Inconsistent results from miticide applications are creating a fair amount of speculation about resistance.
“In the majority of cases, most miticide treatments seem to be doing well,” says David Haviland, Kern County UC Cooperative Extension farm advisor. However, there are fields where PCAs felt the need to spray back in late April to very early May. Many of those fields already have mites back in them (after earlier treatments) and a lot of coffee shop talk is going around about product failures and resistance.”
That may be a rush to judgment, according to Haviland.
“In most cases, my guess is that the real culprit is a combination of cool weather and dusty leaves,” he says. “When the weather is cool, mites don't feed as actively so they will get less of a dose of products like those containing abamectin. They also don't cycle through their life cycle as quickly, so that means there will be delayed effects of using mite growth regulators.”
“When you add in that fact that most leaves have been really dusty this spring due to a lack of rain, and many of those applications went on at about 100 gallons per acre, the cumulative effect is that miticides applied in late April to early May didn't work like one might expect.”