A strawberry accession with elevated methyl anthranilate fruit concentration is naturally resistant to the pest fly Drosophila suzukii

Abstract

During the past decade, Drosophila suzukii has established itself as a global invasive fruit pest, enabled by its ability to lay eggs into fresh, ripening fruit. In a previous study, we investigated the impact of different strawberry accessions on the development of D. suzukii eggs, in the search of natural resistance. We identified several accessions that significantly reduced adult fly emergence from infested fruit. In the present study, we aimed at understanding the chemical basis of this effect. We first noted that one of the more resistant accessions showed an unusual enrichment of methyl anthranilate within its fruit, prompting us to investigate this fruit compound as a possible cause limiting fly development. We found that methyl anthranilate alone triggers embryo lethality in a concentration-dependent manner, unlike another comparable organic fruit compound. We also showed that a chemical fraction of the resistant strawberry accession that contains methyl anthranilate carries some activity toward the egg hatching rate. Surprisingly, in spite of the lethal effect of this compound to their eggs, adult females are not only attracted to methyl anthranilate at certain concentrations, but they also display a concentration-dependent preference to lay on substrates enriched in methyl anthranilate. This study demonstrates that methyl anthranilate is a potent agonist molecule against D. suzukii egg development. Its elevated concentration in a specific strawberry accession proven to reduce the fly development may explain, at least in part the fruit resistance. It further illustrates how a single, natural compound, non-toxic to humans could be exploited for biological control of a pest species.

Fig 1. Methyl anthranilate concentration in strawberry accessions with various degrees of resistance to Drosophila suzukii.

(A) Fragaria accessions differ in their resistance to Drosophila suzukii (replotted from [7]). (B) MA concentration is variable across Fragaria accessions. The Fragaria accession with highest MA levels, 300, is also one of the most resistant to D. suzukii (gray shading). (C) MA concentration is variable in some Fragaria species.

Fig 2. Drosophila suzukii embryonic survival on methyl anthranilate and isoamyl acetate.

The box plots indicate the median number of embryos per well that have hatched on agar plates with various concentrations of MA (A) or isoamyl acetate (B) after 48 h. An MA concentration of 2.10⁻⁴ (v/v) or higher significantly reduces embryo survival as compared to the control group. No effect was observed when adding isoamyl acetate to the medium. (Box plots with median and 10–90 percentiles, n = 36 for MA, n = 24 for IA, stars indicate significant differences when compared to the control group; *** P-value<0.005, **** P-Value<0.0005 ns: non-significant, see methods for statistics).

Fig 3. Fractions of Fragaria vesca accession 300 carry antagonist egg activity.

The schematic depicts the experimental flow from fractionation to the assessment of fraction activity. Upon fractionation with three different solvents, the fractions were tested in agarose plates onto which fly embryos were let to develop. The hatching rate was reduced for methanol/water and ethyl acetate fractions, but not for the water fraction. All percentages were normalized to the hatching rate of the control.

Fig 4. D. suzukii egg-laying preference on MA substrate.

Female flies were offered the choice to lay eggs on strawberry purée supplemented with different concentration of MA or devoid of MA (egg-laying plate depicted to the right). MA at a concentration of 2.10⁻⁴ (v/v) is attractive for egg-laying, while it is repulsive at 5.10⁻⁴ (v/v). (Error bars represent SEM, n = 12 replicates per MA concentration, stars indicate significant difference from the hypothetical mean of 0; ****: P-value<0. 0005).

Fig 5. Preference index for entering traps containing water or water supplemented with different concentrations of MA.

Flies displayed significant preference for traps containing 2.10⁻⁴ (v/v) MA over water. When testing a concentration of 1.10⁻³ (v/v) MA, flies preferred the water traps. No preference was detected when testing 4.10⁻⁴ (v/v) MA. (Error bars represent SEM, n = 9, stars indicate significant difference from the hypothetical mean of 0; *, P-value<0.05; **, P-value<0.005; ns: non-significant).