Efficacy of Topical Application, Leaf Residue or Soil Drench of Blastospores of Isaria fumosorosea for Citrus Root Weevil Management: Laboratory and Greenhouse Investigations

Abstract

The efficacy of topical, leaf residue, and soil drench applications with Isaria fumosorosea blastospores (Ifr strain 3581) was assessed for the management of the citrus root weevil, Diaprepes abbreviatus (L.). Blastospores of Ifr were applied topically at a rate of 107 blastospores mL−1 on both the larvae and adults, and each insect stage was incubated in rearing cups with artificial diet at 25 °C, either in the dark or in a growth chamber under a 16 h photophase for 2 weeks, respectively. Percent larval and adult mortality due to the infection of Ifr was assessed after 14 days as compared to untreated controls. Leaf residue assays were assessed by feeding the adults detached citrus leaves previously sprayed with Ifr (107 blastospores mL−1) in Petri dish chambers and then incubating them at 25 °C for 2–3 weeks. Efficacy of the soil drench applications was assessed on five larvae feeding on the roots of a Carrizo hybrid citrus seedling ~8.5–10.5 cm below the sterile sand surface in a single 16 cm × 15.5 cm pot inside a second pot lined with plastic mesh to prevent escapees. Drench treatments per pot consisted of 100 mL of Ifr suspension (107 blastospores mL−1), flushed with 400, 900, or 1400 mL of water compared to 500, 1000, and 1500 mL of water only for controls. The mean concentration of Ifr propagules as colony forming units per gram (CFUs g−1) that leached to different depths in the sand profile per treatment drench rate was also determined. Two weeks post-drenching of Ifr treatments, larvae were assessed for percent mortality, size differences, and effect of treatments in reducing feeding damage to the plant root biomass compared to the controls. Topical spray applications caused 13 and 19% mortality in larvae and adults after 7 days compared to none in the control after 14 days, respectively. Adults feeding on a single Ifr treated leaf for 24 h consumed less than the control, and resulted in 100% mortality 35 days post-treatment compared to 33% in the untreated control. Although offered fresh, untreated leaves after 24 h, only adults in the control group consumed them. Ifr CFUs g−1 were isolated 8.5–10.5 cm below the sand surface for the 1000 and 1500 mL drench rates only, resulting in 2%–4% larval mortality. For all the Ifr drench treatments, no differences were observed in percent larval mortality and size or the effect of treatments in reducing feeding damage to the plant root biomass compared to the controls. These results suggest that the foliar application of Ifr may be an efficient biocontrol strategy for managing adult populations of D. abbreviatus; potential alternative larval management strategies are discussed.

In Planta Localization of Endophytic Cordyceps fumosorosea in Carrizo Citrus

“…Through our sand core assessments, we determined that Cfr moved down through the sand column and was able to inoculate the roots; however, Cfr did not remain inside the roots as an EEPF. This finding corroborated with that of Avery et al [ 31 ], where Cfr was able to move through the sand as the water drench amount increased. In contrast to Avery et al [ 31 ], our study differed in that the Cfr fungal inoculum contained conidia, not blastospores and the water drench continued on a daily basis for a month, as compared to only a single drench for < 1 min.…”

“…This finding corroborated with that of Avery et al [ 31 ], where Cfr was able to move through the sand as the water drench amount increased. In contrast to Avery et al [ 31 ], our study differed in that the Cfr fungal inoculum contained conidia, not blastospores and the water drench continued on a daily basis for a month, as compared to only a single drench for < 1 min. Considering D. abbreviatus feeds on the roots of citrus plants, but Cfr was not localized in the roots of Carrizo citrange, it is unlikely for this fungus to act as a reliable biocontrol against this pest as an EEPF through whole plant inoculation of this citrus plant.…”

“…To determine conidial concentrations of PFR-97 at different depths of the soil drench treatment, the methods of Avery et al [ 31 ] were followed. Plastic tips of 25 mL pipettes (Fisher Scientific, Inc., Hampton, NH, USA) were modified by cutting off at the 2 mL mark on the pipette’s tip.…”

“…Student’s t -tests (α = 0.05) were used for pairwise comparisons for assessing plant attributes between treatments. The sand core experiment was also a randomized complete block design [ 31 ]. A one-way analysis of variance (ANOVA; α = 0.05) was conducted for the sand core assessment by using trial replicate as a random effect.…”

Secondary Metabolites and the Risks of Isaria fumosorosea and Isaria farinosa

“…Some of the SMs are mycotoxins, which have risks contaminating foods and impacting human health. There were several reports on the risk evaluations of myco-pesticides and their mycotoxins, mainly involving B. bassiana , M. anisopliae , and their NRP (non-ribosomal peptide) and PK (polyketide) metabolites [ 27 , 28 , 29 , 30 ]. However, there have been few documents about the risks of Isaria fungi and their mycotoxins.…”