Black, white, and metallic plastic mulches reduced D. suzukii larval infestation of fruit by 40–72% (Fig. 3; McIntosh et al. 2021), increased radiance in the UV spectrum (Fig. 4), and modified the low canopy microclimate by increasing temperature, decreasing humidity, and increasing time above temperature and below humidity thresholds for D. suzukii. In oviposition assays in the lab, the white and metallic plastic mulches reduced D. suzukii oviposition in the presence of UV light (Fig. 8) but not in visible light only. On the ground, all three plastic mulches killed 80–100% of larvae and pupae placed on the mulch surface in the field (Fig. 6), and the surface temperature was higher in all the mulch plots compared to the control (Table 3). The reduction in larval infestation of fruit and the high mortality of D. suzukii immature stages on the ground are likely driven by the ways the mulches modify the microclimate to make it less favorable for D. suzukii.
The plastic mulches increased radiance in the UV spectrum, which may be a mechanism reducing larval infestation of fruit. All three plastic mulches increased UV radiance in the full sun, and the white and metallic mulches also increased UV radiance in the shade (Fig. 4). When UV light was present in no-choice oviposition assays in the lab, the white and metallic mulches had the highest UV radiance and reduced oviposition compared to the control (Fig. 8). Our data adds to evidence that UV light affects oviposition behavior in Drosophila. Female D. suzukii and D. melanogaster are attracted to UV light when not in the presence of oviposition substrates (Zhu et al. 2014; Fountain et al. 2020), potentially because UV light is used by insects for navigation within vegetation and orientation relative to the sky (Weir and Dickinson 2012; Cronin and Bok 2016). However, during oviposition, D. melanogaster females avoid UV and lay fewer eggs in substrates illuminated by UV (Zhu et al. 2014). Here, D. suzukii females laid fewer eggs in UV + visible light compared to only visible light (Fig. 8), which is consistent with oviposition behavior in the field. Drosophila suzukii prefers to lay eggs in the darker inner canopy and at dawn and dusk, likely to protect developing progeny from detrimental temperature extremes (Zhu et al. 2014; Diepenbrock and Burrack 2017; Evans et al. 2017).
While UV light can also affect adult D. suzukii, data from this study suggest that reducing adults in the canopy is not likely the mechanism by which larval infestation is reduced by the mulches. Our previous study reported fewer adult female D. suzukii in the canopy above the plastic mulches, and we had hypothesized that UV radiance may be deterring adult flies, thereby reducing larval infestation of fruit (McIntosh et al. 2021). However, in our current study we saw no effect of the plastic mulches on adult D. suzukii numbers (Fig. 3), and instead saw a reduction in larval infestation and found that UV light reflected from the mulches reduced oviposition (Fig. 3, 4, 8).
Canopy temperature and humidity may be secondary factors influencing lower larval infestation in mulched raspberries, since the plastic mulches made conditions warmer and drier, and therefore less favorable for D. suzukii survival, oviposition, and larval development (Kinjo et al. 2014; Tochen et al. 2014, 2016; Hamby et al. 2016; Ryan et al. 2016; Zerulla et al. 2017; Evans et al. 2018; Winkler et al. 2020). In this study, the white and metallic mulches sometimes increased temperature and decreased humidity in the high canopy (Table 2), while others saw no effect of plastic mulches at similar canopy heights (Nottingham and Beers 2020; McIntosh et al. 2021). However, the ability of plastic mulches to influence air temperature and humidity is greater closer to the ground (Ham et al. 1993; Tarara 2000). Indeed, all three plastic mulches increased temperature and decreased relative humidity in the low canopy (10 cm above the ground; Table 2). The daytime mean and maximum temperature were up to 0.5°C and 1.2°C higher and the mean RH was up to 2.2% lower in the mulch plots compared to the control, but all treatments were within the optimal ranges for D. suzukii development (Tochen et al. 2014, 2016; Hamby et al. 2016; Winkler et al. 2020). However, the plastic mulches amplified temperature and humidity extremes during the afternoon when D. suzukii seek refuge from hot and dry conditions in the cooler, more humid low canopy (Hamby et al. 2016; Evans et al. 2017; Van Timmeren et al. 2017b; Jaffe and Guédot 2019; Shaw et al. 2019). Though the daily maximum temperature was inconsistently above the 31°C upper thermal developmental threshold for D. suzukii (Kinjo et al. 2014; Ryan et al. 2016; Zerulla et al. 2017; Evans et al. 2018), the plastic mulches increased the amount of time the low canopy exceeded 31°C each season by 12–32 h in both the black and white mulches and 33–55 h in the metallic mulch (Table 2; Fig. 5). The plastic mulches reduced the minimum RH by 2–4% compared to the control and spent 80–100 minutes longer per day below the 70% RH threshold compared to the control (Table 2, Fig. 5). Thus, the plastic mulches make abiotic conditions in the low canopy less favorable for D. suzukii.
The majority of D. suzukii reach the ground to pupate either by dropping out of fruit or by falling inside of infested fruit (Woltz and Lee 2017). Here we show that the plastic mulches increased the ground surface temperature, which may be the mechanism contributing to nearly complete mortality of immature D. suzukii on the mulch surface in less than three hours. The maximum ground surface temperature frequently exceeded 50°C on the plastic mulches, even exceeding 75°C on the black mulch. The mulch surface temperature was above 31°C for 31–131 h per season more than in the control plots. Similarly, black woven polyethylene weed mat increased the time above the thermal threshold on the ground in blueberry, and pupal emergence was lower above the weed mat than below (Rendon et al. 2019). Further, we observed larvae struggle to crawl on the plastic mulches and visibly desiccate within seconds in lab and field conditions (as shown in the video in Online Resource 1, 2), possibly due to the texture of the mulches. Since larvae crawl < 7.5 cm to find a suitable pupation site in the soil, it is highly unlikely that the larvae will be able to escape the mulch to reach the soil (Ballman and Drummond 2019; Rendon et al. 2019).
We observed a low amount of fallen fruit in this study, with less than ten fallen fruit per meter on most dates. This illustrates the efficacy of the 2–3 d harvest schedule in preventing fallen fruit due to D. suzukii infestation (Leach et al. 2018). The plastic mulches had more fallen fruit than the control plots in mid-September in 2021 and 2022 (Fig. 7), likely due to substantially higher yield in those plots compared to the control (McIntosh et al. 2023, in preparation). Larval infestation of fallen fruit was lower in the metallic mulch compared to the control and numerically lower for the other mulch treatments compared to the control (Fig. 7). Though we did not use this practice in our research plot, sanitation of fallen fruit is an important practice to manage D. suzukii infestation (Leach et al. 2018), and is likely easier to perform on the plastic mulches. The hot surface temperature of the plastic mulches may even solarize infested fruit. Thus, using plastic mulches in combination with frequent harvesting and field sanitation should eliminate the majority of fallen fruit that could contribute to D. suzukii infestations.
In summary, our work adds to substantial evidence that hot and dry microclimate conditions reduce D. suzukii infestation (Tochen et al. 2014, 2016; Guédot et al. 2018). Our data suggest that UV light is also important since it reduces D. suzukii oviposition. Cultural practices that increase UV radiation and temperature in the canopy and decrease canopy humidity should result in a reduction of D. suzukii larval infestation of fruit. However, it can be challenging to develop cultural practices that modify the microclimate at a sufficient magnitude to reduce D. suzukii. For example, canopy pruning, applying black woven polyethylene weed mat, or using overhead irrigation did not influence the microclimate enough to substantially reduce larval infestation (Rendon and Walton 2019; Rendon et al. 2019; Schöneberg et al. 2020). Plastic mulches are highly effective due to the cumulative effects of UV radiance, canopy temperature and humidity, and ground surface temperature. The black and metallic mulches had the highest reduction in larval infestation of fruit and highest increase in marketable raspberry yield compared to the control (McIntosh et al. 2023, in preparation). Plastic mulches may be effective for managing D. suzukii in different regions and crops if they are selected to maximize impacts on the microclimate.