Spider mites are considered to be the most important invertebrate pests of commercial field-gro wn rose plants, but sampling methods and treatment thresholds have been subjective. This study shows that roses exhibit a higher tolerance for spider mites than previously thought. Quality rose plants were produced with fewer acaricide treatments by using a rapid presence/absence field sampling method and treatment thresholds for spider mites. Kern County produces the majority of U.S. rose plants,

more »

... vesting about 2,000 acres of rose plants annually in the Wasco area. Most of these are cultivars destined for use as outdoor plants and most are grown on a 2-year production cycle. Rootstock cuttings are planted in the field in December or January, then budded with scion varieties from March through early June. Early in the second year, the plants are cut back to the bud union to force the scion bud. Canes (branches) of the scion variety grow through the summer, then the plants are dug, graded and shipped beginning in November. Wholesale prices reach as high as $2.50 per plant for some varieties. There are typically 24,000 plants per acre, so each field represents a substantial investment. Rose growers in the southern San Joaquin Valley consider several species of spider mites to be their most troublesome invertebrate pests. These mites cause loss of leaf chlorophyll and, presumably, loss of plant vigor. High mite populations on second-year plants can cause defoliation, resulting in sunburn that renders the plants unfit for market. First-year plants appear more tolerant of spider mites. In the past, growers controlled mites by treating rose fields with acaricide four to eight times per season, with as many as 13 applications in some years to prevent damage to the high-value crop. However, no published studies address sampling methods, mite thresholds, or the impact of spider mite populations on the final grade and yield of rose plants. The value of the crop and perceived need to differentiate the product from competitors make pesticide applications seem relatively inexpensive, but frequent use of acaricides may create problems. Repeated applications are suspected of selecting for resistant spider mite populations; in other words, changing the spider mite population so that the hardier mites predominate, which can make them difficult to control in other crops. Worker safety and environmental concerns also argue for reducing acaricide applications when possible. The number of acaricides on the market also has dropped substantially, providing further incentive for examining alternative treatment methods. The 1990-1991 study In previous work, we found that using a sampling technique known as presence/absence (PA) was useful in assessing spider mite populations in rose fields. The PA technique is rapid, gives quantitative data, and data are correlated to mite counts obtained by brushing. r P u > --X m 7 High populations of spider mites can render a rose plant unmarketable. Below, a pest control advisor uses the presencelabsence sampling technique to monitor spider mites. To evaluate the effects of different mite population levels on rose plant growth and yield, we began a field study in 1990. We set treatment thresholds of 40% infested for firstyear plants and 25% for second-year plants. Two rows of 'Dr. Huey' rootstock were planted in January according to customary industry practice. Plants were then budded to the 'Peace' variety later in the spring. Two guard rows buffered the plants to the west, with numerous other rows acting as buffers to the east. Randomized plots were divided into two %foot rows. We sampled the center 20 feet of each plot weekly, selecting 10 leaves from 10 rose plants in each row for a total of 20 leaves per plot. These leaves were noted for PA of spider mites. The species present were predominately Pa-38 CALIFORNIA AGRICULTURE, VOLUME 49, NUMBER 3