Psyllid Lineup

Identifying the pests that carry zebra chip

Published in the June 2012 Issue Published online: Jun 10, 2012 Joseph E. Munyaneza, USDA-ARS Yakima Agricultural
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Potato Psyllid eggsThe potato psyllid, Bactericera cockerelli, is as an economically important insect pest of potatoes, tomatoes and other solanaceous crops in the western U.S., Mexico, Central America and New Zealand. This insect has historically been linked to psyllid yellows disease, but more recently has been shown to be the vector of a bacterium that causes zebra chip (ZC) disease of potato and severely damages other solanaceous crops. ZC is an emerging and economically important disease that has devastated potato growers throughout the psyllid's range, often leading to the abandonment of entire fields.

Potato psyllid adults are small, measuring about 0.1 inch (2.5 mm) long, and resemble tiny cicadas, largely because they hold their wings roof-like over their bodies. Body color ranges from pale green at emergence to gray or black within few days following emergence.

Potato Psyllid eggProminent white or yellow lines are found on the head and thorax, and dorsal whitish bands are located on the first and terminal abdominal segments. These white markings are distinguishing characteristics of the psyllid, particularly the broad, transverse white band on the first abdominal segment and the inverted V-shaped white mark on the last abdominal segment. Adults are active in contrast to the largely sedentary nymphal stages and readily jump when disturbed. Adult longevity ranges from 20 to 62 days, and females usually live two to three times longer than males. Females lay 300 to 500 eggs over their lifetime.

The dark yellow or orange eggs are deposited singly, principally on the lower surface of leaves near the leaf edge. Often, females will lay numerous eggs on a single leaf. The eggs are mounted on a short stalk and hatch within three to seven days. The yellowish-green nymphs are flat with a scale-like appearance. There are five nymphal growth stages, of similar appearance other than size. The compound eyes are reddish and prominent. A short fringe of wax filaments is present along the lateral margins of the body.

Potato Psyllid nymphPotato psyllid nymphs may easily be confused with the nymphs of whiteflies, although the former move when disturbed. Because nymphs prefer sheltered locations, they are mostly found on the lower surfaces of leaves and usually remain sedentary during their development. Nymphal development time depends on temperature and ranges from 12 to 24 days. Both nymphs and adults produce large quantities of whitish excrement.

The potato psyllid is native to North America and is common in the southwestern U.S. from western Texas to California. The psyllid also occurs in Mexico and Central America. Contrary to previous reports, the insect does occur in Washington and Oregon, where it usually colonizes potato fields in late June and early July. The origin of psyllids colonizing potatoes in the Pacific Northwest has not been established, but it is suspected that the insects migrate into the region from southern California.

Also, there is evidence that the potato psyllid is able to overwinter in the Pacific Northwest, especially on Solanum dulcamara (bittersweet nightshade) and local populations may contribute to psyllids colonizing potato fields in this important potato growing region of the U.S. The psyllid was accidentally introduced into New Zealand, apparently sometime in the early 2000s.

Potato Psyllid adultThe potato psyllid seems to be adapted for warm but not hot temperatures. Optimum psyllid development occurs at about 80 degrees F. Oviposition, hatching and survival are reduced at 90 degrees and cease at 95 degrees F. A single generation may be completed in three to five weeks.

In North America, driven primarily by wind and hot temperatures in late spring, the psyllid annually migrates from its overwintering and breeding areas in southern and western U.S. and northern Mexico into northerly regions of the U.S. and southern Canada. Besides solanaceous species, the potato psyllid has been shown to reproduce and develop on some Convolvulus species, including field bindweed and sweet potato.

The potato psyllid is one of the most destructive potato pests in the western hemisphere. Historically, damage to potato crops is thought to have been due to its association with "psyllid yellows" disease, presumably caused by a toxin that is transmitted during the insect's feeding activities, especially by nymphs. The nature of this toxin has not yet been demonstrated. Above-ground plant symptoms of psyllid yellows include arrested growth, erectness of new foliage, chlorosis and purpling of new foliage with basal cupping of leaves, upward rolling of leaves throughout the plant, shortened and thickened terminal internodes resulting in rosetting, enlarged nodes, axillary branches and aerial tubers. Below-ground symptoms include setting of excessive numbers of tiny misshaped potato tubers, production of chain tubers and early breaking of dormancy of tubers.

Tuber infected with ZCIn recent years, potato, tomato and pepper growers in a number of geographic areas have suffered extensive economic losses associated with potato psyllid outbreaks. Damage is due to a previously undescribed species of a bacterium named "Candidatus Liberibacter solanacearum" (also known as "Ca. L. psyllaurous"), vectored by potato psyllids.

Psyllids acquire and spread the pathogen by feeding on infected plants. The bacterium is also transmitted transovarially in the psyllid to the offspring, which contributes to the spread of the disease between geographic regions by dispersing psyllids, and also helps maintain the bacterium in geographic regions during the insect's overwintering period.

Foliar symptoms associated with liberibacter in potato closely resemble those caused by psyllid yellows and purple top disease (caused by leafhopper-transmitted phytoplasmas). However, tubers from liberibacter-infected plants develop a defect referred to as "zebra chip." Characteristic symptoms of ZC in potato tubers consist of collapsed stolons, browning of vascular tissue concomitant with necrotic flecking and streaking of internal tissues. Upon frying, these symptoms become more pronounced and chips or fries processed from affected tubers show very dark blotches, stripes or streaks, rendering them commercially unacceptable. ZC was first documented in 1994 near Saltillo, Mexico, then in southern Texas in 2000. Infected fields have since been documented in several other southwestern and central states of U.S. The disease was documented for the first time in Oregon, Washington and Idaho late season in 2011. ZC is also found in Central America and New Zealand.

Fried Zebra Chip ChipsMonitoring the potato psyllid is crucial to effective management of this insect pest. The adult populations are commonly sampled using yellow sticky cards, pan traps, sweep nets or vacuum devices, while egg and nymph sampling requires visual examination of foliage. Typically, psyllid populations are highest initially along field edges, but if not controlled, the insects will eventually spread throughout the crop.

Potato psyllid control is currently dominated by insecticide applications (see control options at http://potatoes.com/Research.cfm) but even with conventional insecticides, this pest is difficult to manage. Good coverage is important because psyllids are commonly found on the undersides of leaves. Also, the different life stages require the use of specific insecticides, as it has been shown that chemicals that control adults do not necessarily control nymphs or eggs.

Therefore, caution is needed when selecting insecticides, to ensure that a product effective against the targeted life stage is used. Psyllids have been shown to develop insecticide resistance, thus alternative strategies should be considered as much as possible. No potato variety has so far been shown to exhibit economically sufficient resistance or tolerance to ZC. Several natural enemies of the psyllid are known but little is known on their effectiveness against the insect or preventing ZC.

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