Impact of the fungal pathogen, SPFG, on the Salvinia molesta Mitchell biological control agent, Cyrtobagous salviniae (Coleoptera: Curculionidae)

Authors: Grodowitz, Michael; ELLIOTT, ROBERT - Former ARS Employee; DIAZ, RODRIGO - Louisiana State University; Boyette, Clyde; Weaver, Mark; Stetina, Kenneth - Ken

Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2021
Publication Date: N/A
Citation: N/A

Interpretive Summary: SPFG, a fungal pathogen being evaluated as a management strategy against the invasive floating fern giant salvinia, was evaluated for both contact-based mortality and plant-quality mediated impact to the biological control agent Cyrtobagous salviniae. Generally, SPFG had little, if any negative impact. Only when mixed with a wetting agent did significant negative impacts occur and then these were minimal. Using SPFG in areas where C. salviniae is established should cause only limited decreases in the extant weevil population.

Technical Abstract: Giant salvinia, Salvinia molesta Mitchell, is a free-floating aquatic fern native to southeastern Brazil. It was introduced into the U.S. in 1995 and 1998 in North Carolina and Texas, respectively and is now found from Hawaii to Florida to Puerto Rico. While several control strategies are employed, one of the more successful is the use of the insect biological control agent Cyrtobagous salviniae Calder and Sands (Coleoptera: Curculionidae). However, this weevil is only minimally effective in the more northern extremes of salvinia’ s distribution in the U.S. Recently, a native pathogenic fungus in the Family Botryosphaeriaceae, designated SPFG because of patent and proprietary issues, has shown great promise in the control of giant salvinia. Despite the potential of SPFG as a control tool, no information is available on its impact on salvinia weevils; this is especially important since these two management options have the potential to be used simultaneously in the same area. Therefore, the objective of this study was to determine the direct (contact-based mortality) and indirect (plant-quality mediated) impact of SPFG on adult weevils. Overall, SPFG had only minor impact to giant salvinia weevil adults. In direct toxicity tests survival of adult C. salviniae was not different among water alone (86.7+3.4%), adjuvant/water (Silwet® L77, 81.4+5.0%), and SPFG (79.7+5.6%) treatments. However, only 58.0+5.6% percent of the weevils survived direct application of the combination of SPFG and Silwet® L77. No significant differences were observed in terms of dispersal from either adjuvant only treated plants (15% movement) as well as those plants treated with SPFG alone (24% movement) in comparison to the water treated plants (12%). However, significant differences were observed in comparison to the control, adjuvant only, and SPFG alone for the treatment combination of SPFG and Silwet® L77 with over 45% of the weevils moving away from the treated plants. While significant impacts were observed in both indirect and direct assays these were generally small and shouldn’t preclude using SPFG in combination with the salvinia weevil.