Phosphine is a commodity fumigant used worldwide since sometime after World War II. It's primarily used to kill pests in food or other commodities like tobacco, animal feed, lumber, etc.
Fumigants are not sprays as many believe, they are gasses. Similar to oxygen in the air we breathe, fumigants can penetrate, and travel places a “spray” cannot.
Essentially, phosphine gas is applied into a special enclosure designed to hold the gas in. The commodity inside the enclosure is exposed to a certain concentration for a certain amount of time to kill the target pest.
The gas is then released back into the environment where it breaks down into naturally occurring compounds.
During application if the gas is not held at a minimum concentration or for not enough time, some insect survival will occur. The “strongest” of the population will survive and pass on their genetic traits to their young. Since Phosphine has been used on many commodities all over the world for decades, some populations of pests in some geographic areas have become resistant or tolerant to Phosphine.
Many interperet this news to mean that Phosphine no longer works. This is not true. While certain populations are more tolerant than they used to be, they are still controlled when proper fumigation practices are used. Plus, some populations are not resistant at all. There are some specific populations which have become extremely resistant to Phosphine or maybe they never really worked well on them at all. When dealing with these populations, a different fumigant should be used such as Sulfuryl Fluoride (SF). SF can also be used to reverse moderately resistant populations.
At the end of the day, this should remind us that we need to do a good job when applying pesticides for this and many other reasons.