I admit that scouting soys when just the cotyledons are out makes for tough pest injury diagnostics. It does take some years of experience to really get to know what you are looking at. Three pests that commonly feed on the cotyledons include seed corn maggot, bean leaf beetle and slugs. Occasionally, the feeding damage on the cotyledons is easy to distinguish amongst the three but often you need to look for other signs of the pest being present to know for sure. And there could be cases where there is a a combination of pests present, particularly in cool wet springs and if there is sufficient crop residue on the soil surface.
Seed corn maggot can feed on the cotyledons. Their feeding tends to leave small caverns and tunnels on the cotyledons. These tunnels however can be along the edge of the cotyledon, making it look like something with much bigger “jaws” has taken a bite out of it. To know for sure if it is seed corn maggot damage, you need to dig the seedling plant out of the ground and look for the tell tale signs of mining along the stem of the seedling.
The small white maggot could also still be present in the seed or stem. Cruiser does work well on seed corn maggot though so if feeding is extensive in a Cruiser treated field, it is likely due to another pest. Fields at high risk of seed corn maggot injury are those that were freshly tilled with heavy crop residue present, had recently applied manure, were deep planted or have had poor emergence from cool, wet conditions.
Bean leaf beetle can also feed on the cotyledons. I like to think of them as meticulous feeders, since they tend to eat in neat little circles, scraping off the surface of the cotyledon. These circular patches on the surface of the cotyledon can blend into each other and start to resemble the same kind of damage the slugs can make though. Even if the soybeans have been treated with Cruiser, you will see some feeding, as the beetle needs to take a bite of the plant before it dies. If populations are high enough, the damage can be quite noticeable. Fields at high risk are those with a history of infestations and those neighbouring alfalfa fields.
You might see a few adults present on or near the plants while scouting, though you have to look at least a metre ahead of your step, as they tend to run and hide or drop down into cracks in the soil as soon as they sense your approach. In fields not treated with Cruiser, the threshold is 16 beetles per foot of row during the seeding stage and especially if plants are being cut off. Foliar control options can be found here: http://www.omafra.gov.on.ca/english/crops/pub812/2blb.htm
Slugs are the last of the primary culprits that can feed on cotyledons. They have a radula for a mouth..picture dentures for the upper jaw that are filed to a sharp, fine saw-toothed blade. So they essentially hack at their food, tearing small chunks of plant tissue away as they feed. Less scraping, more hacking. Cruiser seed treatment does not work on them as they are not insects. A tell tale sign that slugs are the culprit is to look for a silvery trail (slime trail) left on the surface of the plant or nearby soil surface (though a recent rain will wash off the trails).
You can also leave a few small pieces of plywood or shingle on the soil surface for a day or two and return to turn them over and see if slugs are underneath. Fields are risk are those with history of slug damage, fields following wheat with red clover, fields with a fair amount of trash on the soil surface particularly close to the seed bed and of course, wet field conditions. Slugs present in the spring are the same slugs that were present last fall so wet fall conditions help build populations for the following year. Unfortunately, there is no economically feasible control for slugs, other than moving trash away from the seed bed before planting.
Once the unifoliate and trifoliates start to emerge, the diagnosis becomes much easier. Seed corn maggot really only feed on the leaves when they are still just developing and tend to leave a “snakehead” shaped seedling remaining. Bean leaf beetles perfect circular holes become very apparant on the leaves while in the case of slugs, the leaves start to look skeletonized or look similar to hail damage with leaf tissue missing while the leaf veins remain.