Ecology and Movement of the Wheat Curl Mite

Gary L. Hein
University of Nebraska
Panhandle Res. & Ext. Center

The wheat curl mite is an eriophyid mite of considerable importance to wheat on the Great Plains. This mite transmits two viruses (wheat streak mosaic virus and high plains virus) that cause disease in winter wheat throughout the Great Plains and the Pacific Northwest. These diseases are the most important disease problems in winter wheat in the Great Plains. Management of the mite relies on the ability to manage mite populations during the over summering period between wheat maturity in the summer and the emergence of the new crop of winter wheat in the fall. The most important of these over summering hosts is volunteer wheat that has resulted from hail storms that have shelled out and germinated the wheat before harvest (see 'Short Range Mite Movement' below).

Wheat Curl Mite (150-250 microns)

The management of this mite is complicated by its ability to move, and the study of the movement of this mite is complicated by the size of the mite. This mite is extremely small (ca. 150-250 microns in length) as are all eriophyids. The study of these mites requires a great deal of labor for counting or the development of special techniques that allow the adequate sampling of the mite either from their host plants or from the air as they are moving. We are trying to develop techniques that allow us to better monitor these mites in the field so that we can study the ecology of the mite.

Impact of mite movement through the season within different cropping systems:
During the last two seasons we have monitored mite movement in different areas through the season and have found differences between the level of movement found in an area where wheat is the primary crop and in areas of where irrigated corn is one of the major rotational crops. In 1999 we expanded this study to include mite movement adjacent to dryland corn and foxtail millet. Changes in the federal farm program have increased the diversity of dryland crops in the region with the acreage of dryland corn increasing significantly.

Our mite movement sampling has relied on the use of freshly emerged greenhouse plants as trap plants. The trap plants are set out in the field and exposed to any potential mite movement for seven days and then covered and returned to the greenhouse. Plants are sampled upon collection and the number of mites counted on these plants. The remaining plants are grown out in the greenhouse to allow for mites to build up, transmit virus to the plants, and the virus titre to increase.

As expected, movement of mites in the wheat system peaks just prior to harvest and drops rapidly to zero until the early fall when a low level of activity occurs (see below). The level of activity in the fall is likely due to the proximity of a significant source of mites from volunteer wheat. In the irrigated cropping system, a smaller peak occurs around the time of wheat harvest when mites are likely traveling longer distances to be present in significant numbers around corn and subsequently infesting the corn. A peak from corn seems to occur in the fall just before the corn begins to dry down and/or freeze. The extent of this mite activity is surprising in that infestation levels of 100% of our trap pots has occurred during a time when mite populations were assumed to be rather low. We are currently analyzing the 1999 data which preliminarily indicates that mite numbers are higher in dryland corn than in dryland wheat. The presence of more mite activity in dryland and irrigated corn indicates that there is potential for greater virus transmission in these cropping systems. However, more work needs to be done into the occurrence of widespread infections in these cropping systems.

Factors that trigger mite movement off of plants and subsequent disease spread:
We are investigating the factors that trigger mite movement off of their host plants, particularly volunteer wheat. Field trials have indicated that mite movement appears to be tied to plant condition. Plots with plants showing the most visible signs of plant stress (e.g. yellowing) had the greatest mite activity level within them. The concentration of chlorophyll as measured by using a SPAD meter has demonstrated usefulness in monitoring plant quality. Even though the highest number of mites seems to be coming from plants that are in decline, as measured by a reduction in chlorophyll, the total number of mites coming from volunteer wheat over a long period of time likely will result from that volunteer that will stay healthy the longest. This results from plants staying healthy longer and allowing more time for very high mite populations to build up. Subsequent movement off these plants, when measured over time, does account for greater mite activity.

We are also continuing to characterize the relationship between mite movement in the field and Roundup applications. Our observations indicate that Roundup when applied to volunteer wheat acts to increase the potential for viral infections in neighboring wheat. We do not see a rapid increase in movement off of Roundup treated plants, but a gradual increase in movement off the Roundup treated plants. This appears to be the result of the onset of plant deterioration or senescence resulting from the Roundup treatment. Mites are capable of surviving on the Roundup treated plants and continue to move off the plants until the plants have totally dried up. We are currently concluding these studies and analyzing the last year of the study and evaluating virus transmission in our Roundup treated plots.

[1999 Research Index | 1999 Meeting Minutes ]