What impact have hot and dry conditions had on soybean pests and the soybean crop overall? Drs. Seth Naeve, Extension soybean agronomist and Robert (Bob) Koch, Extension entomologist, joined Dr. Angie Peltier, Extension educator-crops to discuss these issues on the August 5th Strategic Farming: Field Notes program.
Even though areas of the state caught some rain recently, good portions of the state are still dry or very dry. When soybeans are under moisture stress, stomata (tiny openings or pores that facilitate gas exchange and evaporative cooling) on the undersides of plant leaves will close.
When stomata close, the plant stops taking in carbon dioxide and releasing oxygen, reducing the capacity of the soybean plant to photosynthesize. This means the soybean plant isn’t able to capture new energy that could be used for vegetative growth or seed growth, which ultimately results in lost yield potential.
Evaporative cooling, which normally occurs as water leaves the plant surface, is greatly reduced when stomata are closed. As the soybean canopy increases in temperature, soybean plants will start to flip their leaves to reveal the lighter underside. Seeing a field of soybean with flipped leaves indicates the crop is under stress.
The critical period for soybean lasts from R4 to R7. Naeve states that although drought stress may have cut off top-end yields, there is still plenty of time to put on yield if we get timely rains to fill pods on the plant.
Hot, dry conditions favor twospotted spider mites Twospotted spider mites are a pest that do well under hot, dry conditions. There are a number of reasons for this. Plants stressed by hot and dry conditions become better quality food sources for spider mites. Spider mite reproductive rates also increase when temperatures reach the upper 80s and 90s (degrees F). Lastly, naturally-occurring fungi can normally prevent an outbreak when rainfall is adequate and temperature and relative humidity are favorable. These fungi perform best when temperatures are below 85°F and relative humidity is over 90% for at least 12 hours in a day. Hot, dry conditions hinder the development of these fungi and as a result, spider mite populations are not suppressed.
More details on twospotted spider mite identification, scouting, thresholds and treatment options can be found on the recent MN Crop News article “Hot, Dry conditions trigger spider mite reports in soybean” at z.umn.edu/mncropnews.
Keep an eye on soybean aphid
To date, soybean aphid populations have been relatively low and scattered this year. Soybean aphid is favored by temperatures in the 70s to low 80s (degrees F), so cooler temperatures may lead to higher numbers in fields. This is also the time of year aphids are developing winged forms, so there can be movement between fields.
Remember, treatment is not recommended unless the economic threshold is reached (i.e., average of 250 aphids per plant, 80% of the plants have aphids, and the population is increasing).
Yield has not been hurt at this point and you have at least 4 to 7 days to make an application before yield MIGHT be hurt. Treating when soybean aphid populations are below threshold levels can result in an unneeded expense, can flare other pests like spider mites depending on the product used, results in selection pressure for resistant populations, and can also hurt populations of beneficial insects and pollinators depending on the product used.
Scout regularly for soybean aphid through the R5 stage of soybean (beginning seed), using the treatment threshold to guide application decisions. It gets more difficult to make an application decision once soybeans reach the R6 stage and preharvest intervals can come into play.
— By Liz Stahl, Extension Educator – Crops, Seth Naeve, Extension Soybean Agronomist, and Robert Koch, Extension Entomologist
