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A print-friendly version of this newsletter is available to download here: SW MN IPM Stuff 2020-02
It appears that we are living in interesting times.
Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather.
Trying to keep things in perspective
Whether waiting out the pandemic confined to our homes or out on the essential service front lines, most folks now ponder the coronavirus and its current and potential effects on our lives and livelihood.
Are the ever-changing estimates of infection rates, mortality predictions, and flattened curves we are bombarded with right… or wrong? At some point, hindsight will tell us. For now, it is important to remember that these numbers are based on mathematical models; models with limited data on actual infection rates, and on the physical, physiological, and pharmaceutical factors moderating the COVID-19 virus and the disease it causes. They are only predictive models and the odds of the currently calculated best/worst case scenarios being correct are low. They will continue to change as new information is generated and incorporated. When this year’s pandemic is over, the models will be pretty good at describing what happened. Model outputs are seldom entirely accurate, being close enough to be useful is often good enough. So for now, be safe and avoid fear - don’t be stupid either.
Well…enough about the difficulties of predicting viral disease spread among the world’s population. While busy flattening the curve at home, I had some time to consider what crop pest problems the 2020 growing season will bring. Some problems, such as weed populations and SCN infestations are chronic and fairly easy to incorporate into management plans. On the other hand, predictions for most pests are fraught with considerable uncertainty.
Will we have insect pests in 2020 Minnesota crops?
The short answer to answer the question posed in the title is YES. While in general, crop pest species are more problematic in some years than others, they are still field specific problems to some extent. Growing season weather has a large effect on both crops and their insect pests. The following predictions, um, guesses, on a few key crop pests, contain general biological principles but at this time lack the information on key variables needed for accurate predictions.
Soybean aphid (SBA)
The 2019 fall populations of SBA on SW and SC MN buckthorn were much higher than the previous year. It is prudent to consider many Minnesota soybean aphid populations pyrethroid insecticide resistant, complicating insecticide management decisions (see: Insecticide options for resistant soybean aphid).
While we had several brief bouts of sub-zero temperatures in southern Minnesota, they were short-lived. In NW Minnesota, temperatures very briefly flirted with -29oF, the average temperature that a lab study showed SBA eggs freeze. Even in NW MN, snow cover might have moderated temperatures for eggs on lower canopy buckthorn buds. This might indicate higher early season aphid populations in 2020 but…
A major, and yet unknown variable, for 2020 SBA populations will be spring and summer weather. Warm, moderately dry conditions, will favor aphids. Wet, cool conditions favor fungal pathogens and heavy rain events, particularly when soybeans are small, can suppress SBA populations temporarily. Although aphids leaving buckthorn can colonize volunteer soybeans, early soybean planting can provide many more plants to choose from. Season-long, winged soybean aphids colonize new fields. Even when early-spring aphid populations struggle, they can receive reinforcements. Particularly during late July and early August, winged aphids can infest fields in distant areas.
Like most insects, we can use accumulated temperature to estimate how quickly SBA can complete a generation and in the case of soybean aphid, roughly predict how many nymphs a female can produce. A final variable that we cannot quantify ahead of time are populations of lady beetles and other predators and parasitoid wasps. Their populations depend on their own interactions with weather, prey, and parasitism.
All these variables could be incorporated into predictive models for SBA populations but it gets complicated, doesn’t it?
Black cutworm, green cloverworm, thistle caterpillar, potato leafhopper, cereal aphids, and other migrant insects
Insects, capable of flight, can move short distances within, or between fields, sometimes much farther if they take advantage of weather systems. Unable to survive a typical Minnesota winter, the adults of migratory species must make the trip north each spring. Each year’s populations depend on the insects’ survival and reproductive success in southern wintering areas. These insects then need spring weather patterns that can help move them into Minnesota. Once they arrive, the immigrants need to find suitable host plants and subsequent favorable weather for survival and reproduction.
Watching for the weather systems capable of bringing migrant insects can provide clues for timing scouting. In some cases, we can track when, where and how many insects arrive with pheromone traps, light traps, or timely field scouting. That’s when we’ll start getting a fix on 2020 problems from these insects. In the case of black cutworm, pheromone traps detect moths arriving in MN and degree-day development models for cutworm development and help determine when to start and stop scouting. For 2020, trap captures and cutting predictions will be available at the Cooperative Black Cutworm Reporting Network.
Migratory plant diseases?
Unlike insects, plant pathogens lack nervous systems and behavior to direct their flight. Yet, they too can move long distances on wind, water, and soil. The long-distance transport of wheat rust spores northward on Great Plains winds is an example.
Some pathogens overwinter in, move from plant to plant, or migrate with their insect vectors. Bean pod mottle virus can overwinter in the gut of its vector, the bean leaf beetle. Migrating into Minnesota from areas where both insect and disease overwinter, aster leafhopper can transmit the aster yellows phytoplasma to a wide range of crops and cereal aphids may bring barley yellow dwarf virus. High insect vector populations provide a clue to a possible increased prevalence of their associated plant diseases but do not guarantee an epidemic.
Soybean gall midge
While we still know relatively little about this insect, we do know that it overwinters as a larvae near the soil surface. We don’t yet know how cold a temperature this insect can tolerate but should be reasonably confident it will survive the winter. A large number of 2019 soybean fields in SW Minnesota were not tilled last fall. Potentially, this might increase the survival of overwintering gall midge larvae but empirical data is lacking. Damage tends to be worse in early planted soybeans so an early spring is likely to increase damage. With normal or early planting progress this spring, Minnesota could possibly see more widespread infestations and damage from this insect than we have in previous years. How’s that for a guess?
European corn borer
Damage from the introduced European corn borer (ECB) once caused economic losses to many corn fields across the Corn Belt. Since the late 1990s, the widespread use of transgenic Bt traits has effectively suppressed populations of this once abundant insect. Statewide, populations are very low. However, ECB can still be found in Minnesota and neighboring states, most often in areas historically planted to a low percentage of Bt corn. Some fields have had economic infestations in recent years.
Fall ECB surveys provide information on overwintering populations and some estimates of overall risk for damage in the following season. Even in areas where corn borers are present, infestation levels will vary among fields without Bt traits. Using information from a cornfield’s stage of development and a degree-day model, timely scouting can help find those problem fields. Stay tuned to this newsletter, and to the VegEdge site for ECB light trap data, and degree-day predictions. A more thorough discussion of Minnesota ECB populations and their management can be found in reducing Bt trait acres in 2020.
Rootworms continue to be a significant corn insect pest. Generally, populations are low. However, there are individual fields where populations of northern corn rootworm (NCR), western corn rootworm (WCR), or both caused losses during 2019. Unlike ECB where moths emerging in the spring fly to a nearby field that meets their requirements, rootworm larvae and their damage are confined to the fields where eggs were laid last summer – possibly two or three years ago in the case of extended diapause NCR.
A rough estimate of a field’s risk of future injury can be obtained by scouting it in late July and August when beetles are active. Some populations of both WCR and NCR are documented to be resistant to one or more Bt proteins. This means that Bt fields, particularly those planted to corn longer-term, need your scouting attention too.
During the growing season, I’ll try to pass on what I learn about 2020 insect, disease, and agronomic issues and try to provide some less than perfect, but hopefully close enough to be useful, advice in these newsletters. As always, I would appreciate hearing your observations from the field.
Happy trails and keep safe,