Integrated Biological Control of the Sugar Beet Weevil Asproparthenis punctiventris with the Fungus Metarhizium brunneum: New Application Approaches

Abstract

The mass occurrence of the sugar beet weevil (Asproparthenis punctiventris, previously Bothynoderes punctiventris) has been endangering sugar beet cultivation in Austria for centuries. Exacerbated by climatic and political changes (warmer, drier spring and limited access to chemical pesticides), new approaches are needed to counter the problem. The aim of our work was to test whether the bioinsecticide Metarhizium brunneum Ma 43 (formerly M. anisopliae var. anisopliae BIPESCO 5/F52) can be used as a sustainable plant protection product against the sugar beet weevil. Our goal was to control the pest in all its development stages through multiple applications. Therefore, GranMetTM-P, a granular formulation of M. brunneum Ma 43, was applied in spring to establish the fungus in the soil, whereas GranMetTM-WP, a liquid formulation of the production strain, was used in early summer on trap ditches and leaves to target the adult weevils. Soil and plant samples as well as weevils were collected during the planting season from the trial sites to evaluate the development of the fungus and the mycosis of the treated weevils. In addition, data on hibernating weevils and their emigration from untreated field sites was collected. In all field sites, the Metarhizium spp. abundance increased above the background level (<1000 CFU g−1 soil dry weight) after application of the product. With an increasing number of treatments per plot, and thus an increased contact possibility between pest and the fungus, a rise in the mycosis rate was observed. In conclusion, the various Metarhizium application strategies, which are already available or in testing, must be implemented to ensure control in both old and new sugar beet fields. Metarhizium is a further asset in the successful control of this sugar beet pest.

Keywords: abundance; entomopathogenic fungus; mass application; mycosis; pest control.

Figure 1

Time-Mortality analysis of A. punctiventris weevils inoculated with the fungal bioinsecticide. Mean values (black line) and minimum and maximum values (red lines) are shown. Dotted lines indicate LT₅₀ and LT₉₀ values. Results are obtained by a probit analysis corrected by control group values [26].

Figure 2

Development of Metarhizium spp. abundance in soil in all trial sites. (A) continuous granular application, (B) granular application with spray application of the trap ditch, (C) application after sugar beet cultivation, (D) application before sugar beet cultivation. Arrows indicate application time (dashed arrow indicates the spray application), and dashed line shows maximum indigenous density without soil disturbance.

Figure 3

Metarhizium spp. CFU isolated from leaves after spray treatment. (A) CFU in percent calculated from mean value of day 0 in the field, (B) CFU per cm² leaf surface in the pot trials. Each dot represents an individual value of a single plate.

Figure 4

Minimum spanning network (MSN) showing the relationship between the SSR genotypes isolated from the soil of the field sites before (A) and after (B) spray application of M. brunneum Ma 43. Circle sizes are proportional to the number of isolates belonging to one MLG, the thickness of the line is proportional to genetic SSR-based similarity of genotypes.

Figure 5

A. punctiventris beetles caught with pheromone traps, which were installed at regular intervals of 10 m along the edges of the fields. Thirteen trial sites with an area size between 2.4 ha and 13.1 ha were sampled throughout Lower Austria in the planting season 2021. Ranked number of weevils per location is plotted against the location in which they were caught in degrees on a compass rose (360°; E = East, S = South, W = West and N = North).

Figure 6

Mycosis evaluation of sugar beet weevils from (A) control sampled separately, (B) control sampled from pheromone traps, (C) field site B (emerging beetles after two-time application) and (D) field site C (immigrating beetles that had just crossed the sprayed trap ditch). The larger grey/red circle indicates percentage of mycosis with different fungi, and the smaller green/yellow circle shows percentage of different Metarhizium species isolated from weevils.