April 2012
Science of psyllid biology By Alan Schreiber, Agriculture Development Group, Inc., Andy Jensen, Regional Research Director, for the Idaho, Oregon and Washington Potato Commissions and Silvia Rondon, Oregon State University

Zebra chip documented in new spots in 2011

Little is known about the biology of potato psyllids in the Pacific Northwest. Even less is known about its management in potatoes grown in this region.

The following article is based on information on potato psyllids from other states, limited research and observations on potato psyllids and zebra chip in the Pacific Northwest and our general knowledge of the pest, potato pest management and insecticides registered on potatoes. This document has been reviewed by more than a dozen entomologists and researchers working on potato psyllid and it is our best attempt at providing growers and potato pest management decision makers with information regarding management of this pest.

The potato psyllid is a phloem-feeding insect that has an extensive host range of at least 20 plant families, but reproducing mostly on the potato and nightshade family (Solanaceae). This insect has been very costly to cultivated solanaceous crops in the United States, Mexico, Central America, and more recently in New Zealand.

In recent years, a new potato tuber disease, zebra chip (ZC), has caused millions of dollars of losses to the potato industry in the southwestern United States, particularly Texas. However, ZC was for the first time documented in Idaho and the Columbia Basin of Washington and Oregon late in the 2011 growing season. This disease is characterized by development of a dark striped pattern of necrosis in tubers. The pathogen associated with ZC is the bacterium Candidatus Liberibacter solanacearum that is vectored by potato psyllid.

Potato psyllids pass through three life stages: egg, nymph and adult. All life stages are difficult to detect. The adults look like small cicadas, about 0.08 inch (2 mm) long. They are closely related to aphids and leafhoppers and have clear wings that rest roof-like over the body. Although predominantly black, the potato psyllid has white markings. The first abdominal segment shows a broad white band, the last segment has an inverted white “V”. Psyllids jump quite readily when disturbed.

The football-shaped eggs (Fig. 2B) are extremely small, slightly larger than leaf hairs, and on a short stalk. They are usually on the underside and along the edges of leaves and are usually laid in the upper plant canopy. A 10X hand lens is required to see them.

The insect (adult or nymph) aquires the bacterium when feeding on an infected plant. Once infected the insect is always a carrier of the bacterium. Eggs laid by an adult with the bacterium are also infected. The disease usually takes about 3 weeks from infection to produce symptoms in the foliage and tubers.

Psyllid nymphs look like immature soft scale or whiteflies. Unlike whiteflies, when disturbed, they move readily. They are flat and green with a fringe of short spines around the edge. Immature psyllids go through five stages in as little as 13 days in warm temperatures.

Potato psyllid can damage a plant even if it is does not carry the pathological bacterium since these insects feed directly on the plant and may weaken it. As they feed, psyllids inject toxins with their saliva that can cause leaf yellowing or purpling, smaller and fewer tubers, and misshapen tubers. This physiological condition has been dubbed “psyllid yellows” disease and is generally less damaging than ZC.

First identified in northeastern Mexico in 1994 and south Texas in 2000, ZC has now been reported from California, Idaho, Kansas, Nebraska, New Mexico, Oregon, Wyoming and Washington. Also, the disease was recently reported from New Zealand. Plants affected by ZC exhibit a range of symptoms that are similar to potato purple top and psyllid yellows, including stunting, chlorosis, leaf scorching, swollen internodes near apical portions, axillary bud and aerial tuber proliferation, necrosis of vascular system, and early death. The name “zebra chip” refers to the characteristic brown discoloration of the vascular ring and medullary ray tissues within the tubers that is enhanced when tubers are sliced and fried into chips or fries.

Potato psyllids will feed on and transmit the disease-causing organism to all varieties of potatoes. While there are differing susceptibilities across potato varieties, virtually all varieties will express symptoms of ZC.

The bacterium affects the phloem tissue, causing the foliar symptoms described above and higher than normal sugar concentrations in tubers. When cooked, the sugar caramelizes and forms dark brown stripes. The defect is
harmless to consumers, although taste is different, but renders the tubers unmarketable. This disease that is not restricted to potato chips. In addition, the ZC organism can significantly reduce yields and tuber size.

Potato psyllid has long been known to be present throughout the Pacific Northwest. It has been thought to not overwinter in the PNW, but instead migrate into the area on air currents from the south. There is a preliminary study by Chapman, Strube and Bextine that shows that the psyllids in the PNW are genetically related to those from California.

The historical lack of ZC in PNW potatoes, even though the insect has long since been present here, is probably due to the insects in previous years not carrying the pathogen.

It is possible that psyllids have carried the disease in previous years in the PNW, but disease symptoms were attributed to other causes such as viral infections. The insect is known to overwinter from California to southern Texas and northern Mexico. Research is underway to confirm whether potato psyllid overwinters in the PNW. Andy Jensen has found potato psyllid adults overwintering in the Boise, Idaho area up until the time of this writing (February, 2012). They were found in Solanum dulcamara L. (a.k.a. bittersweet nightshade.) This is a perennial weed native to Eurasia. It can be found in fence rows, pond margins, low woods and roadsides throughout the Pacific Northwest.

Psyllids are typically first detected in PNW potatoes in July, but the timing of the ZC outbreak in 2011 suggests that it first colonized the earliest affected fields in mid June.

Sampling. Use yellow sticky cards to detect first occurrence of population. Place yellow sticky cards such as those used for beet leafhopper in the field, near the edge. We do not have a recommended number of cards per field, but the more cards per field the more likely one will detect psyllids. Spatial and temporal studies indicated that both psyllid abundance and ZC incidence progress over time faster on the edges than in the infields early in the season. In California, 70% of the potato psyllids can be found on the field edges. As the season progresses psyllids become more evenly distributed throughout the field. Placement of cards outside of fields, such as is recommended for beet leafhopper, is not effective for monitoring potato psyllids.

A second sampling method is to collect ten leaves from ten locations from the first ten outer rows of the field. Collect full sized leaves from the middle of the plant. A hand lens is required to see psyllid eggs and nymphs. Count the number of adults and nymphs, and if you have very good eyesight, eggs. The nymphs reside on the underside of the leaf. Eggs are most commonly present on the leaf’s edge. Be aware that adult potato psyllids are active and will fly or jump away when disturbed.

Use of yellow sticky cards in the perimeter of potato fields is expected to be useful for early detection of psyllids. Leaf sampling is useful for monitoring eggs and nymphs. Texas A & M University recommends a mixture of using sticky cards and leaf sampling.

A third method, again originating out of Texas, is use of a sweep net. Take 100 sweeps from around the field perimeter. This method will only collect adults.
Action Threshold. No action threshold exists for psyllids in potato. We suggest that detection of potato psyllids at any level, in any life stage, is the threshold for action until we learn more about how ZC will impact PNW potatoes.

Non-Chemical Control.
There are no effective non-chemical control tactics for potato psyllids, although research is underway in this area.

Chemical Control.
Fortunately there are a number of insecticides registered on potatoes that have activity against potato psyllids in the adult and/or immature stages.Some insecticides with activity against adults and nymphs will also have activity against the eggs. Season long weekly applications are used in areas (e.g. Texas) where ZC has been problematic. We are not expecting to follow the same lengthy period of control since psyllids are not thought to be in potato fields in the early part of the season.

Resistance Management.
If a neonicotinoid is applied at planting do not apply another Group 4a product later in the season. Do not apply a single mode of action more than twice in a row against potato psyllids or any other insect. For example, do not apply a pyrethroid insecticide more than twice in a row. Do not apply Agri-Mek, Monitor, Provado, Actara or any other product more than twice in a row. For example, do not apply Movento twice and then apply Oberon.

Potato psyllid has developed resistance to insecticides. For example, growers in Texas stopped using Monitor because it lost efficacy after a few years of heavy use. In the PNW, the first life stage detected is usually the adult. Some insecticides have activity against adults, while others do not. In psyllid control programs in other states, when adults are detected, it is recommended that a product with activity against adults be used first. Also, growers may also have other insect pests to control at the same time as psyllids.

For a detailed table on insecticide treatments, please consult the April issue of Spudman.

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