From The Field

The effectiveness of insecticides and the development of resistance has been studied in the German cockroach population at the Lincoln Terrace Apartments in Roanoke, Virginia, for about 15 years. Working in the 340 apartments that make up this large complex, which has real-life cockroach infestations, has provided insight and information that a laboratory environment and laboratory-reared cockroaches could not. The conditions in small, artificial populations in the laboratory, without the pressures of finding food and harborage and avoiding toxic material, may be quite different from what happens in apartment kitchens and bathrooms.

Although the conditions faced by the German cockroaches in Lincoln Terrace may not be the same as other locations, the environment, housekeeping, and general pest control operations are representative of most urban areas. Apartment kitchens and bathrooms are treated two to three times per year with liquid or bait insecticides by a trained pest control worker from the Roanoke Redevelopment and Housing Authority. The Urban Pest Control Research Center has researched the German and American cockroach populations, and has worked regularly with the cockroach control program in Lincoln Terrace since 1975.

Since 1990 the detailed history of chlorpyrifos and cypermethrin against the Lincoln Terrace German cockroaches has been reported here in PCT. The main events have been:

• The history of insecticide use and the development of cypermethrin resistance (PCT, October 1990).

• Chlorpyrifos resistance in response to use patterns, and the limited value of rotation as a resistance management scheme (PCT, December 1992). • The reversal of resistance to several pyrethroids — including cypermethrin, cyfluthrin, and fenvalerate — in the Lincoln Terrace cockroaches, and the prospects of further use of these insecticides (PCT, September 1994).

This article will report on research conducted in Lincoln Terrace in the last two years (1994 and 1995). During this time the levels of cypermethrin and chlorpyrifos resistance have been closely monitored, and the insecticides' levels of effectiveness in controlling German cockroaches have been evaluated. The encouraging message is that some of the mistakes (exclusive use of one insecticide) made with the cockroach population have been "repaired"; cypermethrin and chlorpyrifos resistance has stabilized; and these two insecticides are part of a pest control program that integrates different insecticides and application methods.

CHLORPYRIFOS: PAST. In 1981 the application of chlorpyrifos in the apartments provided about 61% reduction of infestations, and resistance to this organophosphate was considered moderate. For the next five years chlorpyrifos was the primary insecticide used for cockroach control, and in 1984 it provided 63% reduction of infestations. From July 1986 to July 1990 the use of chlorpyrifos was discontinued and replaced with cypermethrin. However, from 1990 to 1993, chlorpyrifos was again made the primary insecticide in the apartments, and during this time it provided about 54% reduction of German cockroach infestations.

CHLORPYRIFOS: PRESENT. In 1994 the use of chlorpyrifos was integrated with other insecticides for cockroach control in the apartments. In 1995 it provided 61% reduction of the German cockroach infestations. Chlorpyrifos continues to be used in the cockroach control program in Lincoln Terrace, but now it is included as one of several control strategies; the others include the use of pyrethroids and baits. New formulations of this insecticide have increased its acceptance by the apartment residents.

CHLORPYRIFOS: FUTURE. The level of chlorpyrifos resistance and control performance has remained relatively stable for about the last 15 years, and this is not expected to change. Although this insecticide has been used for German cockroach control for about 20 years, there are few records of control failure resistance. For most insect pests, organophosphate resistance is characterized by low to moderate resistance levels. In the Lincoln Terrace population the level has changed little in the last 15 years, regardless of the use pattern (exclusive and long-term use or long-term absence).

CYPERMETHRIN: PAST. In 1981 cypermethrin was used in Lincoln Terrace apartments on an experimental basis, and at that time it provided about 90% reduction of infestations. In 1985 and 1986 this pyrethroid became available for commercial use. From July 1986 through July 1990 it was the only insecticide used to treat the apartments, and applications were made about twice yearly. In 1988 there was evidence that cypermethrin had lost some effectiveness; only 74% reduction was achieved. After three years' absence (during 1990 to 1993 no pyrethroids at all were used), cypermethrin was returned to Lincoln Terrace for cockroach control, and it provided about 76% reduction.

CYPERMETHRIN: PRESENT. In 1994 cypermethrin was returned to use in Lincoln Terrace apartments, along with the use of chlorpyrifos and other insecticides. At this time the level of resistance had been reduced, the flushing action returned, and there was renewed confidence in this pyrethroid. In 1994 and 1995 cypermethrin provided a range of 59% to 62% reduction of apartment infestations. Cypermethrin is now used in the cockroach control program in Lincoln Terrace, but like chlorpyrifos, it is one of several strategies. CYPERMETHRIN: FUTURE. There were some dire predictions when pyrethroids were introduced to household pest control:

• Widespread resistance would quickly develop.

• This entire class of insecticides would be "lost" for German cockroach control.

Clearly, these predictions have not come true. Cypermethrin and the other pyrethroids appear to be providing effective levels of cockroach control. The lessons learned in Lincoln Terrace are that German cockroach populations may develop resistance to pyrethroids, just as they have for nearly every other insecticide developed for household use. It seems that an early sign of pyrethroid resistance is the loss of flushing action; PCOs may consider watching for this. Populations with high levels of resistance may be rehabilitated by removing all pyrethroids for an extended period e.g. two to three years. There is no evidence that a rotation program (alternating the use of different insecticides) would have prevented the cypermethrin resistance in Lincoln Terrace.

CONCLUSIONS. Resistance has been (since 1951) and will continue to be an important characteristic of German cockroach populations around the world. This genetic powerhouse of a cockroach seems to have the capability of surviving whatever control strategy we devise. But the "super roach" the one we can't kill has not appeared. There may be more of them than us, but our brains are bigger and we are still winning.