Thrips gather in large numbers on onion leaves, causing wrinkled or ribbony shoots. White or gray spotting, called silver top, may eventually cover the entire leaf, giving it a sickly appearance. Thrips are a primary pest of onions, reducing photosynthesis and yield by cutting open leaf cells to suck up their sap. Emptied of chlorophyll, the dead cells leave bleached silvery streaks. If the onion plant lives through an early infestation, the bulbs may be distorted and undersized. Thrips are too small to be readily seen by the naked eye, so the silver top appearance is the best indicator of an infestation.
Spray with insecticidal soap or pyrethrins from the time seedlings are set out. Thrips are also susceptible to horticultural oil, which works both by suffocating and by repelling thrips. We recommend Plant Guardian Houseplant Insecticidal Soap, Pyola Insect Spray and Oil-Away Supreme Insecticidal Spray.