Factors Driving 2015 Pest Populations

Over the last couple of years, corn and soybean growers in the Midwest have dealt with noticeably fewer pests. Although there have been some localized outbreaks, many pests have figuratively dropped-off-the-radar. In my lifetime, we have seen some significant devastation and yield reductions resulting from stalk borers, Japanese beetles, cutworms, soybean aphids, gray leaf spot, anthracnose, and weeds such as foxtails, milkweeds, and waterhemp, just to name a few. It was only about 10 years ago that we saw entire fields of corn laid flat on the ground in mid-June as a result of root damage from corn rootworm larvae. Such spectacles have become rare.

In a survey I conducted in 1995 on corn and soybeans, University pest specialists in the Midwest listed 30 insects, 29 diseases, and 36 weeds that were considered serious threats to crop production. Although a similar follow-up survey has not been conducted, many pests such as jimsonweed, European corn borers, and some seedling blights have lost their status as significant pests, while others such as white mold, herbicide resistant Palmer amaranth, and the Western bean cutworm, have now been added to the list.

And while it’s common to attribute the ‘apparent’ reduction in pests and pest damage to GMO traits, that would be only part of the story. Another part of the story is that while GMO traits significantly reduce many pests, other pests, particularly ones that are pesticide resistant, have expanded to fill the gap. In addition, there are other factors affecting which pests are becoming more or less troublesome including: an improved awareness by growers of how to best use pesticides; changing agronomic practice;, and lastly a change in weather patterns.

Keep in mind that when we talk about how weather affects pests, all pests can be grouped into one of two principal categories: indigenous (occurring naturally in a region) and those which are migratory or air-borne. While the survivability and impact of indigenous pests on crops (all weeds and many insects and crop diseases) is greatly affected by temperature and rainfall, the severity of migratory and air-borne insects and diseases are also affected by prevailing winds and storm patterns.

It is evident from recent weather patterns that a wet spring and saturated soils have reduced the survivability of indigenous insects such as corn rootworms, white grubs, and bean leaf beetles. However, it is also evident that the wet soils and persistent dews we see in many areas of the Midwest have fostered diseases such as white mold, Sudden Death Syndrome, and Frogeye Leaf Spot.

As an example of how weather can affect diseases, see the map below which shows cumulative rainfall from the 16th to the 31st of July 2015. This 14-day cumulative rainfall map indicates rainfall in excess of average amounts (dark blue & purple areas with 200% +) over the west central Cornbelt and in select areas of western Kentucky, Indiana, and Ohio. In these areas, the stage is set for continued development of stem and foliar diseases such as septoria brown spot, frogeye leaf spot, and white mold on soybeans, and northern corn leaf blight and southern rust on corn. Meanwhile, Wisconsin and most of Illinois have gotten a reprieve by receiving less than average rain during this same period. Although this isn't going to stop disease infection from spreading in those areas (they had lots of rain earlier in the season), it can reduce current disease virility and obviate the need for additional treatments.


The distribution and appearance of soybean aphids, corn borer moths, western bean cutworm moths, and southern rust are all driven by wind direction, wind speed, and storm events. This map can also be useful in estimating migratory and air-borne pest movement. While that particular map records only a short portion of the day's surface winds for July 31, 2015, other wind and meteorological maps, in combination with this one, can help us understand pest movement. The integration of these kinds of weather data are included in Morning Farm Report and drive Agrible’s analytics.