Pathogenicity of La Jolla Virus in Drosophila suzukii following Oral Administration

Abstract

Drosophila suzukii (Ds) is an invasive pest insect that causes severe and widespread damage to soft fruit crops. Chemical control based on topical insecticides is inefficient and harmful to consumers and the environment, prompting interest in the development of biological control measures such as insect viruses with narrow host specificity. We previously described a strain of La Jolla virus (LJV) found in moribund Ds specimens in Germany. We demonstrated a pathogenic effect following the intrathoracic injection of LJV into adult Ds flies. However, the development of an effective biocontrol product based on LJV would require the characterization of (1) virulence following oral delivery, particularly in larvae, and (2) stability under different pH and temperature conditions reflecting realistic exposure scenarios. Here we describe the pathogenicity of LJV following oral delivery to Ds adults and larvae. The oral infection of Ds adults with LJV reduced survival in a concentration-dependent manner, whereas the oral infection of Ds larvae caused the arrest of development during pupation. LJV remained stable and infectious following exposure to a broad pH range and different temperatures. We, therefore, demonstrated that LJV is promising as a candidate biological control agent against Ds.

Keywords: Drosophila suzukii; Iflavirus; La Jolla virus (LJV); biological pest control.

Figure 1

Standard curve of LJV. The y-axis shows the cycle threshold value (C_T), which represents the number of PCR cycles needed to reach the threshold of the fluorescent signal. The x-axis represents the log₁₀ of the absolute number of LJV RNA copies. The PCR efficiency is 98%, and the limit of quantification is 4 copies of the template corresponding to a C_T value of 37.

Figure 2

Survival of female Ds flies following the oral administration of LJV. Black line = untreated control, orange line = LJV low concentration (LC) of 10⁵ GE/mL, and red line = LJV high concentration (HC) of 10⁶ GE/mL. There was a significant difference in survival between LJV-fed flies and the untreated control (**** p < 0.0001).

Figure 3

The quantification of LJV following oral administration to adult Ds flies. (a) High concentration (LC, 10⁶GE/mL). (b) Low concentration (HC, 10⁵ GE/mL). T-test analysis revealed no significant differences between the virus load measured on day one compared to day two or three.

Figure 4

Characteristics of Ds larvae following the oral administration of LJV. The untreated control group is shown in shades of gray (black = larvae, dark gray = pupae, and light gray = adults). The LJV-infected group is shown in shades of red (pink = larvae, dark red = pupae, bright red = adults). The data were analyzed by two-way ANOVA. Statistically significant differences are shown at *** p < 0.001.

Figure 5

Survival of flies after exposure to LJV pre-treated in buffers over a broad pH range. Black = untreated negative control, red = LJV-fed positive control without pretreatment. LJV high concentration (HC, 10⁶ GE/mL) was used. Other lines represent LJV-fed flies following different pH treatments, as shown in the color key. Statistical analysis (Kaplan–Meier) indicated a significant difference (**** p < 0.0001) between the negative control and flies exposed to LJV following all pre-treatments except pH 10 and 12.5.

Figure 6

Absolute quantification of LJV following pH pre-treatment and oral delivery to adult Ds flies. The control (LJV without pH pre-treatment) is shown on the x-axis as “Virus” followed by the pH pre-treatment groups. The viral load was measured from day 0 to day 3 (D0–D3).

Figure 7

Survival assay following the oral delivery of LJV pre-treated at different temperatures. LJV high concentration (HC, 10⁶ GE/mL) was used. LJV was incubated at 20, 25, or 30 °C for 3 h (solid lines) or 8 h (dashed lines). Statistical analysis (Kaplan–Meier) indicated a significant difference (**** p < 0.0001) between the negative control and all flies exposed to LJV.

Figure 8

Absolute quantification of LJV following temperature pre-treatment and oral delivery to adult Ds flies. The control (LJV without pre-treatment) is shown on the x-axis as “Virus” followed by the temperature pre-treatment groups. The viral load was measured from day 0 to day 3 (D0–D3).