. "Symptoms and yield losses from SCN can be quite severe in dry years as opposed to years with adequate rainfall."
While it has been more than 20 years since there has been widespread and severe damage from SCN in Iowa, Tylka says people should not let their guard down.
"It is not uncommon for farmers to grow high-yielding, SCN-susceptible soybean varieties in fields with low or moderate SCN infestations," he said. "As we approach spring planting season, increased vigilance about SCN is warranted for the 2012 growing season because of the dry soil conditions statewide."
SCN can cause increased yield loss to susceptible soybean varieties under dry soil conditions, even when SCN egg population densities are low. Tylka highly recommends planting SCN-resistant varieties in 2012 because of the dry soil conditions.
Overall, the mild winter could cause some issues with insects, while for others it might not be an issue. Hodgson and Tylka agree that scouting for insect problems is very important to ensure the crop is free of problems in order to reach its best possible yield.
Jennifer Carrico can be reached by phone at 515-833-2120, or by email at firstname.lastname@example.org.