The lesser grain borer and the red flour beetle are two of the major pests in stored grain in the United States, and are often combated with phosphine fumigation. These fumigations are starting to fail, raising questions about their effectiveness. Resistance in stored-product insect pests has become a major problem in many countries, with very high levels of resistance found in some parts of Asia and Africa and, more recently, in Australia and South America. Drs. George Opit, Thomas Phillips, Mahbub Hasan and Mr. Michael Aikins have conducted studies to see whether this is true in the United States. Their results were recently published in Journal of Economic Entomology, under the title “Phosphine Resistance in Tribolium castaneum and Rhyzopertha dominica from stored wheat in Oklahoma.”
In 2009, reports of phosphine fumigation failures in the United States along with the concern caused by high levels of phosphine resistance in many countries motivated a reinvestigation into the level of phosphine resistance in the red flour beetle and lesser grain borer from six counties in Oklahoma. These field insects were tested for phosphine resistance. Nine populations of red flour beetle and five populations of lesser grain borer collected from silos, concrete bins and steel bins were tested. Insects were tested for resistance using an internationally accepted method called FAO Method No. 16. Phosphine gas concentrations in fumigation containers were verified by using a gas chromatographic-flame photometric detector.
Before this most recent survey, The United Nations Food and Agriculture Organization’s Bruce R. Champ and C. E. Dyte conducted a “Global Survey of Pesticide Susceptibility of Stored Grain Pests” in 1972 through 1973. They found that about 10% of stored-product insect populations sampled in different countries contained phosphine-resistant individuals.
Another survey, similar to the one conducted in 2009, titled “Pesticide Resistance in Tribolium castaneum (Coleoptera: Tenebrionidae) and Rhyzopertha dominica (Coleoptera: Bostrichidae)” was conducted by Larry Zettler and Gerrit Cuperus in 1990. It found that 1 out of 8 red flour beetle populations and 8 out of 12 lesser grain borer populations collected from 10 Oklahoma counties had phosphine resistance. The report cited phosphine control failures in grain storage structures and suggested that these could have been due either to phosphine resistance or to inefficient fumigation. Even with these findings, use of phosphine in Oklahoma had continued with no effort to document whether the resistance to this fumigant has increased since 1990.
The results of the 2009 survey showed that 89% (8 out of 9) of the red flour beetle populations and 100% (5 out of 5) of the lesser grain borer populations were resistant to phosphine. In fact, in one red flour beetle population and three lesser grain borer populations, at least 80% of individuals fumigated in a test to detect resistance survived the fumigation.
When a field population is 100 times or more resistant than the susceptible population, this is called “strong” phosphine resistance. When dose-response tests were conducted to determine the level of phosphine resistance in these four populations, the red flour beetle population was 119 times more resistant to phosphine. Three lesser grain borer populations, were 254, 910 and 1,519 times more resistant! It is likely that resistant populations of stored-product pest species, like these in Oklahoma, occur in other parts of the U.S.
New fumigation levels
Highly resistant populations mean that the amount of aluminum phosphide tablets needed in order to properly fumigate stored grain has risen exponentially. The label rate for aluminum phosphide tablets used for space or bulk fumigations in vertical storages such as concrete silos and steel bins is 30 to 140 tablets/1,000 cubic feet; or 40 to 180 tablets/1,000 bushels of grain, respectively.