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Pests not bother soybeans this year

The drought has not been good for crops this year, though it?s too soon to say how much it will affect soybean yields.


Iowa State University field agronomist for southeast Iowa Joshua Michel said corn has borne the brunt of the drought-induced stress.


?We?re seeing heat stress to corn along Highway 34,? Michel said. ?But the soybeans are hanging in there.?


Rain was hard to come by early in the growing season, and it didn?t help that June and July were hotter than normal.


?Soybeans were doing well until [early August], and since then southern Iowa has seen very limited moisture,? Michel said. ?The first thing you?ll notice is that their leaves start to droop and they turn a sickly gray color.?


 


Cannibals


In a drought, the soybean plant is forced to cannibalize nutrients from its own leaves to save its pods. The lowest leaves are the first to go. Not only that, the plant will start aborting some of the pods it can?t sustain until full maturity.


?The plant is trying to fill those pods with seeds, but if it?s under stress, it has to determine what it can support and what it can?t. In fields that experience severe stress, we?ll see undeveloped pods, or the beans will be underweight.?


Michel said August is a critical month for soybean. Beans grow best when the weather turns cooler and they get plenty of rain. Unfortunately, the early part of August delivered neither of those demands.


?This last shot of rain we got [around Aug. 20] will go a long way in helping the beans fill those pods,? Michel said. ?I?m cautiously optimistic that soybean yields will remain pretty good this year.?


Michel said a plant?s life cycle can be divided into growth phases: vegetative and reproductive. Vegetative growth describes what the plant does in its early life as it grows taller and wider. Reproductive growth refers to the process of filling out its pods with seeds.


?We don?t want the reproductive growth phase rushed,? he said. ?If it is, you?ll see lower seed weights. You might have pods that are not full, either.?


 


Pests? Not bad this year


Crops grew faster than normal in the early part of the year, which meant they were a step ahead of the insects that plague them. Insects have evolved so that their life cycle matches the plants they like to eat. Under normal circumstances, they can do great damage by eating a young soybean plant?s leaves or stem.


However, since the soybeans grew fast this year, they were hardier and better able to fend off attacks when the bugs tried to eat them.


That said, Michel said he still investigated fields hit by the usual suspects: Japanese beetles, bean leaf beetles, aphids and spider mites. For diseases, frog eye leaf spot is a common one, and Michel saw some of that, too.


?Overall, this was not a bad year for pests,? Michel said. ?We had an excellent planting season because it was very warm and humid. Plants were able to stay 1-1.5 weeks ahead of the insects.?


Another factor that has kept disease down is that most of them need moisture to grow, but with a lack of rain, they haven?t gotten a foothold in the fields.


?We normally see them in early July, but the environmental conditions were not favorable to them,? Michel said.