Fortunately this has not been a bad year for cereal leaf beetle. Unfortunately, in places where the beetle has been found, it has been developing very slowly. The beetle develops quicker in warmer weather, which means that it will reach its pupal stage quicker, drop in the soil, and will not feed any longer on your wheat. Some consultants have noticed fields that have hung around the 15 larvae per 100 tiller mark for several weeks now, below the recommended treatment threshold of 25 larvae per 100 tillers. These beetles have now reach a large size and are feeding on the flag leaf, which provides most of the nutrients needed for filling the grain heads. Therefore, these fields are experiencing some level of loss due to the beetle. The question is how much?
NCSU Small Grains specialist Randy Weisz has noted that the wheat is developing very slowly, much slower than normal. This coupled with the slow development of the beetle, could lead to a situation where beetles are stripping leaves and the wheat is not able to outgrow the injury. In general, cooler years tend to favor development of the wheat over the beetle, but this does not appear to be the case this year.
The graph to the left is based on the actual data that our treatment threshold comes from (Ihrig et al. 2001). Each point represents the number of large larva (4th instar) per stem. Note that our treatment threshold is 0.25 larvae per stem (or 25 larvae per 100 tillers). Some of the points below 0.25 larvae per stem represent fields where there were few cereal leaf beetle and lots of yield loss. In one case, yield loss approached 20%. Other cases represented fields where there were few larva and there was actually a yield gain! All this is saying is that yield loss due to cereal leaf beetle is highly dependent on environmental characteristics, including wheat development.
That being said, with conditions the way they are, you may consider treating for cereal leaf beetle at levels lower than normal, especially if they have been there for several weeks and are feeding on the flag leaves. Remember that average yield loss due to drive down after jointing is 3.3%. Please take this into consideration. Fields with some cereal leaf beetle around (possibly at levels lower than 10 larvae in 100 tillers) probably will not benefit from a treatment.