To kill the red flour beetles that are 119 times more resistant than “normal” insects, it takes fumigating with a concentration of 377 parts per million (ppm) of phosphine gas for 72 hours to kill 99% of the resistant insects. This concentration is equivalent to 15 tablets of aluminum phosphide/1,000 cubic feet. However, at least 30 tablets/1,000 cubic feet; or 40 tablets/1,000 bushels need to be used to kill red flour beetles with this level of resistance because aluminum phosphide labeling recommends these minimum numbers of tablets be used in vertical storages.
In the case of lesser grain borers, which were 254, 910 and 1,519 times resistant in three different collections, concentrations of phosphine estimated to kill 99% of these individuals were 573, 2,054 and 3,431 ppm, respectively. These concentrations are equivalent to 23, 83 and 138 tablets/1,000 cubic feet; respectively. Again, at least 30 tablets/1,000 cubic feet or 40 tablets/1,000 bushels need to be used. To maintain the concentrations of phosphine gas mentioned here for three days under field conditions would be difficult due to gas loss from the storage structure, gas sorption to grain and incomplete reaction of the tablets.
Because a larger number of tablets are going to be required to kill resistant insects, it is likely that the current highest label dose rate for phosphine tablets is no longer effective for the highly resistant populations. Resistance to phosphine discussed here applies to cylinder-based phosphine products as well.
Phosphine gas is an important tool for the management of stored grain pests and many other pests of durable stored agricultural products, and the occurrence of phosphine resistance in pest populations presents challenges to the continued effective use of this fumigant. The presence of highly phosphine-resistant populations of lesser grain borer and red flour beetle in Oklahoma could be an indication of the same in other parts of the United States and North America and an indication that it could develop in areas where resistance may currently not exist but phosphine is commonly used.
What is now required is a survey of key stored-grain insect pest species across the U.S. to determine the presence and extent of phosphine resistance. In addition, a national resistance monitoring program for the U.S. needs to be developed. Resistance monitoring is crucial for a number of reasons, including providing an early warning of resistance, feedback on the success of resistance management activities, diagnosis of phosphine control failures and obtaining information on the likely impact of new phosphine resistance. Resistance monitoring is an important part of keeping the proportion of susceptible individuals in a population as large as possible to ensure continued effective use of phosphine.
Also needed is a phosphine resistance management strategy, such as a rotation plan with other active ingredients or products that may allow for applying phosphine as infrequently as possible to delay the development of resistance where it currently does not exist and to destroy resistant stored-product insect populations where they exist. Research will soon be under way to determine if alternative fumigant gases (e.g., sulfuryl fluoride) and residual applied long-acting insecticides (a mixture of chlorpyrifos-methyl [21.6%] and deltamethrin [3.7%]) can be used to kill phosphine-resistant grain pest populations so the use of phosphine can be preserved for the future.
Dr. George P. Opit is an Assistant Professor of Entomology at Oklahoma State University. His research includes the study of integrated pest management and biological control of stored-product arthropod pests, including psocids associated with grain storage, handling, and processing facilities and product warehouses in the U.S. Dr. Thomas W. Phillips is a Professor of Entomology at Kansas State University. He specializes in biology, the management of pests attacking stored food products, and chemical ecology.