Alternative Methods to Manage Silverleaf Whitefly [Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae)] in Zucchini (Cucurbita pepo L.)

The study was carried out at the Horticulture Farm, University of Georgia, Tifton Campus (31.446 N and 83.477 W, and an altitude of 116 m) in the fall of 2019 and 2020. The soil is loamy sand and has a pH of 6.5. A historical annual mean air temperature of 18.9 °C and precipitation of 1208 mm (www.georgiaweather.net/). In 2019 (23 Aug. to 15 Oct.), average air temperatures were 33.5 °C (maximal), 26.8 °C (mean), and 20.1 °C (minimal), and cumulative rainfall was 85 mm. In 2020 (12 Aug. to 6 Oct.), average air temperatures were 30.0 °C (maximal), 25.1 (mean), and 20.2 °C (minimal), and cumulative rainfall was 221 mm. Weather data were collected from a University of Georgia nearby (< 300 m) weather station (University of Georgia Weather Network).

Zucchini ‘Spineless Beauty F1’ (Harris Seeds, Rochester, NY) was direct-seeded on two rows per bed formed on 1.8-m centers, 30 cm between plants within the row and 36 cm between rows. Beds were covered with 1.52 m wide and 25 μm thick low-density polyethylene plastic mulch. A single drip tape (John Deere Ro-Drip, 40) with 20.3-cm emitter spacing and 1.0 L·h⁻¹ emitter flow was placed 5 cm deep in the bed center. Irrigation water was applied when cumulative crop evapotranspiration was 12 mm (ca. 2–3 days). The plot size was a 4-m long bed (26 plants per plot).

The number of whiteflies per leaf was estimated visually as a whitefly index (WFindex) using the following scale: 1 – no whiteflies, 2 – small number of whiteflies (< 250), 3 – medium number of whiteflies (250–500), 4 – high number of white-flies (500–750), and 5 – extremely high number of whiteflies (> 750). The WFindex was evaluated on the abaxial side of three randomly selected mature leaves per plot during the early morning when whiteflies had little activity. The WFindex determinations were based on five evaluations made twice weekly, starting 30 days after seeding. Plants were monitored weekly for symptoms of viral diseases.

Three trials were conducted in the fall of 2019 and one in the fall of 2020, using similar production practices to evaluate various approaches to managing whiteflies. All treatments were replicated four times and arranged in a randomized complete block design.

Zucchini was planted on 23 Aug. 2019. The row cover was installed immediately after planting and removed on 24 Sep. 2019. There were four treatments [two plastic mulches (white and black) and two row-cover treatments (with or without row cover)]. Plastic mulches were from Berry Plastics (Evansville, IN). The mulches were made from low-density polyethylene, 1.52 m wide, 25 μm thick, with a slick surface. The row cover was white (3 m wide, 18.7 g·m⁻², and 95% light transmission; AgroFabric Pro19; Seven Springs Farm Supply, Check, VA) and was laid in the field immediately after planting.

Zucchini was planted on 26 Aug. 2019. There were six treatment combinations [3 plastic mulches (black, gray-on-black, and white-on-black) × 2 kaolin particle clay treatments (treated and untreated)]. The plastic mulch (gray-on-black) was from Pliant (Chesterbrook, PA) and had the same parameters as the others. The kaolin particle clay [‘Surround^® WP’ (Novasource, Phoenix, AZ)] was applied on 4, 12, and 17 Sept. 2019 at 112 kg·ha⁻¹. Leaf temperature and mulch film temperature were measured (3 readings per plot) with an infrared thermometer at 1400 HR on 4 Oct. 2019.

Zucchini was planted on 30 Aug. 2019. Plants were grown on low-density white-on-black polyethylene plastic mulch. There were three treatments which were liquid dish soap [2% (Dawn dish soap, Procter and Gamble)], neem oil [1% (Georgia Organic Solutions, Blakely, GA)], and the control (water). Soap, neem oil, and the control were applied on 18 Sep. 2019. This trial was repeated in the fall of 2020 (planted on 12 Aug.). Treatments (soap, neem oil, and water) were applied on 21 and 28 Aug. and 8 Sep. 2020.

Fruits were harvested (10 plants per plot) and graded as marketable and nonmarketable according to the standards of the United States Department of Agriculture (Anonymous 2016). Nonmarketable fruit included discoloration, mechanical injury, and damage caused by insects and diseases.

Except for the whitefly index, data were analyzed using the General Linear Model from SAS (ver. 9.4, SAS Inst., Cary, NC). Means comparisons were performed using Tukey's test (P < 0.05). The WFindex was analyzed using a logistic regression model for the ordinal response variable. Treatment effects were tested by comparing the odds of whitefly index values in different treatment groups. Instead of mean comparisons, odds ratios were used to detect the differences between groups of treatments. A significant difference in the WFindex between treatments was concluded when the 95% confidence interval of the odds ratio did not include 1. The analysis was performed by LOGISTIC SAS procedure.

Plastic mulch color had no effects on plant FW or WFindex and had an inconsistent impact on fruit yield. While in Trial #1, there were no significant effects of plastic mulch color on fruit yield, in Trial #2, marketable fruit number and yield were reduced on black plastic mulch by 23% and 20%, respectively, compared to white mulch. The reduced fruit yields are probably because of supraoptimal root-zone temperature conditions under black mulch during the fall season in Georgia, as previously reported for tomatoes (Díaz-Pérez & Batal 2002) and other crops. Plastic mulch color did not affect the WFindex, suggesting that root-zone temperature did not influence whiteflies.

The WFindex was reduced in plants that had been covered. The low presence of whiteflies did not increase plant FW or fruit yield. Plants under row cover may have enhanced plant growth and reduced low insect populations (Acharya et al. 2020). However, the effect of row cover on the enclosed environment depends on net composition, covering method, and season. During the summer, a cover composed of polyethylene terephthalate increased the air temperature (Hamasaki 2013).

The benefits of particle clay are inconsistent in the literature. In the present study, particle clay reduced the WFindex. Our findings agree with studies showing that particle clay controls several insects (De Smedt et al. 2015; Glenn & Puterka 2002; Hidayat et al. 2013). Our results showed that particle clay decreased leaf and mulch surface temperatures. The diminished leaf temperature resulted from the reduced infrared radiation captured by leaves covered with particle clay. Under the high ambient temperature conditions, the decreased leaf temperature may have improved fruit growth, leading to increased fruit yields. Particle clay increased marketable fruit number and yield but did not impact plant fresh weight. In bell and jalapeno peppers (Capsicum annum L.), particle clay did not affect leaf gas exchange, the incidences of Phytophthora blight (Phytophthora capsici) and Pythium spp., and fruit yields (Russo & Díaz-Pérez 2005).

Neem oil and soap decreased the WFindex in both 2019 and 2020. This reduction in whitefly populations explains the increased zucchini fruit number and yield in 2019. In 2020, however, there were no differences in fruit number and yield among insecticidal treatments. The impact of whiteflies and viral diseases on zucchini may vary yearly due to interactions with environmental factors (personal observation). Neem oil is efficacious for thrips (Thysanoptera) control in mango (Mangifera indica L.) and can be used in combination with imidacloprid (Aliakbarpour et al. 2011). In corn, neem extract was as effective as fipronil in controlling Diabrotica speciosa larvae (Boiça et al. 2017). As part of the IPM program that included chemical pesticides, neem oil effectively controlled pests and increased tomato fruit yields (Boiça et al. 2007). In zucchini and tomato leaves, detergents caused more than 85% mortality of sweet potato whitefly compared to water (Butler et al. 1993). Detergents, soaps, neem oil, and other plant-derived oils applied on the undersurface of leaves provided adequate control of whitefly on cotton (Gossypium) (Puri et al. 1994).