Skip to main content
ARS Home » Northeast Area » Ithaca, New York » Robert W. Holley Center for Agriculture & Health » Emerging Pests and Pathogens Research » Research » Publications at this Location » Publication #321552

Title: Efficacy of spray applications of entomopathogenic fungi against western flower thrips infesting greenhouse impatiens under variable moisture conditions

Author
item Wraight, Stephen
item UGINE, TODD - Cornell University
item RAMOS, MARK - Former ARS Employee
item SANDERSON, JOHN - Cornell University

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2016
Publication Date: 2/26/2016
Citation: Wraight, S.P., Ugine, T.A., Ramos, M.E., Sanderson, J.P. 2016. Efficacy of spray applications of entomopathogenic fungi against western flower thrips infesting greenhouse impatiens under variable moisture conditions. Biological Control. 97:31-47.

Interpretive Summary: In previous tests of the common insect pathogenic fungus Beauveria bassiana against western flower thrips infesting greenhouse crops of impatiens, we were unable to achieve greater than 40¬–50 percent control, and pest populations continued to grow rapidly. We hypothesized that the micro-environments on the smooth, waxy foliage and exposed flowers of impatiens were too dry to support Beauveria activity. Tests were therefore conducted to determine if efficacy of this and other insect pathogenic fungi could be increased by manipulation of greenhouse moisture conditions. This was achieved via controlled sprays of water onto greenhouse floors for a period of 40 hours (from the evening through the second night) following fungus application. Multiple applications were made at 3–4-day intervals, and efficacy was assessed from samples of flowers and foliage from treated vs. untreated impatiens. Average relative humidity (RH) over all test days was a strong predictor of control of thrips on foliage but not on flowers. Mean RH during the 40-hour periods following sprays was a stronger and more consistent predictor. The single variable most closely correlated with pest control was the average number of hours during the 40-hour post-spray periods when RH was greater than or equal to 80 percent. Maintaining RH at 80 percent for at least 25 hours after sprays resulted in greater than 70 percent control of thrips on foliage. Substantially longer periods (30–35 hours) were required to achieve equivalent control on flowers. Control greater than or equal to 70 percent in both foliage and flowers prevented pest populations from increasing. Results show that use of insect pathogenic fungi to control thrips may be difficult on some greenhouse crops, but control can be achieved by increasing greenhouse moisture levels following treatments.

Technical Abstract: Efficacy tests of three entomopathogenic fungi (Beauveria bassiana strain GHA, Metarhizium brunneum strain F52, and Metarhizium anisopliae s.l. strain ESC-1) were conducted against thrips infesting greenhouse crops of single impatiens under variable moisture conditions. Fungal conidia suspended in 0.01 percent Silwet® were applied at a high rate of 2x1014 conidia/hectares per acre at 3–4-day intervals, and efficacy was assessed from samples of both flowers and foliage. During tests, greenhouse moisture conditions were modified via controlled sprays of water onto the floor beneath the greenhouse benches for a period of ca. 40 hours (from the evening through the second night) following each application (post-spray period). Efficacy of the pathogens expressed as percent reduction of the combined populations of thrips nymphs and adults relative to spray-carrier controls was ultimately correlated to greenhouse environmental conditions. Mean relative humidity (RH) during the tests was a strong predictor of control of thrips on foliage (R2=0.733) but not on flowers (R2=0.435). Mean RH over the post-spray periods was a more consistent predictor (R2 foliage=0.745; R2 flowers=0.753). The single best predictor of control was the mean number of hours during post-spray periods when RH was greater than or equal to 80 percent (R2 foliage=0.89; R2 flowers=0.862). Efficacy of the three fungi was similar under comparable moisture conditions. Maintenance of 80 percent RH for at least 25 hours during post-spray periods resulted in greater than 70 percent control of thrips in foliage. Substantially longer periods (35 hours) were required to achieve equivalent control in flowers. Control greater than or equal to 70 percent in both foliage and flowers prevented pest populations from increasing. Aspects of impatiens that create a challenge for thrips control using fungal pathogens are described and discussed.