Overfertilizing St. Augustinegrass could encourage chinch bugs, UF researcher warns

March 3, 2008

GAINESVILLE, Fla. — A little fertilizer can perk up a St. Augustinegrass lawn as spring arrives, but homeowners who overdo it may find they’re growing more than grass.

A University of Florida study suggests that repeatedly using large amounts of nitrogen fertilizer can ignite a population explosion of Southern chinch bugs — the No. 1 insect pest of St. Augustinegrass, the state’s most popular turfgrass.

The findings were presented in Jacksonville today at an Entomological Society of America meeting.

“Everything in moderation,” said Eileen Buss, an associate professor of entomology with UF’s Institute of Food and Agricultural Sciences. “When we try to overly manage a natural system we get the balance out of whack.”

UF turfgrass experts advise homeowners to use no more than 1 pound of slow-release nitrogen fertilizer per 1,000 square feet of lawn, a recommendation found in the document “St. Augustinegrass for Florida Lawns,” available at http://edis.ifas.ufl.edu/LH010.

In the study, Southern chinch bugs produced the most eggs on St. Augustinegrass fed the equivalent of 2 pounds nitrogen per 1,000 square feet per month.

That rate is a worst-case scenario, she said, but not unrealistic because people sometimes deliberately overfertilize in their zest to have the greenest lawn in the neighborhood.

That more-is-better approach has become riskier in the past five years because Southern chinch bugs in Citrus, Escambia, Flagler, Hillsborough, Lake, Orange and Volusia counties have developed resistance to pyrethroids, the class of pesticides commonly used to control the insects, Buss said.

Resistant chinch bugs may be able to survive exposure to bifenthrin, a pyrethroid that is the top choice for Southern chinch bug control in Florida. However, pyrethroids should still perform well against nonresistant populations of Southern chinch bugs.

Buss co-authored the study with turfgrass specialist Laurie Trenholm, an associate professor of environmental horticulture, and doctor of plant medicine student Megan Gilbert.

Conducted at a UF research facility in Citra, it involved chinch bugs raised on St. Augustinegrass that received nitrogen fertilizer at one of five rates, equivalent to 0, 0.5, 1, 2 or 4 pounds of nitrogen per 1,000 square feet of turf, applied monthly.

The chinch bugs were paired to reproduce, each pair placed in a cage containing St. Augustinegrass fertilized at the same rate the insects previously experienced. The results showed females raised on grass given 0 or 0.5 pounds nitrogen produced 15 to 20 eggs per week; those on grass given 1 or 2 pounds produced 25 to 35. Females on grass given 4 pounds briefly produced 45 eggs per week, then declined to 20.

Adult female Southern chinch bugs live about two months, and produce eggs the entire time.

Buss said female chinch bugs produce more eggs on healthy St. Augustinegrass, which accounts for the differences in egg production. Future research may examine the role of the nutrients phosphorus and potassium in chinch bug population growth, and the possibility that overfertilization may reduce turfgrass resistance to chinch bugs.

Though this study was conducted in a laboratory rather than a yard, Buss says the results are relevant to homeowners. Southern chinch bugs don’t move around much, staying in the same area unless they can’t find food.

And with the insects producing a new generation every four to six weeks during Florida summers, increased egg-laying could lead to rapid population growth in overfed lawns.

Buss said she’s not sure how applicable the results are in other Gulf Coast states where St. Augustinegrass is commonly grown, due to environmental differences.

Studies on other chinch bug species elsewhere in the United States show that it’s common for nitrogen fertilizer to boost egg production, said chinch bug expert Fred Baxendale, an entomology professor with the University of Nebraska-Lincoln.

“What Dr. Buss is seeing is in line with prior research,” he said. “I think her research is interesting, I think it’s valid and it needs to be taken further.”