Bechinski, Edward John

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71£ Section III. Root-feeding maggots, soil arthropods and other problemsHOW TO INTERPRET WIREWORM COUNTS FROM BAIT STATIONS IN POTATOESE.J. BechinskiDepartment of Plant, Soil &Entomological SciencesUniversity of IdahoMoscow, Idaho 83844208/885-5972Development of recommendations:Iderived guidelines for using bait stations to determine if insecticides are needed for wirewormcontrol in potatoes. Ibegan by using the data of Gibson (1939) to compute statistical modelsthat predict wireworm damage as a function of larval density. Gibson monitored damage topotatoes at Yakima caused by the sugarbeet wireworm, Limonius californicus, and the PacificCoast wireworm, Limonius canus. These also are the most important species in Idaho potatofields. My best-fit model wasequa. 1 p =1- exp (-1.65784 x 154S33)n = 6, P = 0.0001, r2 = 0.992where p isthe predicted proportion ofwireworm-damaged tubers at harvest and x is the meannumber ofwireworm larvae per 1-ft2 X 18-inch deep soil sample prior to planting. In Idaho, 3%damaged tubers is a commonly accepted upper limit for wireworm injury. Equation 1 predictsthat tuber damage will exceed 3% if wireworm density atthe beginning ofthe season is greaterthan 0.076 larvae per 1-ft2 (1 larva per 13.2-ft2). Onsager (1969) earlier had derived a linearmodel from Gibson's data, but his model is not as biologically realistic because it predictswireworm damage even when no larvae are present. Damage predictions from Equation 1aremost accurate for wireworm densities < 0.5 larvae per 1-ft2 soil sample. Beyond that range, themodel is extremely conservative; it overestimates tuber damage. The practical consequencesof that limitation are nil because 0.5 larvae per 1-ft2 is well in excess of the economicthreshold.Iconverted economic thresholds from larvae per 1-ft2 soil sample to number of larvae from baitstations by analyzing the data ofWilliams (1993). He worked with a third species, Limoniusinfuscatus, within a dryland pea and wheat cropping system during 1985 -1989 atWalla WallaCounty, Washington, and Umatilla County, Oregon. His data set consisted of wirewormdensities from paired soil core and bait samples. Baits were 1:1 wheat and corn seed. Thesewere buried 6-inches deep during early March and were covered with ablack polyethylene sheet.Wireworms were recovered by screening 3 to 5 weeks later.Correlation analyses generally failed to detect statistically significant relationships betweennumbersof wireworms from soil core samples and bait stations. However, by reorganizing thedata into frequency classes (percentage of bait stations with 0, 1, 2, 3, and 4 or morewireworms) and then plotting these values against absolute density from soil cores, it waspossible to assign a risk ofeconomic damage to each infestation class. The result is Table 1,HOW TO INTERPRET WIREWORM COUNTS FROM BAIT STATIONS. As previously, Table 1defines economic damage as 3%wireworm-damaged tubers at harvest.72TABLE 1. HOW TO INTERPRET WIREWORM COUNTS FROM BAIT STATIONSAverage no.wirewormsperbait stationRiskofeconomicdamageIPMrecommendation0 wireworms low(less than 1 chancecontrol not neededin 10) orverify infestation level via soil samplingup to 0.5 moderate(1 chance in 3)up to 1.0 (less than 50:50) sample soil and usedecision cardup to 2.0 probable(more than 50:50) >up to 4.0 high(75 to 90% chance)apply insecticide at plantingmore than 4 extreme do not plant potatoesIf baits do not detect wireworms, Table 1 suggests that insecticide application probably is notneeded. At the other extreme, more than 2 wireworms per bait station indicates a high chancethat damage will exceed 3%. If the average count falls between these limits, Vi-ffsoil samplesshould be taken to more precisely determine population levels. Because soil sampling is tediousand time-consuming, I also developed a sequential decision card (Figure 1) that minimizes thenumber of cores required to make a control decision.Practical recommendations for baiting (excerpted from University of Idaho Current InformationSeries "IPM Guide to Wireworms in Potatoes" [in press, due March 1994]):Baiting involves placing food lures in the soil. The carbon dioxide gas given off by sprouting orfermenting baits attracts wireworms from the surrounding soil. One effective bait isa 1:1 mixtureof whole wheat and corn. Coarse chopped carrots, potatoes, oatmeal, or wheat flour also work.Pre-soaking whole grain baits in water for one day increases their attractiveness. Wrap bran andflour baits in a nylon stocking so you more easily can inspect them later.Bury a fist-size portion of bait 4 to 6 inches deep and cover it with soil. Mound soil 4 to 5-inches over the top in the shape of a dome so rainwater runs off; this keeps the bait from rottingand makes it easier to examine. Bait either during the fall before planting or in the spring.Regardless, soils must be moist and temperature at 6-inches must be at least 45° F. Baits donot work well under cool or dry soil conditions because wireworms stop moving under those73conditions. When baiting during cool springs, cover each bait with a3-foot square piece ofclearplastic. This "solar heater" warms the soil and increases wireworm activity, especially earlyduring the season.Put out 25 or more baits for each 30 acres of field size. The more bait stations used, the betterthe chances of detecting wireworm infestations. Flag the location of each bait station.Randomly spread-out baits across the entire field. Consider how previous field history mightaffect wireworm levels. For example, if part of the field had been a pasture, or if grassy weedspreviously were a problem, bait those areas separately from the rest of the field. Wirewormsmay be a problem only in those areas.Dig up baits and the surrounding soil after 5to 10 days and examine for wireworm larvae. Tearthem apart on a tarp or sieve them through 1/4-inch hardware cloth. Calculate the averagenumber of wireworms per bait station.TABLE 1 recommends control if expected tuber damage at harvest is 3% or more. This is anarbitrary level. When planning wireworm control, consider tolerances for defects set in yourcontract or by the planned end-use. Allow for additional types of external defects such asmechanical damage, growth cracks or rhizoctonia.There are limitations to using bait stations. Cool soil, dry soil, wet soil and soils high in organicmatter or natural foods all reduce the effectiveness of baits. For example, if you bait two fieldsand detect wireworms in one field but not in the other, you cannot know for sure if wireworminfestations really differ between fields. Infestations might have beenthesamein both fields, butcool temperatures in one field or dry soil in the other could have prevented larvae from movingthrough the soil to the bait. The only way to be sure about actual infestation levels is take soilsamples.Literature CitedGibson, K.E. 1939. Wireworm damage to potatoes in the Yakima Valley of Washington. J.Econ. Entomol. 32:121-124.Onsager, J.A. 1969. Sampling to detect economic infestations of Limonius spp. J. Econ.Entomol. 62: 183-189.Williams, LH. 1993. Unpublished data from Ph.D. dissertation. Division of Entomology,University of Idaho, Moscow.74Numberofcoresexamined12345678910111213141516171819202122232425DO NOTTREATif total isless thanvXvX-Xv.wv.v.v.RUNNINGTOTAL:total no.wirewormsill!designates that a decision is not possibleTREATiftotalexceedsNumberofcoresexamined26272829303132333435363738394041424344454647484950DO NOTTREATif total isless thanRUNNINGTOTAL:total no.wirewormsTREATiftotalexceedsFIGURE 1. Sequential decision card for wireworm control in potatoes.Dig 1/4 -ft2 cores (6X6 inches square by shovel or 6 3/4 -inch diameter post-hole digger) to a depth of 12 inchesor more. Separate wireworms by screening soil through a pair of sieves. Use 1/4-inch hardware cloth stapled to awooden frame for the upper sieve and 8 to 16 mesh window screen for the lower sieve. Take soil samples from acrossthe entire field. Use one decision card for fields up to 30 or 40 acres. Divide larger fields into uniform 30 to 40 acreblocks and use a different decision card in each. If you know that previous cropping conditions favored buildup ofwireworms in certain parts of the field, you may want to sample those areas separately with a different card. Each timeyou dig and sieve a 1/4-ft2 sample, write on the card the number ofwireworms recovered. Record your results as arunning total, the total number of wireworms recovered from all soil cores. Compare your running total with the valuesin columns labelled DO NOT TREAT and TREAT. Then make one of these three decisions:1. No insecticide needed - If your running total is less than the value in the column labelled DO NOT TREAT, thewireworm infestation probably is below the economic threshold. No wireworm control is needed. You do not needto dig and screen more soil samples. Note that the card does not list values listed for the first 28 samples. This isa safeguard to prevent you from making the wrong control decision. It takes 29 consecutive soil cores without anywireworms before you safely can conclude that no insecticide is needed.2. Apply insecticide - If your running total is greater than the value in the TREAT column, the wireworm infestationsprobably is greater than the economic threshold. Unless you apply an insecticide, there is a good chance thatwireworm damage will be greater than3% damaged tubers. You do not need to dig and screen more cores. The cardrecommends insecticide application ifaverage infestations exceed 0.08 wireworms per foot2. This economic thresholdhas been chosen from research studies to keep wireworm injury below 3% damaged tubers at harvest.3. Continue sampling - If your running total is equal to or between the values in the two columns labelled DOTREAT and TREAT, you cannot make a control decision. Continue to dig and screen soil samples until either (1) thecard gives a treat/don't treat recommendation or (2) you have inspected a total of 50 cores. If your running totalremains between the DONOT TREAT and TREAT columns after 50 samples, the infestation is too close to the thresholdto accurately classify as either control/don't control. Control most likely is needed.

How to Interpret Wireworm Counts from Bait Stations in Potatoes

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How to Interpret Wireworm Counts from Bait Stations in Potatoes

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