RESISTANCE IS FUTILE

The Consolidated Mosquito Abatement District (District) provides mosquito control to portions of Fresno and Kings Counties in the Central Valley of California. The District includes some of the prime agricultural land in the world, as well as a large urban zone (portion of the Fresno metropolitan area). Thus the mosquito control program incorporates urban, suburban residential and rural operational elements.

Among the several species that demand District attention in the form of surveillance and control efforts are Culex quinquefasciatus Say and Culex tarsalis Coquillett. Cx. tarsalis is the primary vector for both Western equine and St. Louis encephalitis viruses. However, the major pest mosquito within the District is Ochlerotatus (nee Aedes) nigromaculis (Ludlow).

Oc. nigromaculis is not native to California. In fact this mosquito was not found and identified in the state before 1937 (Bohart and Washino 1978). But it rapidly spread throughout the Central Valley where it found an available and widespread niche, that was readily and very successfully exploited. This multivoltine, floodwater mosquito is associated with evanescent water habitats such as irrigated pastures, alfalfa fields, orchards and vineyards. In the District Oc. nigromaculis primarily breeds in irrigated, permanent pastures. Pastures offer the mosquito a complete habitat package, from mating and larval development sources to preferred blood meal hosts (bovines and other large mammals).

These mosquito breeding pasture sources are increasingly coming into closer proximity to residential areas, as suburban development continues to encroach upon rural areas and farm land. Adding to the problem are the creation of new small pasture sources as suburbanites desire country living on small acreages with a house and a horse and access to ditchwater. It is in these habitats that Oc. nigromaculis flourishes. Unfortunately the mosquito is a vicious day time and crepuscular biter and is not averse to biting people, which can generate numerous service calls when mosquitoes emerge from a pasture.

Although the District routinely lists, surveys and treats these pasture sources, occasional misses and failures occur. Pastures are typically flood irrigated every two weeks from April through September and each flooding can produce a generation of Oc. nigromaculis, as many as ten generations or more per year. This produces a lot of treatment applications. During peak summer months with day time temperatures in the 100 degree Fahrenheit range, Oc. nigromaculis can develop to adult stage if water stands longer than just three days in a pasture. This short larval development interval leads to more chances for misses, timing and control failures. Added to the sheer numbers of treatment applications, the overall effect is pressure for development of resistance to control agents.

In fact Oc. nigromaculis is notorious for developing resistance to chemical insecticides. It was the first mosquito in California to develop resistance to DDT (Bohart and Murray, 1950) and soon after showed resistance to a wide variety of other chlorinated hydrocarbon insecticides. With the common use of organophosphate insecticides in the 1950’s by mosquito control programs and in agriculture, resistance to parathion in Oc. nigromaculis was soon detected (Lewallen and Nicholson 1959). Cross-resistance to other organophosphates and carbamates soon followed and by the 1970’s there were few chemicals available for Oc. nigromaculis control.

The stage was set for the introduction of methoprene into mosquito control. Methoprene proved to be highly effective in control of Oc. nigromaculis in irrigated pastures (Schaefer et al. 1975). It soon became the control agent of choice for many mosquito abatement districts in the Central Valley of California.

Complete reliance on methoprene for Oc. nigromaculis in irrigated pastures continued without major concern until September 1998. At this time control failures were noted with Altosid® Liquid Larvicide (ALL), methoprene, applied to a pasture west of the City of Fresno. By the following season control failures were noted in additional pastures, including a pasture (Dice Field) within the Consolidated MAD. The District carefully observed and evaluated aspects of formulation, application and biotic and abiotic field conditions to determine the cause of the failures. Attention was given to such factors that could cause failures as problems with methoprene formulation, operator error in application, improper timing of application, coverage errors, incorrect evaluation of efficacy and flooding regime anomalies. In all cases, after closely scrutinized methoprene applications were made, large numbers of adult mosquitoes emerged from the suspect pastures. Because possible contributory factors could not explain the failures, the possibility of methoprene resistance in the Oc. nigromaculis populations in these pastures was considered.

In late summer of 1999, Dr. Anthony Cornel of the University of California, Mosquito Control Research Laboratory in Parlier, CA, was contacted and asked to evaluate whether there was indeed resistance. A protocol was developed to compare suspect pastures with pastures that had shown no evidence of control failures. Three methoprene formulations (ALL, XR-G and Pellets) were used in the study. Immatures were allowed to develop to pupal stage in the field, collected and taken to the laboratory for evaluation.

In the suspect pastures, percent mortality of treated immatures was extremely low, often lower than untreated (control) mortality levels, less than 10% mortality. The study showed that the Oc. nigromaculis populations in selected pastures did indeed have high levels of tolerance to methoprene (Cornel et al. 2000).

The following mosquito season, Dr. Cornel continued to study the Oc. nigromaculis population at the Dice Field in the District. A laboratory methoprene susceptibility assay was developed to compare naïve Oc. nigromaculis, populations that had not been exposed to methoprene applications, with methoprene tolerant populations. For the assay, larvae were either collected directly from the pasture or were reared from eggs oviposited from newly inseminated females collected from the pasture.

Results from the methoprene susceptibility assays showed that Oc. nigromaculis from the Dice Field had 3,100-fold higher LC₅₀and 1,533-fold higher LC₉₀levels than susceptible populations. In fact, Oc. nigromaculis in these pastures were highly resistant to methoprene (Cornel et al. 2002). By way of comparison, the earlier report of methoprene resistance in Oc. taeniorhynchus (Wied.) on Captiva Island showed 14.9 fold tolerance to methoprene (Dame et al. 1998).

Since 1999, immature mosquito populations at the Dice Field have been treated only with Bacillus thuringiensis israelensis (Bti) and mosquito larvicide oil applications. Studies are under way to determine whether this methoprene resistant Oc. nigromaculis population can revert to susceptibility in the absense of methoprene exposure, in this case with applications of alternative control agents. Preliminary results indicate that the level of resistance has diminished.

Elsewhere in the District a pesticide rotation program has been employed to alternate one-half season with Bti and one-half season with methoprene. It is hoped that this rotation program will forestall further spread of resistance within the District. The District is continuing to search for alternative insecticides and methods to control Oc. nigromaculis. The District also continues to work with landowners to more effectively manage this very important breeding habitat, the irrigated permanent pasture.

REFERENCES CITED

Bohart, R.M. and W.D. Murray. 1950. DDT resistance in Aedes nigromaculis larvae. Proc. Calif. Mosq. Contr. Assoc. 18:22-3.

Bohart, R.M. and R.K. Washino. 1978. Mosquitoes of California, 3^rded Berkeley,CA, University of California,153 pp.

Cornel, A.J., Stanich, M.A., Farley, D.G., Mulligan, F.S. III and G. Byde. 2000. Methoprene tolerance in Aedes nigromaculis in Fresno County, California. J. Am. Mosq. Contr. Assoc. 16:223-6.

Cornel, A.J., Stanich M.A., McAbee, R.D. and F.S. Mulligan III. 2002. High level methoprene resistance in the mosquito Ochlerotatus nigromaculis (Ludlow) in Central California. Pest Manag. Sci. 58:791-8.

Dame, D.A., Wichterman, G.J. and J.A. Hornby. 1998. Mosquito (Aedes taeniorhynchus) resistance to methoprene in an isolated habitat. J. Am. Mosq. Contr. Assoc. 14:200-3.

Lewallen, L.L. and L.M. Nicholson. 1959. Parathion-resistant Aedes nigromaculis in California. Mosq. News. 19:12-4.

Schaefer, C.H., Miura, T., Wilder, W.H. and F.S. Mulligan. III. 1975. Evaluation of new chemicals as mosquito control agent. Proc. Calif. Mosq Contr. Assoc. 43:75-7.