Overwintered Drosophila suzukii Are the Main Source for Infestations of the First Fruit Crops of the Season

Abstract

The mechanisms allowing the widespread invasive pest Drosophila suzukii to survive from early spring until the availability of the first fruit crops are still unclear. Seasonal biology and population dynamics of D. suzukii were investigated in order to better understand the contribution of the early spring hosts to the infestation of the first fruit crops of the season. We identified hosts available to D. suzukii in early spring and assessed their suitability for the pest oviposition and reproductive success under field and laboratory conditions. The natural infestation rate of one of these hosts, Aucuba japonica, was assessed over springtime and the morphology of the flies that emerged from infested A. japonica fruits was characterized under field conditions. Then, these findings were correlated with long-term monitoring data on seasonal reproductive biology and morphology of the pest, using a cumulative degree-days (DD) analysis. Field sampling revealed that overwintered D. suzukii females were physiologically able to lay eggs at 87 DD which coincided with the detection of the first infested early spring hosts. The latter were continuously and increasingly infested by D. suzukii eggs in nature from early spring until the end of May, in particular Aucuba japonica. Individuals emerged from most of these hosts were characterized by a poor fitness and a rather low success of emergence. In the field, only few summer morphs emerged from naturally infested A. japonica fruits around the end of May-beginning of June. However, field monitoring in orchards revealed that D. suzukii individuals consisted solely of winter morphs until mid-June. These observations indicate that overwintered D. suzukii females are the predominant source for the infestations in the first available fruit crops of the season. We discuss these findings in the context of possible pest control strategies.

Marking Drosophila suzukii (Diptera: Drosophilidae) with Fluorescent Dusts

“…. By selecting this habitat and seasonal timeframe, it was possible to also release unmarked D. suzukii: there are very few wild D. suzukii in this habitat and any wild drifters would be winter morphs [57] and thus easily distinguished from the laboratory reared summer morphs. In the plot, four trapping grids were deployed with a minimum of 50 m between grids and a minimum of 40 m between grids and plot edges.…”

Morphometric criteria to differentiate Drosophila suzukii (Diptera: Drosophilidae) seasonal morphs

“…The morphometrics we chose were wing length, wing width, and hind tibia length because these are sclerotized body parts. Additionally, a ratio of wing length to hind tibia length was used because D. suzukii size can vary based on diet [27,33], and because temperature-induced phenotypic plasticity in wing size and shape is well known in Drosophila broadly, and D. suzukii specifically [45][46][47]. By using a ratio of wing length to hind length, we correct for potential wing size variation of field-caught versus laboratory-reared D. suzukii that may be associated with body size.…”

“…In laboratory settings, these environmental cues can be simulated to produce winter morphs for experimental purposes; for example, winter morphs can be generated by placing eggs and larvae produced by summer morphs at a constant temperature between 10-15˚C and a photoperiod between 10:14 (L:D) hours and 12:12 (L:D) hours [26][27][28][29].Winter morph D. suzukii are more cold-tolerant than summer morphs, which would allow flies to persist in temperate regions [26,29,30]. Researchers have documented D. suzukii seasonal population dynamics across different regions [12,[31][32][33][34] and in several crops over time [35][36][37]. In addition, research describes the reproductive status and ovary maturity levels of D. suzukii in temperate regions shows that reproduction ceases, or is greatly reduced in cooler months [33,[38][39][40].…”

“…Researchers have documented D. suzukii seasonal population dynamics across different regions [12,[31][32][33][34] and in several crops over time [35][36][37]. In addition, research describes the reproductive status and ovary maturity levels of D. suzukii in temperate regions shows that reproduction ceases, or is greatly reduced in cooler months [33,[38][39][40]. Given these results, and in most temperate regions of the U.S., it is generally assumed that D. suzukii overwinters as an adult winter morph [41][42][43].…”

“…When rearing different morphs in the lab, Shearer et al [25] noted that winter morph males and females had a continuous dark pigmentation on the third and fourth abdominal segment, respectively, that was completely filled. This method has been utilized to determine the seasonal phenology of winter morphs caught in the field [27,33,44]. However, there were some challenges to using color as this was the only metric for identifying winter and summer morphs.…”