IGRs on Fungus Gnats and Western Flower Thrips
The impact of fungus gnat larvae on greenhouse crops hasbeen difficult to assess since feeding damage occurs on the root system withinthe potting medium. The larvae feed on fungi and organic matter in the media,in addition to feeding on healthy and diseased plant tissue. Feeding damage tothe roots of healthy plants has been shown to predispose the plant to infectionby plant pathogens. The ability of adult fungus gnats to transmit pathogens isalso of concern. Fungus gnats have been recorded to vector the pathogenresponsible for verticillium wilt, root and stem rots, damping off, black rootrot and fusarium wilt.
Western flower thrips are also difficult to manage in greenhouseproduction systems. This pest not only causes significant damage to foliage andflowers of susceptible plants but also is capable of vectoring tospoviruses.Western flower thrips management has become extremely difficult because thripshave developed resistance to many of the major classes of insecticides commonlyused in conventional management programs. Another challenge to the managementof western flower thrips is that late second instar nymphs migrate off theplant into the potting medium where the insect remains for two additionalstages until adult emergence. This aspect of the lifecycle makes managementdifficult since these immature stages are not exposed to foliar insecticideapplications. Adults emerging from the medium are capable of dispersingthroughout the greenhouse. In addition, first instar nymphs that fed previouslyon virally infected foliage are capable of transmitting the virus to newsusceptible plants as adults.
An earlier study by Scott Ludwig and Ronald Oetting at the Universityof Georgia, Athens, Ga., found that three insect growth regulators, Precision(fenoxycarb), Adept (diflubenzuron) and Distance (pyriproxyfen), applied atlabel rates to the potting medium, reduced western flower thrips emergence fromthe potting medium. If adult thrips emergence could be reduced by the use ofmedium treatments, fewer applications of foliar insecticides would be needed.
The ability to use only one insecticide to manage both pestscould decrease labor and insecticide costs for growers. In addition, the use ofmedium drenches for thrips control should reduce the number of foliarinsecticide applications required. A reduction in foliar insecticide use wouldalso decrease the likelihood that thrips would develop resistance to insecticides.The objective of this research was to evaluate, under commercial growingconditions, the impact of Precision, Adept and Distance on western flowerthrips and fungus gnat populations when insecticides were applied to thepotting medium of African violets at the rates used for fungus gnat management.
Materials and Methods
The following trials were conducted at a research greenhouseon The Pennsylvania State University campus, University Park, Pa., and atHerman Lederer & Sons Greenhouse, Parker Ford, Pa. In all of the trials,rooted cuttings of African violets were planted in 4-inch pots and fertigatedusing drip tubes. The trials were conducted for 28-35 days to represent atypical African violet production cycle.
The Pennsylvania State University greenhouse has atruss-frame greenhouse covered with corrugated polycarbonate, a concrete floorand an insect screen between the cooling pads and the plants. The trials werereplicated four times with 50 plants (trial one) or 33 plants (trial two) perblock. Blocks were separated by 3 feet.
The greenhouse at Herman Lederer & Sons Greenhouse is aquonset with a soil floor and no insect screen. There were 144 plants perblock, and the trial was replicated six times. In this trial, potato wedgeswere randomly placed in the potting medium 28 days after treatment applicationand checked for the presence of fungus gnat larvae two days later.
The treatments evaluated at label rates were 0.08 g.L-1(0.01 oz./gal.) active ingredient (AI) Precision, 0.02 g.L-1 (0.003 oz./gal.)AI Adept, 0.09 g.L-1 (0.01 oz./gal.) AI Distance and an untreated control. Foreach treatment, a 60-mL (2 fl oz.) drench was applied to each pot at theinitiation of the experiment, and a second drench was made in the Precision andAdept treatments 14 days after the first drench.
The trials were set up as a randomized complete blockdesign. Sampling for thrips and fungus gnats was conducted by the use of oneyellow and one blue 3- x 5-inch sticky card placed in each block. The number ofthrips and fungus gnats on each card was recorded at seven-day intervals.
In all three trials, yellow sticky cards trappedsignificantly more thrips and fungus gnats than the blue sticky cards (seeFigure 1, right). Consequently, the results from the yellow sticky cards wereused in evaluating thrips and fungus gnat populations.
University Trials. There were no significant differencesamong the trials in thrips populations among the four treatments (see Figure 2,right). In the first trial, fungus gnat populations declined in all treatmentsimmediately following the initiation of the study (see Figure 3, page 44).Adept and Distance treatments resulted in lower fungus gnat populations onthree of the five sample periods after treatments were initiated. In the secondtrial, each of the three treatments resulted in significantly lower fungus gnatpopulations on three of the four sample periods after the treatments wereinitiated (see Figure 3, page 44).
Because the medium was kept relatively dry during the firsttrial, fungus gnat populations decreased across all treatments due tounfavorable conditions for larval development. Keeping pots dry is a techniqueoften used to reduce fungus gnat populations. In addition, because thegreenhouse used in this study had a concrete floor, there were no alternativebreeding sites for fungus gnats within the greenhouse. Results from the secondtrial indicated that the use of Adept, Distance and Precision resulted ineffective management of fungus gnat populations.
Commercial Flower Grower Trial. In this trial, the mean thrips populations remained below one thripsper card for all treatments. In contrast to the thrips population, fungus gnatswere high (see Figure 4, above). Fungus gnats did not appear to be affected bythe medium-applied treatments. The potato wedges yielded low fungus gnat larvalpopulations, indicating that the adult fungus gnats being caught on the stickycards were not emerging from the pots. Sticky cards placed under the benches onday 28 and counted on day 35 indicated a high fungus gnat population under thebenches. We speculate that the fungus gnats sampled on the sticky cards weremigrating from the floor. Although the greenhouse was kept clean and the soilfloor was kept dry, fungus gnats were apparently completing development underthe benches.
Adept, Distance and Precision are effective tools formanaging fungus gnats on greenhouse-produced ornamentals. While pesticideapplications for fungus gnats are traditionally only made to the medium inwhich the plants are growing, there may be additional locations that need to betreated to provide adequate fungus gnat management. Results from the greenhousetrial at Herman Lederer & Sons Greenhouse indicated that a pesticideapplication to the soil under the benches was needed. While Adept, Distance andPrecision have been shown to reduce thrips emergence from potting medium inother studies, no definitive results could be obtained from these trials.Additional studies are warranted to further investigate medium drenches as atool for thrips management.
This research was funded by a grant from the PennsylvaniaDepartment of Agriculture Green Industry Grower/Retailer IPM Program and theBedding Plants Foundation, Inc.