Swarms of whitefly have been bugging folks in the northern parts of Oregon’s Willamette Valley recently, their presence strong enough to rate monitoring. Two new species were detected in 2014 – ash whitefly and cabbage whitefly – with ash whitefly suspected to be the more prolific pest.
According to Robin Rosetta, associate professor and Extension entomologist in the Oregon State University Department of Horticulture, two long and hot summers have allowed the whitefly population to increase and “become more evident as they swarm. As these occurrences have been late in summer/fall season, it is not known yet whether this whitefly will reach damaging pest status on their deciduous host plants. References suggest they may be moving onto evergreen hosts on which to overwinter.”
Ash whitefly (Siphoninus phillyreae) has a wide host range, including many ornamentals, native plants and fruit trees. It was first detected in the US in California (1988), and has since spread to Arizona, Florida, Georgia, Nevada, New Mexico, North Carolina and South Carolina – and now Oregon. Early detection and monitoring in these states have helped to keep the pest in check.
Adults have light yellow bodies covered by white wings; eggs are a pale, waxy yellow, and often are surrounded by waxy deposits. Nymphs are nearly translucent, but become more opaque and covered in tufts of white wax. Puparia are covered with tufts of white wax, and have distinct tubercules that form a sort of halo around the edge of their bodies; each of these tubes is capped with a clear wax droplet.
Adult females live for about 30 to 60 days; they lay eggs on the undersides of host plants’ leaves. Nymphs emerge from the eggs and feed on plant sap, eventually pupating and later emerging as winged adults. Both the nymph and adult stages can feed and, being polyphagous, they’ll feed on a wide variety of plants.
Ash whitefly can develop continuously throughout the year, with several generations possible per year. Development may be slowed in cooler temperatures. Preferred summer hosts, such as ash, pear, hawthorn and redbud, may be abandoned for evergreen hosts, where all stages of the whitefly can overwinter.
Heavy infestations can cause leaf wilt, curling tips, early leaf drop and stunted fruit growth; deposits of honeydew and sooty mold may also appear. Even overwintering stages may be harmful: According to Rosetta, “While the overwintering adult stage may not directly damage a plant, it might be a shipment contaminant pest and intervention may be required if plant shipments occur while the whitefly is present.”
To monitor for whitefly, shake or tap branches over which the whitefly is swarming, or slowly turn over a branch or leaf and check for both adult and juvenile stages.
Several biocontrols have proved to be reasonably successful in other states. Releases of the parasitic wasp Encarsia inaron and Clitostethus arcuatus, a lady beetle, have been effective.
“The most successful agent limiting the populations of ash whitefly in California was the tiny wasp, E. inaron, which was found in emergence rates from ash whitefly nymphs at 80 to 98 percent,” Rosetta states. According to the University of California Statewide Integrated Pest Management Program, “ash whitefly is now under good biological control, so it is rarely seen in high numbers.”
Cover and photos: Robin Rosetta, Oregon State University