Professor of Entomology at UC Davis, Frank Zalom, says spraying pyrethroids at this time is not recommended. Pyrethroids are a chemical compound very similar to the organic pyrethrin, but less toxic to mammals and more toxic to insects. Pyrethroids are a very common and effective pesticide, but Zalem says they are also very effective at killing off some harmless insects that actually feed on populations of problem pests.
In fact, Zalem says while hull split is a better time for pyrethroid use, he personally doesn’t recommend pyrethroids for orchard crops at any time.
So for growers now, who need to deal with navel orangeworm, Zalem has some recommendations.
He says if you don’t have heavy population pressure, you probably don’t need to treat in May. The more standard hull split treatment timing would suffice, leaving what he says is an acceptable amount of damage.
If you have heavier navel orangeworm pressure or peach twig borer also present in the orchard, then, he says, a May spray would be good in addition to the hull split spray.