Spring insecticide treatments control adelgids on Douglas fir

Insecticide applications made to Douglas fir Christmas trees vary in their ability to suppress Cooley spruce gall adelgid, a sucking insect that causes needle distortion and discoloration. A field experiment in Napa County showed that organophosphate systemics are ineffective at controlling adelgids, but imidacloprid, a newly registered systemic insecticide, is highly effective. Horticultural oil is effective, but causes slight needle yellowing. This side effect can be mitigated by precisely timing the oil application before bud break when the overwintering adelgids are transforming to adults but before they lay eggs.