Dennehy, Timothy J. : Timothy J. Dennehy is Graduate Student, Department of Entomology, University of California, Davis.

Granett, Jeffrey

Hydroponicallygrown cotton plants and a water barrier ( left ) isolate small cultures of floating islands of spider mites . Below , will in - author Timothy Dennehy prepares leaf - disc bioassay that dicate miticide residual activity not shown by the conventional slide - dip method . In slide - dip assay ( below left ) , mites are im - mersed in miticide test solution . Here , the mites are seen as small dots attached to the slide by double - stick tape . A simplified bioassay system Improved detection of dicofol - resistant Timothy J . Dennehy Jeffrey Granett spider mites in cotton I n the past , resistant pest strains have been suppressingpests without selecting for insec - tance - monitoring and management pro - controlled by replacing noneffective chemi - ticide resistance . IPM programs that incor - grams have not been answered . cals with effectivealternative chemicals . This porate a chemical component , however , may In 1981 , we began field and laboratory tactic is becoming increasingly inappropriate be threatened by the development of resis - studies on documentation and management because tance in pest populations . of spider mite resistance in San Joaquin Val - of escalating costs associated with developing replacement pesticides and inci - No mechanism exists in California for ley cotton . We investigated the variation in dences monitoring the efficacy of cross and multiple resistance . As a of pesticides used in susceptibilityto the widely used miticidedico - result , some workers feel that pesticide resis - the field . Growerswho think they may have a fol by sampling spider mites from fields near tance is the most important problem facing resistance problem must resort to personal Bakersfield , Corcoran , and Lemoore . applied entomology . experience or that of other local growers and In this paper we describe a method that Tactics used in integrated pest management farm advisors in evaluating the inadequacies allowed us to perform resistancebioassayson ( IPM ) , such as biological control , cultural of their chemical control methods . Many numerous single - species spider mite cultures methods , and pheromones , are key factors in questions basic to implementation of resis - from cotton fields . We report on spider mite CALIFORNIA AGRICULTURE , NOVEMBER - DECEMBER 1982 11 In the slide - dip method , female mites were populations found at three locations that ap - Field applications placed on their backs on double - stick tape at - peared quite susceptible to dicofol in topical The contrast in results obtained from the tached to a microscope slide . The slide was assays and yet quite tolerant of dicofol in two bioassay methods presents a problem . then dipped for 5 seconds , totally immersing residual assays . Which method can we relyon to indicate field of 0 , 10 , the mites in dicofol concentrations Floating mite - islands Susceptibility ? If the topical , slide - dipassayis 18,32,56 , or 100ppm . Slideswere allowed to used , then our data would indicate that mites In studying miticide resistance , it is nec - dry for 15 minutes , placed in slide boxes , and in fields from all locations sampled were ap - essaryto isolateand maintain geneticallydif - held at 27 O C . Mortality of mites was assessed proximately equal in susceptibilityto dicofol . ferent spider mite populations . Mite cultures after 24 hours . On the other hand , the residualbioassay indi - are difficult to isolate , because the mites are For the residual assay , leaf discs 20 mm in cates that susceptibility was extremely vari - small and can be carried by air currents . We diameter were excised from cotton cotyledon able both within and between the locations could not use existing methods of maintain - 5 seconds in dicofol con - leaves , dipped for sampled . ing add isolating cultures for our studies , of 0 , 10 , 100 , 1,000 , or 10,000 centrations Cotton growers apply miticides with 20 to because space and labor were limited . The ppm , and placed on moist fiber - cotton beds . floating mite - island method allowed 30 gallons us to of water per acre . Conservative 10 female After the leaf discs were air - dried , estimates are that about 20 percent of the maintain numerous cultures in a relatively mites were transferred to each . Mortality was small laboratory area . mites present in a field are not directly con - assessed after 48 hours at 27 C . Floating mite - islands consisted of Styro - tacted by the spray at the time of application . The slide - dipand residual assaysproduced That 20 percent will either be killed by the markedly different data on susceptibility . Di - foam platforms containing hydroponically or survivethe treat - residual chemical deposit cofol concentrations of 100ppm or less in the grown cotton seedlings . The platforms ment . slide - dip method killed mites from all cul - floated within a cylindrical , clear - sided , 0.01 - Our laboratory studies indicate that the inch - thick acetate - plastic cage . These cages tures after 24 hours , whereas many of the cul - reason for the poor resultssomegrowershave tures contained mites that survived 48 hours were open at the submerged , basal end and obtained with dicofol may be the presence of closed at the top with a piece of finely woven exposure to 10,000 - ppm treatments with the mites that , if not directly contacted by the of which was polyester lining , the mesh residual contact method ( see graphs ) . The miticide , can withstand contact with the resi - smaller than a newly hatched protonymph survivorship rate with the residual assay at due . In essence , we hypothesize that dicofol mite . the 10,000 - ppm concentration varied among Cotton seedling bouquets in test tubes sus - has no or little residual activity in fields con - 0to 100percent . Of 44cultures cultures from pended within the floating mite - islands taining residue - tolerant mites . Field and lab - tested by residual assay , 27 repeatedly had served as nursery plants for the spider mites . oratory studies are under way to test this survivors of the 10,000 - ppm rate . Greenhouse - grown , Acala SJ - 2 cotton seed - hypothesis . lings , grown in vermiculite , were suitable for Conclusions this use when about 4 inches high . Groups of A 10 to 12 seedlings were washed gently to re - The floating mite - island method provided SLIDE - DIPASSAY move the vermiculite from the roots and then significant savings in time and space when , j 77 cultures bulked into bouquets . We took great care to culturing many , small , discrete populations 1 - of introducing spider reduce the chances of spider mites . Field - monitoring studies in 50 zc0g ) 430 0 ) mites inadvertentlyinto mite - islands with re - 1981 with single - speciescolonies detected T placement nursery plants or carrying them urticaeand T pacificuspopulations that were from one culture to another while working readily killed in slide - dip dicofol bioassays with the colonies . ( discriminating concentration of 100ppm or : 20 2 less ) but that repeatedly survived 48 - hour a " 10 Slide - dip and residual assays residual contact with 10,000 ppm dicofol Because of the large number of cultures treatments . 18 32 56 100 a being assayed for susceptibility , we used Dicofol sprays applied to fields containing Discriminating concentration , discriminating concentration method . A dis - residue - tolerant mites may effectively per - ( ppm ) dicofol criminatingconcentration was defined as the a contact miticide . Resistant mites form as B RESIDUAL , LEAF - DISCASSAY lowest concentration of dicofol tested that not directly contacted by the dicofol spray v , I 44 cultures 61.4 resulted in 100 percent mortality of samples may survive the residual spray material so of mites from a culture . We collected mite - that the pest population rebounds rapidly . infested leaves from fields in the three areas The possibility of resistance to residues in and brought them to the laboratory . Single - cotton fields underscores the importance of z 40 - a , species cultures were started from individual good spray coverage . Monitoring programs g 30 - c Tetranychuspacificusor T urticae females of cannot rely solely on slide - dip assays for de - by the mite - island culturing technique . of field resistance . tection of 10 young , adult , female mites Groups TimothyJ . Dennehy is Graduate Student , and Jef - from each culture were treated with each test of frey Granett is Associate Professor , Department 100 1,000 10,000 10,000 + concentration of dicofol ( formulated Kel - Entomology , University of California , Davis . The of authorsgratefullyacknowledge the cooperation Discriminating concentration , thane 18.5 percent EC and 71 ppm Triton Thomas Leigh , U.S . Cotton Research Station , dicofol ( ppm ) B1956 surfactant ) , and discriminating doses Shafter , and Vernon Burton , Extension Entomol - Bioassays showed that mites killed in slide - were recorded . Tests were replicated two to ogist , U.C . , Davis . Project Statistician is Jessica dip tests with 100 pprn or less could survive Utts , Division of Statistics , U.C . , Davis . This re - eight times . 10,000 - ppmresidues ( 10,000 + category in - search was supported in part by a grant from the We used two bioassay methods : the slide - dicates cultures with mites able to survive of California Statewide Integrated Pest University 48 - hour , 10,000 - ppmresidual assays ) . dip and a residual - contact , leaf - discmethod . Management Project . 12 CALIFORNIA AGRICULTURE , NOVEMBER - DECEMBER 1982