Psyllids are small insects that suck plant juices. The adults resemble miniature cicadas and are related to cicadas, leafhoppers and aphids. Several psyllid species are pests of crops such as citrus, olive, pear, potato and tomato.
Description of Zebra Chip
The bacterium that causes zebra chip (ZC) produce necrotic flecking in the flesh of the tuber. The symptoms are similar to net necrosis (caused by Potato leafroll virus), but the symptoms often extend through the length of the tuber. When affected tubers are fried, the disease causes severe darkening of the chip or fry. The tuber defect is severe enough that the disease is a concern for both fresh and process potatoes.
Controlling infected potato psyllids is the primary way to control zebra chip. Non-infected psyllids will not cause zebra chip.
Zebra Chip in Idaho
During 2011, zebra chip (ZC), a potato disease that was new to Idaho and the Pacific Northwest, was found in tuber samples from the Magic and Treasure Valleys. This disease is caused by a bacterium (“Candidatus Liberibacter solanacearum”; Lso) that is transmitted by potato psyllids.
During 2012, a monitoring program was initiated in order to clarify the extent and severity of the threat of ZC in Idaho. We have been sampling potato psyllids annually in commercial potato fields across the state and testing these psyllids for the presence of Lso. Patterns of the distribution and abundance exhibited by potato psyllids generally reflect the temperature and elevation gradient across the state. We typically record higher numbers of potato psyllids in the Treasure Valley (western Idaho) sites relative to Magic Valley (south-central Idaho) sites, and captures of potato psyllids in the Upper Snake region (eastern Idaho) generally are rare.
Incidences of Lso and ZC have not exhibited similarly clear patterns in their geographical distributions. ZC incidence has varied from year to year, but generally seems to reflect the abundance of both potato psyllids and Lso each year. ZC appears to be prevalent only when both high numbers of potato psyllids and Lso are recorded in our monitoring program. 2012 remains a standout year in this regard.
The University of Idaho and Miller Research have been developing a scouting and information transfer program for the Idaho potato industry. Below is a summary of the program and recommendations.
- University of Idaho in collaboration with Miller Research and numerous crop consultants across the state has been conducting a potato psyllid monitoring program funded by the Idaho Potato Commission, ISDA Specialty Crop Block Grants and USDA-TASC grants.
- Potato psyllids are sampled weekly using a combination of methods, but focusing on the use of yellow sticky traps. Adult psyllids are tested for the presence of Lso by PCR.
Notification regarding the distribution and abundance of potato psyllids and incidence of Lso and ZC occurs at least weekly during the growing season via multiple formats, including the following:
Various scouting techniques are used to survey Idaho fields. If you are considering your own potato psyllid scouting program, here are some recommendations.
Ideally, scouting should begin no later than mid-May. The date of initiation may need to be adjusted based upon timing of crop emergence.
No one scouting method is superior to the others, but some methods will be better for certain life stages. In all cases, scouting for potato psyllids also will provide information on populations of other potato insects such as green peach aphids and potato aphids.
You can either evaluate your samples for the presence of potato psyllids yourself or, initially, you may bring them into your local University of Idaho research and extension center to confirm identifications. After submission, subsequent training will occur to allow you to identify potato psyllids. Potato psyllid samples submitted to the University of Idaho can be analyzed for the presence of the zebra chip pathogen. Photographs and key diagnostic characters of the different life stages of potato psyllids can be found at various websites listed above or ordered at http://www.nwpotatoresearch.com.
Yellow sticky cards. This method will detect only adult potato psyllids.
- These cards must be placed within the field but towards the field edge to concentrate on the detection of potato psyllids and reduce the number of other harmless psyllid species on cards.
- Sticky cards need to be checked and replaced at least weekly since other insects and debris will accumulate on cards which make finding psyllids on the cards difficult.
- The cards can be easily attached to a wooden lath stake with a binder clip. As cards are replaced, remember to move the card upwards on the lath as the canopy grows, ensuring that the card is located just above the top of the canopy.
- Before transporting, individual cards should be handled in one of the following ways: placed in a thin, clear plastic bag; separated with a wooden rack; or covered with the paper covering that comes with the card. If using the paper covering that comes with the card, it is imperative that only the waxy side of the paper be re-adhered to the card face. If using a plastic bag, the sticky face of the card should be carefully adhered to the inside face of the bag to allow observation of insects through the plastic. Ensure the card or plastic bag is labeled.
- Stack sticky traps neatly with nonstick paper between them.
- There are several sources or vendors for yellow sticky traps. Choose a vendor by price and service.
Leaf sampling – for eggs, nymphs and adults.
- We recommended that you collect 10 leaves from 10 locations within 30 feet of the edge of the field.
- The best approach would be to sample the expanded leaves towards the middle of the plant. This technique will allow sampling for all life stages including adults, nymphs and eggs. However, adult potato psyllids are active and often jump abruptly when disturbed and may not be observed.
- A hand lens or magnifying glass is necessary since the insects are quite small.
Sweep nets – for adults.
- The recommendation is to take 100 sweeps in the field from various areas and focusing on field edges. Collect all insects from the sweep net into a plastic bag, jar or vial and freeze overnight. Freezing the sample immediately decreases the potential for sample degradation and loss from predators eating the potato psyllids.
Information/mapping of potato psyllid and zebra chip throughout Pacific Northwest. We will update and post information on potato psyllid detection throughout Washington, Oregon and Idaho to allow for tracking of the presence and movement of potato psyllids. Information will be reported by county only and all information will be kept confidential. Both the presence of potato psyllids and whether they are infected with the bacterium that causes zebra chip will be reported.
See the following link for a complete list of insecticides registered in potatoes: http://insect.pnwhandbooks.org.
Recommendations for high-risk areas and/or highly risk-averse growers:
- We recommend an at-planting application of a neonicotinoid insecticide (seed treatment, in-furrow or at-hilling).
- Follow up with a foliar insecticide application to target adult and nymph potato psyllids once the at-planting material nears the end of its efficacy period. Refer to the “Potato Psyllid information and management guidelines” report published on the Northwest Potato Research site http://www.nwpotatoresearch.com for foliar insecticide recommendations. Note that some insecticides target adults, nymphs, eggs or all life stages. Select an insecticide that is most effective for the psyllid life stage requiring control measures as well as other insects that may be present at the time (e.g., Colorado potato beetles, aphids, mites, etc.).
- Avoid foliar neonicotinoid insecticide applications if this class of insecticide was used at planting or at hilling. This is important to avoid insecticide resistance. At-plant neonicotinoids have a long period of control, exposing susceptible insects for several weeks. This period and intensity of exposure may be adequate to foster insecticide resistance in any one (or more) of the pests in potatoes. Adding more foliar applications with the same class of insecticide will only increase this selection for resistance. It is important to remember that Colorado potato beetles, aphids and other insect pests are also being affected by sprays targeting psyllids. Relying too heavily on any one mode of action will select for resistance not only in potato psyllids, but in all of these pests. Avoiding insecticide resistance in these pests is a critical part of psyllid management decisions.
- Avoid pyrethroid insecticides for potato psyllid control. Pyrethroids may flare potato psyllid populations by enhancing egg laying by females and/or killing beneficial insects that attack potato psyllids. Pyrethroids also may flare aphid and mite populations by releasing them from control by natural enemies.
Recommendations for areas in Idaho that might be at lower risk for ZC:
- Follow traditional insecticide program previously used, but consider an at-planting or at-hilling neonicotinoid insecticide if it is not a part of your current insecticide program.
- Keep current on survey results in areas with positive detection of infected and non-infected potato psyllids.
- If at risk, follow a foliar program as described above and as outlined in the “Biology and Management of Potato Psyllid in Pacific Northwest Potatoes” report published in the Potato Progress.