Movement of the Wheat Curl Mite

Gary Hein
State Representative
University of Nebraska/PREC, Scottsbluff, NE 69361.

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 hailstorms that have shelled out and germinated the wheat before harvest. 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 continue to study the ecology of this mite.

Wheat curl mite survival off the plant: Wheat curl mites under go continual generations and thus, populations must be maintained on green plant material. The mites must rely on air movement to move them to new or different hosts. The major factor in disease epidemiology is the existence of green plant material to host mites from harvest of one wheat crop in the summer to the emergence of the next wheat crop in the fall. The length of this over summering period and the environmental conditions during this period are major factors in determining the potential for disease problems to develop. The hosts for the mites during this time are termed the 'green bridge' for the mites. Recent work on wheat streak mosaic virus has indicated that the virus has nearly as much variability within a field as it has from county to county or across the state (Nebraska). These results suggest that mixing of virus populations does occur over longer distances and the only way for mixing to occur is for the mites to move with the virus, thus implying longer range movement. Movement of mites throughout the ecosystem has normally been thought to be due to short range movement because the mites are prone to desiccation and would not survive more than a few hours off the plants. Studies have been conducted to determine the length of time the mites can survive off the plant. Mites were held at different temperatures in high and low humidity conditions in isolation chambers, and the survival of the mites monitored over time. At temperatures above 20 degrees C and low humidity mites survived less than 12 hours, but as temperatures were reduced and humidity increased the mites survived considerably longer with the some mites surviving to five days at 5 degrees C. Figure 1 shows the influence of temperature on mite survival. These results indicate that mites certainly could survive long enough to move longer distances.

Impact of mite movement within different cropping systems through the season: During the last two seasons we have monitored mite movement in different areas though 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. 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 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 (Figure 2). 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. A peak from corn seems to occur in the fall just before the corn begins to dry down. 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 virus presence in our trap plants to determine the importance of this phenomenon to the epidemiology of the viruses involved.

Factors that trigger mite movement off of plants and subsequent disease spread: We are investigating the reasons that trigger mite movement off of the plants. Field trials this season indicate 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. We are attempting to correlate mite movement to leaf chlorophyll content as a quantitative measure of plant condition. We are also continuing to characterize the relationship between mite movement in the field and Roundup® applications. Limited observations indicate that Roundup when applied to volunteer wheat acts to increase the potential for viral infections in neighboring wheat.

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