resistance in spider mites

Elizabeth E. Grafton-Cardwell 0 Jeffrey Granett 0 Timothy J. Dennehy

Thestrawberry mite, two-spotted spi- atively few fields: 10 to 30 percent cide would be most effective. The rapid der mite, and Pacific mite are the domi- of those randomly sampled during 1982- bioassay can also be used after an acari- nant spider mite pests of cotton in Califor- 85. If resistance becomes widespread be- cide application to distinguish between nia's San Joaquin Valley. The damage cause of mismanagement of these acari- loss of efficacy due to field failure and they cause ranges from stippling of leaves cides, however, and they are replaced that resulting from other factors. Finally, to defoliation, influencing plant growth with broader spectrum that temporarily avoiding the use of a problem and production. kill natural enemies, the excellent inte- gives susceptible mites a Three acaricides that allow beneficial grated management program pres- chance to survive and interbreed with re- insects and mites to survive are currently ently used in San Joaquin Valley cotton sistant mites, resulting in a lower fre- registered for control of spider mites in will be disrupted. It is therefore impor- quency of resistant individuals in the next cotton. One of them, sulfur, is effective tant to implement resistance manage- generation. only against the strawberry mite, Tetran- ment practices quickly for both dicofol Several areas of research had to be ychus turkestani Ugarov and Nikolski. and propargite. completed in developing the rapid bio- The other two, dicofol (Kelthane) and pro- In the past, acaricide resistance was assay. We first performed laboratory pargite (Comite), have been used success- detected by commercially treating a field bioassays to characterize the levels of re- fully for 15 to 20 years to control all three and assessing spider mite densities 7 to 14 sistance in different populations of spider species of spider mites. In the last few years, however, we have begun to detect populations of two-spotted spider mite, T. A rapid bioassay that detects buildup of resistance urticae Koch, and Pacific mite, T. pacifi- cus McGregor, that are resistant to dico- can help growers avoid the expense and hazard of fol, propargite, or both. wasted treatments. Several characteristics of the spider mitelcotton system in the San Joaquin Valley make acaricide resistance more days after the application. If resistance mites throughout the San Joaquin Valley. manageable than in many other situa- was a problem, however, this was an ex- Then field trials determined which levels tions. First, growers keep acaricide use pensive way to find out. Also, this method of resistance detected in the laboratory down to one or two applications per sea- did not distinguish between lack of effec- corresponded to resistance of populations son, resulting in a moderate selection tiveness due to resistance and that.due to in the field that cause the acaricide to lose pressure. Second, they avoid broad-spec- other factors, such as poor coverage, mi- effectiveness. Finally, the rapid bioassays trum that remove natural en- gration of spider mites from neighboring were developed so that growers could de- emies. Third, , and thus pesti- crops, low natural enemy populations, termine, in 24 hours, which cotton fields cide practices, in the San Joaquin Valley and plant water stress. Our laboratory had significant levels of resistant spider are diverse; the use of many different has worked for the past five years on the mites. acaricides lowers the selection pressure development of a bioassay test that grow- We report here on the development of of any one. Fourth, because spider mites ers can use for an inexpensive, fast as- the rapid bioassays for detecting dicofol are very mobile and can drift from field sessment of resistance in individual cot- and propargite resistance in spider mites to field on air currents, interbreeding of ton fields before applying an acaricide. in San Joaquin Valley cotton. susceptible and resistant mites is en- The benefits of a quick method for de- hanced. Finally, inheritance of resistance tecting resistance are both short-term and Rapid bioassays to dicofol and propargite is not dominant, long-term. If resistance is a problem at a The accurate evaluation of resistance and breeding with susceptible mites will specific location, the grower will avoid depends on random sampling of mite-in- rapidly reduce the frequency of resis- the expense and hazard of a wasted acari- fested leaves. One to three species of spi- tance in a population. cide application. If more than one acari- der mites may be found in a cotton field, The result is that high levels of dicofol cide is available, the grower can make a and several levels of resistance may ex- and/or propargite resistance occur in rel- more accurate decision on which acari- ist. Thus, 80 to 100 leaves are needed from

8 CALIFORNIA AGRICULTURE, JULY-AUGUST 1987 Rearing did not have as serious an effect on their level of resistance to dicofol. Also, if we store them in cold tempera- tures for more than 48 hours before test- ing, susceptibility to both acaricides in- creases. The rapid bioassay should thus be conducted within 48 hours of removal of spider mites from the field. Validation of bioassays Our final step in validating the rapid bioassay was to use it before a commer- cial application of acaricide and see if it correctly predicted efficacy or lack of ef- ficacy. We chose 16 cotton fields that were to receive a propargite application. We sampled each field twice using pres- ence-absence sampling (percentage of 25 randomly collected leaves infested with mobile stages of spider mites) and con- ducted the propargite rapid bioassay im- mediately before the commercial appli- cation. One week to ten days after Damage caused by strawberry spider mite (at left), one of three mite pests of cotton, ranges from treatment, we returned to the fields and stippling of leaves, as shown here, to complete defoliation The mites are hard to control without repeated the presence-absence sampling. killing their natural enemies, and they can quickly develop resistance to some pesticides If the mites survived well in the pro- pargite-treated dish, we predicted that they had high levels of resistance to the each field. Only one adult female mite is After 24 hours, the mites are examined acaricide and that the percentage infesta- removed from each leaf. This type of for activity. For dicofol, if more than 20 tion would change very little after the ap- sampling ensures that the spider mites percent of the spider mites can walk the plication. Alternatively, we expected a are a cross-section of the various species distance of one body length when prodded large change in the percentage infestation and resistance levels present in the field. with a camel's hair artist's brush, then of the cotton leaves if little or no resis- The mites can be stored in a refrigerator significant levels of resistance exist in tance had been detected. or ice chest (above 40°F) for up to 48 that cotton field, and we recommend Spider mites from seven sites (fig. 1) hours before testing. against use of dicofol. In the propargite had shown 25 percent or greater survival For the dicofol rapid bioassay, small, test, the edge of the dish is tapped vigor- in the dish and thus had significant levels tight-fitting plastic petri dishes are treat- ously on a hard surface to knock the mites of resistance to propargite. Although 30 ed with 56.2 parts per million (ppm) ac- to one side. If more than 30 percent of the percent survival was the threshold we de- tive ingredient (AI) formulated dicofol in spider mites can move their legs vigor- scribed earlier for highly resistant popu- 95 percent ethanol (weight/volume). For ously, then significant levels of resistance lations, 25 percent resistance was includ- the propargite rapid bioassay, the dishes exist in that cotton field, and we recom- ed in this group, because it is a borderline are treated with 1000 ppm A1 formulated mend against the use of propargite. response and may also cause loss of field propargite in ethanol. The control dishes We want to avoid the error of discon- efficacy of the acaricide. In four out of are treated with ethanol alone. One milli- tinuing use of dicofol or propargite when seven of the sites, the percentage infesta- liter of solution is placed in the test dish, resistance levels are low and the acari- tion actually increased. In site 7, a broad- the dish is closed, the liquid is swirled cides are still effective in controlling spectrum applied with the pro- around the inside of the top, bottom, and most of the spider mites. The dicofol and pargite had an acaricidal action, and mite sides, and the excess is poured off. The propargite rapid bioassays were there- densities were greatly reduced. dishes are then air-dried and kept in the fore designed to detect only those popula- Of the nine sites in which the spider refrigerator (40" to 50°F) for at least one tions of spider mites that have at least 20 mites showed 20 percent or less survival week and then used as needed. When and 30 percent resistant individuals, re- in the propargite-treated dish, six showed stored in this way, the dishes remain ef- spectively. at least a 50 percent reduction in percent- fective for three months. If survival of the spider mites in the age infestation after treatment (fig. 2). When the test is set up, the dish con- rapid bioassay dish is very close to 25 per- Site 11 was next to a cornfield, and so, taining ethanol only, 56.2 ppm dicofol, or cent, other factors that influence acari- although the spider mites were suscept- 1000 ppm propargite is placed on paper cide efficacy must be carefully weighed, ible, the propargite appeared to fail be- toweling on a block of frozen blue ice. The such as coverage, plant stress, and the cause more spider mites were blowing paper prevents condensation, and the ice number of natural enemies, before a deci- into the cotton from the corn. Site 15 re- cools the mites and prevents them from sion to use the acaricide is made. For ex- ceived a propargite application for spider leaving the dish. Twenty adult females ample, if the natural enemies are at high mites in combination with a are removed from 20 sampled leaves and densities and resistance is borderline, application for Lepidoptera insect con- placed in a dish for each concentration control will probably be achieved. trol. Since tend to cause mite tested. The test dishes are closed and left We have found two sources of error in populations to flare up, the propargite ap- in indirect light or in darkness for 24 handling spider mites that alter their re- plication did not lower the percentage of hours at 70" to 85°F for the dicofol-treat- sponses to dicofol or propargite. If we spider mite-infested leaves. ed dish and 80" to 85°F for the propar- rear the spider mites in the laboratory, With a few exceptions, the propargite gite-treated dish. their resistance to propargite decreases. rapid assay was a very good predictor of

CALIFORNIA AGRICULTURE, JULY-AUGUST 1987 9 100

90 - 90 -

80 - 0 Pretreatment 80 Pretreatment =Posttreatment F3 Posttreatment

70 - 70 - - CI - c - t - (I) 60 (I) 60 t - a - - 2 50 -Q) 50 U P 1 - 3 - - 0 40 8 40 c- - 30 - 30 -

20 - 20 -

to - 10 - D - 0- 0 L 2; 2; s sg s In Lo P Lo Lo 0 m N -l - .. PretreatmentI survivors of rapid bioassay Pretreatment survivors of rapid bioassay Rapid bioassay was a good predictor of resistance to propargite In were fairly close 3f sites from which spider mites showed sus sites from which spider mites showed high resistance in laboratory ceptibility to propargite. fig 2 (right) six had a significant reduction in bioassays fig 1 (left) pretreatment and posttreatment infestations infestation after treatment

the effectiveness of commercially applied creased to a level where the acaricide will acaricide resistance in San Joaquin Val- propargite. The spider mite populations be effective again. ley cotton. that did not respond in the manner pre- Of the 54 spider mite-infested leaf dicted by the rapid bioassay demonstrat- Commercial use samples received from various counties ed that factors other than resistance, such During the 1986 field season, with the in the San Joaquin Valley, approximately as mite migration, natural enemies, and support of the California Department of 50 percent revealed spider mite popula- use of broad-spectrum pesticides, can also Food and Agriculture, we cooperated with tions significantly resistant to dicofol, influence the apparent effectiveness of Dellavalle Laboratories in Fresno in a pi- propargite, or both (table 1). For the re- the selective acaricides. lot program with the rapid bioassays. We maining sites, the advisor used Dellavalle’s facilities and courier now knows to look for other factors that Timing service and trained a technician to pro- contribute to lack of effectiveness. The rapid bioassays are most useful if cess mite-infested leaf samples. Pest con- Cotton is the only crop in which the conducted immediately before an acari- trol advisors brought in these samples for dicofol and propargite rapid bioassays cide application is needed to determine resistance assessment, because they were have been validated. Propargite is used in which one will be effective. If the test pre- concerned about a lack of effectiveness in other crops, however, such as almonds dicts resistance to one acaricide, the other particular fields. The results therefore do and seed alfalfa, and so we have begun one should be used. If resistance to both not represent the true distribution of work to validate that bioassay for those acaricides is detected, another selective crops. The situation for perennial crops is acaricide should be used if available. TABLE 1. Number of “problem” cotton fields more difficult, because factors other than which produced susceptible or resistant spider Broad-spectrum pesticides should be mites when tested with the rapid bioassay resistance, such as natural enemies and avoided if at all possible, because they de- during 1986 water stress, exert greater influence on stroy the natural enemies. acaricide efficacy than they do in cotton. SamDles tested with dicofol or DroDaraite Because spider mite species and resis- ~~~ ~ Elizabeth Grafton-Cardwell is Postdoctoral Re- tance levels can change during a season, Dicofol’ Propargitet seacher, and Jeffrey Granett is Associate Professor, one early-season measurement of resis- Department of Entomology, University of Califor- nia, Davis; and Timothy J. Dennehy is Assistant Pro- tance is not sufficient to predict efficacy County Susc. Resist. Susc. Resist. fessor, Department of Entomology, Cornell Universi- of an acaricide throughout the season. If ty, Geneva, New York. The authors thank Bonnie Merced 3 1 0 0 Beffa, Steve Lorenzato, and Sherita Normington for more than one acaricide application is Madera 3 0 1 0 technical assistance; Rohm & Haas and Uniroyal needed, the rapid bioassay should be re- Fresno 1 0 1 0 Chemical Companies for providing acaricides; and peated. Tulare 3 4 3 8 Dellavalle Laboratories for use of their facilities. Kings 5 10 9 5 They are also grateful to the growers and pest con- The rapid bioassays can also be used Kern 7 7 5 5 trol advisors for use of cotton fields. This project was 10 days or more after an acaricide appli- supported in part by funds from the UC/IPM pro- Total 22 22 19 18 gram, CPCSD, Cotton Incorporated, and the Environ- cation to see if noneffectiveness is due to * Resistance IS defined by >20% of the spider mites mental Monitoring and Pest Management Division of resistance or some other factor. The rapid walking after 24 hours on 56 2 ppm dicofol the California Department of Food and Agriculture. t Resistance is defined by >30%of the spider mites The rapid bioassay dishes and testing service are assays are also useful the following sea- moving vigorously after 24 hours on 1000 ppm currently available from Insect Lore Products, P.O. son to see if resistance levels have de- propargite Box 1535, Shafter, Ca 93263.

10 CALIFORNIA AGRICULTURE, JULY-AUGUST 1987