David Owens, Extension Entomologist, owensd@udel.edu
A large percentage of field corn and soybean is treated with an insecticide seed treatment – usually a neonicotinoid insecticide such as thiamethoxam, clothianidin, or imidacloprid (ex: Cruiser, Poncho, and Gaucho). In today’s market and social climate, it begs the question – where do they pay and what role do they have in agriculture?
Seed treatments are generally effective for a very short window, no more than 3-4 weeks. There are a few scenarios generally recognized to be at risk for significant early season pests: fields with a recent incorporation of manure, compost, or cover crop and fields that have been fallow or in pasture/meadow. The first scenario presents an elevated risk for seedcorn maggot, the second for wireworms and grubs. A third scenario for corn is cool weather. Slow growing corn is more susceptible to insect damage than fast growing plants. A fourth scenario, that has been less well studied, is planting green – I am thinking of armyworm and cutworms (controlled by most trait technology in corn that is not in soybean). We do not have much data on this scenario. The literature suggests that predators (and weed seed eating beetles) are much more abundant in planted-green fields.
In soybean, outside of the first two scenarios (recent long fallow/pasture, recent organic matter incorporation), university data in NC, VA, MD, and PA has not seen significant yield contributions by seed treatments. In soybean, some defoliation early, up to 30%, can be compensated for without any yield decrease. Early severe insect outbreaks in the 3-4 week window that seed treatments are effective are extremely unusual. I am not going to say that it can’t happen, just that it is very rare.
That is for soybean. What about corn? Soil pests pose greater challenges to corn than they do to soybean because corn is planted at lower seeding rates and the growing point remains underground for a long period of time. Corn also has much less compensatory ability. As I noted above, slow growing corn is at greater risk for stand damage. Recently, there has been a trend in the region to start increasing seed treatment rates. I do not have much information on relative costs of a high seed treatment rate over a low rate. Maryland data suggests that an in-furrow insecticide may actually be a little cheaper than increasing the seed treatment rate for some products. Maryland and North Carolina data also suggest that in a typical, lower soil-insect pressure field, there is no significant contribution of the seed treatment to yield. Conservatively, in tilled soil where pest pressure tends to be light, a low rate may be all that is needed. If you plant corn without a seed treatment or have not been satisfied with low rates of neonicotinoid seed treatment and are concerned about soil pests, an option you may want to look into is an in-furrow pyrethroid. Some formulations (usually designated “…LFR”) are compatible with fertilizers, check the labels for guidance.
When insects are present, there is often a complex present. A useful chart rating product efficacy towards various corn pests can be found here: http://blogs.ext.vt.edu/ag-pest-advisory/files/2014/10/Field-corn-insecticide-seed-treatment-chart.pdf. I also need to point out that one pest no seed treatment works on is the infamous slug. Slugs are not insects, their biology is very different and the neonics do not impact them the same way they do for insects. However, predators attacking slugs come into contact with the neonic in the slug and are killed. So we may be shooting slug control in the foot. In cool, wet fields with slug history, scouting is key.
Recent articles and posts on this topic can be found here: http://ento.psu.edu/extension/field-crops/fact-sheet-Effectiveness-of-Neonicotinoid-Seed-Treatments-in-Soybean and from NC corn: https://entomology.ces.ncsu.edu/2018/04/are-insecticidal-seed-treatments-and-in-furrow-insecticides-worth-it-in-corn/?src=rss. Please note that billbugs and sugarcane beetles are an unusual pest group primarily in NC’s blackland farms on former swampland.
One final thought: y’all are great experimenters on your farms. You have to be to learn how an adjustment might fit with your operation. If you are thinking about adjusting early season insect control practices, try it out on a small area in a few fields. See how it works, and let us know.