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Grubs in Turfgrass: To Treat or Not to Treat

New grub "decision rule" ready for implementation; could reduce the need for insecticides by 50-80 percent

Professional turf managers in New York want to use IPM. No longer comfortable with the convention of preventive pesticide sprays, they have asked for new guidelines. They seek help in determining whether grubs are a sufficient threat to the health of their turf to warrant action. Their questions led Cornell entomologists Mike Villani and Jan Nyrop to work on a "decision rule" for grubs. The rule, formulated in 1995 specifically for the grub species European chafer and for home lawn settings, is based on a combination of a risk assessment of each site and 20 plug samples taken throughout the lawn to determine grub densities. The risk rating was formulated after gathering data on lawn characteristics such as slope, age, amount of shade, and grass variety. Table 3 below shows how risk is assessed and which kinds of lawns will require sampling for grubs and possible treatment.

In 1997, after four years of background work, Villani and Nyrop are confident that the decision rule is reliable and can reduce the need for insecticides by 50 to 80 percent.

Table 3. Risk rating system for European chafer larvae on residential lawns

% of Lawn in Shade

% of Lawn that Is Kentucky Blue Grass

Risk Category

Need to Sample?

 >60%

 <30%

 1

 no

 >60%

 30-60%

 2

 no

 30-60%

 <30%

 3

 no

 30-60%

 30-60%

 4

 no

 >60%

 >60%

 5

 yes

 30-60%

  >60%

 6

 yes

 <30%

 <30%

 7

 yes

 <30%

 30-60%

 8

 yes

 <30%

 >60%

 9

 yes

NOTE: Lawns with risk categories less than or equal to 4 NEED NOT be sampled or treated.