Bean leaf beetles caused a lot of frustration and yield loss in 2006
Palle Pedersen, Soybean Extension Agronomist
Originally published in Iowa Soybean Review December 2006

One nice thing that I recognized this year was farmers complaining about their yield when a field was in the 50’s. That is a good mentality to have. Four years ago when I came to Iowa we were complaining about 35 to 40 bushels. So we are moving in the right direction. However, it is not easy to “swallow” a field in the 50’s when the neighbor was in the 60’s planting his field on the same day. It is not about inputs, but a lot of the time it is small things like variety selection and careful scouting that makes the difference.

Looking back at the 2006 growing season we lost some “easy” bushels, which is frustrating since it could have moved us up to the 2005 yield level – probably even higher. In the last two issues, I have discussed the large yield loss we had from soybean cyst nematodes (SCN) this past year because of the wrong variety selection. However, another source of yield loss for growers mainly in central and southern Iowa was bean leaf beetles. I heard many farmers in Story, Boone, and Greene County struggled to get above 50 bushels. Many believed it was from the dry conditions and high temperatures. It was not. Beside SCN, bean leaf beetles and bean pod mottle virus were the large yield robbers in many areas in central Iowa in 2006. The level of bean leaf beetles this year was the highest we have seen since 2002. Some fields around Ames were so bad they needed to be treated twice since the background level of beetles in the area was so high.

Besides causing a significant yield loss, bean leaf beetles also cause a tremendous seed quality issue that we have not seen for many years from the apparent high incidence of bean pod mottle virus that it can transmit. This is going to be a tremendous problem for the seed companies next year. Especially for those companies that grew seed beans in central Iowa in 2006. In addition to the virus infected seed, the pod feeding from the second generation of beetles caused many seeds to be moldy since seed was vulnerable in August and September from the wet weather. That led many fields untouchable for up to 2 weeks because of saturated soils which made the whole bean leaf beetle/bean pod mottle virus/open pods/moldy seed situation even more frustrating.

This year, if we do not get a really cold winter, with no or little snow cover, then there is a large chance for similar or higher levels of bean leaf beetles next year. Some of the old recommendations suggest delaying planting to avoid the overwintering generation of bean leaf beetles. The problem in 2006 was that even soybean planted during the last week of May went above threshold in central Iowa. We ca not afford to delay planting since we will lose too much in yield potential. However, we can manage them very easily. So, just because soybean aphid is not in your area it does not mean that you should not scout. Bean leaf beetles can not reproduce as fast as soybean aphid, but timing of managing them is as critical as is the case with soybean aphid. It is all about scouting.