Strategy for Japanese beetle traps: Detection then removal. Late each May, I place a red insecticide strip and a pheromone lure into a specially designed Japanese beetle trap that resembles a colorful torpedo or bomb (Figure). The lure works to attract Japanese beetles two different ways, one portion holds a female sex pheromone that attracts males looking to mate and the other portion mimics a floral scent which is thought to attract hungry beetles.
Each season as I wait for the Japanese beetle adults to make their way up and out of the soil the lure fills up with newly emerged June bugs and bumble bees. On Thursday, June 26 the trap was filled with approximately 40 adult Japanese beetles. After the first Japanese beetle adults of the season have been detected, I IMMEDIATELY REMOVE the trap and lure from the field so as to not attract even more of these destructive pests into the area. Crop producers and homeowners are also encouraged to removed their traps upon first detection.
Japanese beetle identification. Japanese beetles are oval-shaped and have an iridescent metallic sheen. They have a green head and thorax and copper colored outer wings with a green stripe down the center. There are also five white 'racing stripes' along each side of the beetle under the wings towards the posterior end, and two on the rear end. These are actually small white tufts of hair called setae (Figure).
Economic thresholds vary by crop. Economic thresholds and control recommendations may be vastly different in a home garden setting than for a cash grain operation. Rhonda Ferree, Horticulture Extension Educator, has summarized information on the Japanese beetle paying special attention to helping homeowners combat this pest on a blog called Japanese Beetle News.
Scouting. Damage on corn and soybean plants can be alarming, particularly if individual leaves are in tatters. When adult beetles emerge, they have two main interests: eating and mating. When they find a good food source and are looking for a mate, they call all of their friends to the area by releasing pheromones.
By just observing the outer edges of the field, we may end up over-estimating beetle density and the potential threat to grain yields throughout the field. Particularly with the lower commodity prices now and forecasted in the near future, producers are scouting more than just the outside edge of the field to make the best decisions about whether to consider an insecticide spray.
Soybean. For soybean, treatment recommendations depend upon the growth stage of the plants. Thresholds for treatment are 30% defoliation before bloom and 20% defoliation between bloom and pod-fill.
It may be difficult to train the eye to estimate defoliation, but a photo from Dr. Marlin Rice of Iowa State University, may help with visualizing the differences among the different levels of defoliation.
Corn. Corn recommendations are more straight forward. Consider using an insecticide if during silking if you have 1) three or more beetles per ear and 2) silks are clipped shorter than ½ inch and 3) pollination is less than 50% completed. Dr. Mike Gray recently summarized results from Japanese beetle research out of Tennessee and Missouri that continues to support these recommendations.
Additional Information:Japanese Beetle Fact Sheet