Homan, H. W.

O'Keeffe, L. E.

Undetermined

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LYGUS BUG INJURY IN LENTIL FIELDS AND THE OCCURRENCE OF CHALKY SPOTL. E. O'Keeffe and H. W. HomanExtension Entomology, University of Idaho, Moscow 83843Recently, R. J. Summerfield and F. J. Muehlbauer, USDA, ARS atWashington State, implicated lygus bugs as the causative agent of "chalkyspot syndrome" in lentils. Seed lots containing chalky spot and otherdamaged seeds in excess of 2 percent are graded Number 2 lentils and thosein excess of 3.5 percent are given a sample grade and discounted in themarket place. Summerfield and Muehlbauer also suggested that economicyield loss was probable based on growth chamber studies. They reportedincreased prevalence of shrivelled, unfilled pods (13-15%); promoted abscissionof immature pods on "Chilean" lentil (22%); and increased seed abortion(4-20%) by caged lygus onto pot-grown plants in comparison with lygus-freecontrols.We monitored lygus bug populations in 21 northern Idaho and easternWashington lentil fields by sweep-netting. A field sample consisted of75, 180 sweeps and was taken twice a week during bloom, pod set and podfilling. Portions of six fields were treated with malathion (1 lb ai/a)applied by air in 5 gallons of water. At harvest combined seed sampleswere taken near sampling sites within fields and in treated and untreatedfield portions. Composited field samples were assessed for percent chalkyspot by the Fedreal Grain Inspection Service, USDA at Moscow, ID.Adult lygus bug populations generally occurred at low levels (near5/25 sweeps) and did not fluctuate appreciably as the season developed.At the end of the season adult lygus increased in number in the majorityof fields. In 3 instances populations exceeded 1 lygus/sweep but thesehigh numbers were observed for only a short time and were related to nymphschanging to adults within the fields. Numbers of lygus nymphs began increasingabout the time first pods were formed, reached peak populations withintwo weeks and then declined as they changed to adults and crop maturityoccurred. Nympha1 lygus bug populations occurred at levels of less than15/25 sweeps in 5 fields; near 25/25 sweeps in 6 fields; near 50/25 sweepsin 6 fields and more than 75/25 sweeps in 3 fields. The higher and loweramounts of chalky spot in lentil seed was associated with the highestand lowest field infestations of lygus bugs but there were notable exceptions.One field had relatively low season-long lygus numbers but had twice theamount of chalky spot of other fields studied.Chemical treatments were effective in reducing lygus bug numbersto low levels (less than 2/25 sweeps) and reduced the percent chalky spotin seed from treated field portions when compared to untreated fieldareas. However, the amount of reduced chalky spot associated with treatmentwas not economically important in most cases. It appears that chemicalapplications, applied when lygus populations were increasing or had peakedin number, were too late to be of much value in preventing the occurrenceof chalky spot.

Lygus Bug Injury in Lentil Fields and the Occurrence of Chalky Spot

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Lygus Bug Injury in Lentil Fields and the Occurrence of Chalky Spot

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