Issue No. 15, Article 4/July 1, 2005
Twospotted Spider Mite Infestations in Soybeans Intensify as Drought Conditions Persist
Observations of spider mite infestations in soybeans are common across many areas of the state. Unfortunately, the scenario that is unfolding is eerily reminiscent of 1988. During that drought year, an estimated 6 million acres of soybeans received at least one insecticide application to reduce spider mite densities. Many fields received two applications of an insecticide. Spider mites also were a significant challenge in 1983; however, infestations during that season began during the third week of July and persisted through early August. In 1988, mite issues began to surface in June, similar to the current growing season. Because of the prolonged drought in 1988, spider mite populations persisted throughout much of Illinois well into the month of August. By then the loss of yield was substantial, and many producers remained convinced that the insecticide applications did not result in a significant return on their investment. But who knew that the lack of rain would persist for much of the summer? Many producers are now beginning to ask many of the questions that were raised in 1988 with regard to spider mite management. The current threshold is to consider a rescue treatment when 20% to 25% of soybean leaves are discolored before pod set. After pod set occurs, a rescue treatment should be considered when 10% to 15% of the leaves are discolored. With regard to the insecticides labeled for control of spider mites in soybeans, not much has changed in 17 years: Dimethoate 4EC--1 pint of product per acre or *Lorsban 4E--1/2 to 1 pint of product per acre.
In an effort to improve our management recommendations for spider mites, we designed an experiment in 1988 to measure how soybean plants responded to mite infestations. On August 4, 1988, we evaluated the effect of a mite infestation on a field of soybeans (Williams 82). Five different areas (20 square meters per area) within the field were sampled for mites, and a LiCor 6000 gas analyzer was used to estimate the net photosynthetic rate, stomatal resistance of leaves, and transpiration rate. All of the plant measurements were taken on leaves between 11 a.m. and 1 p.m. on a clear day. The same leaves also were used to estimate chlorophyll content and mite densities. The infested leaves from each of the five areas were selected based on the following visual damage-rating scale: (1) leaves normal green, with no apparent mite damage; (2) leaves paler green, some yellow mottling evident; (3) yellow mottling more prevalent, tending to cover leaf surface with a few necrotic areas apparent; and (4) leaf extensively mottled, with numerous necrotic areas. As spider mite injury intensified (greater leaf-injury scores), net photosynthetic capacity decreased, stomatal resistance increased, transpiration rate decreased, and total chlorophyll content decreased (Table 6).
During our investigation, we noted that soybean leaves with ratings of 2 to 4 had similar numbers of mites. Yet the physiological stress on plants increases considerably, from a rating of 2 to 4. Leaves with an injury rating of 4 had photosynthetic rates that were roughly three times less than leaves with ratings of 1. In addition, leaves that were rated 4 on our damage scale had approximately half as much chlorophyll as leaves rated 1. Stomatal resistance was more than twice as great on leaves rated 4 compared with those rated 1. As stomatal resistance increases, the rate of gas exchange through the leaf surface decreases; consequently, leaf temperatures increase, further exacerbating the population explosion of mites.
This investigation provided some information that may be of help in making treatment decisions for mites during this growing season. According to the leaf-damage scale that we evaluated, a rating as low as 2 resulted in some physiological responses of the soybean plant that would likely result in yield loss, particularly if mite and drought stresses continued over a protracted period of time.
If rescue treatments are warranted for mites, how much control can we expect from the very limited choice of products labeled for this impressive pest? Some information obtained from Dr. Ames Herbert, an entomologist with the Department of Entomology, Tidewater Agricultural Research and Extension Center, VPI, may help shed some light on this question. Dr. Herbert reported that dimethoate (if applied properly) significantly reduced the number of adult mites and immatures; however, the residual activity of the product was compromised on drought-stressed plants. He indicated that the systemic activity of dimethoate was reduced on plants that were "shut down" physiologically. So we should anticipate potential residual problems with dimethoate if the hot and dry conditions persist.
Dr. Herbert also noted that if dimethoate is mixed with alkaline water (high pH), it is subject to alkaline hydrolysis. He reported that the effectiveness of dimethoate may be reduced by as much as 50% in just 48 minutes if mixed with water that has a pH of 9. Determining and adjusting the pH of your spray solution may be a critical factor in optimizing mite control this season. Dr. Herbert reported that Lorsban 4E at the higher rate (1 pint product per acre) was effective in killing mites by contact; however, repeated applications were often necessary in as few as 3 to 5 days to kill recently hatched mites. Recall, Lorsban 4E does not have systemic activity. I wish there was more optimistic news to report on the insecticide efficacy front.
We will continue to provide updates on the spider mite outbreak as the season unfolds. Let's hope for some soakers.--Mike Gray