Monitoring adult Delia platura (Diptera: Anthomyiidae) in New York State corn fields using blue and yellow sticky cards

Corn (Poaceae: Zea mays L.) is a vital crop in New York State, with a significant impact on both grain and silage production. In 2022, New York State producers harvested over 1 million acres of corn, valued at USD $544 million (USDA/NASS 2022). Among the early-season pests that pose a particular concern for corn, Delia platura (Meigen; Diptera: Anthomyiidae) stands out due to its unpredictable nature and the limited rescue options available once seed damage occurs (Bessin 2019; Hodgson 2016; Koch and Wold-Burkness 2015; Krupke et al. 2017; Schmidt et al. 2017). Producers typically become aware of the damage when affected plants fail to germinate, leading to visible stand losses (Schmidt et al. 2017). A 10 % stand loss represents a 1–5 % decrease in yield and an economic loss of $8 to $40 per acre (Shields 2021). In New York State, the first generation of adult D. platura emerges from the soil typically by early April, followed by mating and oviposition in May. This period coincides with the planting of corn and edible beans in most areas of the state, providing an opportunity for the developing maggots to feed on these recently planted crops (NYS IPM 2024). Traditionally, D. platura has been controlled using insecticides containing neonicotinoids such as thiamethoxam and clothianidin (Grout et al. 2020; Krupke et al. 2017; Wise et al. 2014). However, with the enactment of the ‘Birds and Bees Protection Act’ in New York State (Holyman-Sigal et al. 2023), to protect pollinators by limiting the use of neonicotinoid pesticides on certain crop seeds, ornamental plants, and turf, these insecticides will no longer be a possible control method for D. platura.

Sticky traps are an important tool in the integrated pest management (IPM) strategy toolkit for a wide range of insect pests. These traps supply an early warning signal of pest presence and population density, facilitating the development of pesticide-free control strategies (Bashir et al. 2014; Epsky et al. 2008), such as adjusting planting times to lower-risk periods (Nault et al. 2011; Silver et al. 2018). Sticky trap cost-effectiveness and low labor requirements make them ideal for early detection and monitoring of D. platura adults. Blue and yellow are colors that have been shown to effectively attract D. platura in the field (Finch 1992; Vernon and Borden 1983). For this reason, we used blue and yellow sticky cards to monitor adult D. platura in corn fields across the state to make recommendations as to which sticky trap color is more effective at trapping D. platura.

In 2022, a total of 596 (298 yellow, 298 blue) commercial dry touch, lure-free, rectangular, small-sized, sticky cards (10.2 cm W × 16.5 cm L cm; Solida, Saint- Ferréol-les-Neiges, Québec, Canada) were placed in 52 corn fields within 22 counties in New York State (Figure 1A). At each corn field, two wooden stakes were placed 30 m from each other. Each stake had one blue and one yellow sticky card secured using a 6-inch clamp, with the lowest card placed 25.4 cm off the ground (Figure 1B). To ensure the traps did not interfere with corn planting, we placed them at the edges of each field. Field crop specialists placed sticky cards in the field on 19 April 2022, and replaced them weekly until 4 June or 2 July 2022, for a total sampling time of 6–10 weeks. Each set of sticky cards was labeled with the date and the trap identification number, wrapped individually in parchment paper, and mailed to the laboratory in resealable plastic bags. All bags with sticky cards were stored at 4 °C until identification. D. platura individuals (both females and males) were counted on one yellow and one blue sticky card out of the two pairs of sticky cards placed in each field at each time point. The second pair of cards was a back-up in case the first pair was damaged or got lost. We identified D. platura using the taxonomic key for Delia species (Savage et al. 2016).

Figure 1:

Spatial distribution of planted corn (yellow) and 52 sampled corn fields (black dots) in 22 counties in New York State in 2022 (A) and visualization of an individual corn field with blue and yellow sticky cards placed at the edge (B).

The effectiveness of blue and yellow sticky cards in trapping adult D. platura in the field was assessed using a negative binomial generalized linear mixed model (glmer.nb) to account for overdispersion (variance/mean > 1) in the data. The response variables were the total number of adults, females, and males per card, the predictor was the color of the card, and sites were treated as the random effect to take into account the non-independence of the repeated measurements within a site. Data analyses were conducted in R 4.1.2 (R Core Team 2021), with the glm.nb model from the MASS package (Venables and Ripley 2002).

During the sampling season, a total of 16,944 adult D. platura individuals were trapped, comprising 8,110 females and 8,834 males. The blue cards captured a mean (±1 SE) of 44.1 ± 3.93 D. platura adults compared to the yellow cards which captured a mean (±1 SE) of 12.7 ± 1.45 adults (see Figure 2A; z = −13.9, p < 0.001). Both females (Figure 2B; z = −11.5, p < 0.001) and males (Figure 2C; z = −13.9, p < 0.001) were captured more frequently on the blue sticky cards than on the yellow cards.

Figure 2:

Adult Delia platura (mean ± 1 SE) trapped on blue and yellow sticky cards between 19 April and 2 July 2022 in 22 counties in New York State. Total D. platura (A), D. platura females (B), and D. platura males (C). The numbers on top of each bar are the mean number of flies trapped on each card color. Significance at ***p < 0.001. Number of observations = 596 (298 blue, 298 yellow), sites = 52.

Delia species feeding on various crops have been monitored using different trapping methods (water pans, sticky cards), traps of varying shapes, sizes, and colors, with differences in color intensity and reflectance, in combination with or without lures, and varied orientations and angles from the ground (Finch and Collier 1989; Harris and Miller 1983; Vernon 1986; Willett et al. 2020). In this study, we used blue and yellow sticky cards without a lure to monitor D. platura in 52 corn fields covering the main areas in New York State growing corn (Figure 1A). Blue sticky traps consistently outperformed yellow traps in capturing higher numbers of D. platura, and this result is unlikely to be due to random variation, based on the high negative z-scores. Our results align with a previous study in canola (Brassica rapa cv. Tobin; Brassicaceae) indicating D. platura preference for blue sticky cards (Vernon and Broatch 1996). Our study differed from Vernon and Broatch (1996) in several aspects. First, we used commercial sticky cards instead of white cardboard painted and coated with non-toxic insect-trapping adhesive. The use of commercial cards allowed us to make recommendations that others can easily follow as the cards can be purchased and do not need to be manufactured. Second, we conducted a broader study, sampling 52 corn fields across New York State over 10 weeks, in contrast to Vernon and Broatch (1996), that sampled one canola field for 14 days. However, we did not explore the orientation of sticky cards, nor did we consider the impact of background color (i.e., brown/gray of the soil, green of the plant, or yellow of the tassels) in combination with sticky card color on captured individuals, both factors considered by Vernon and Broatch (1996). A similar study to ours, testing the use of yellow and blue cards to catch Delia planipalpis (Meigen) found that yellow cards were more effective than blue cards in trapping this species (Lasa et al. 2024). This is contrary to what we found in our study and indicates that species within the same genus can respond differently to colors, making it clear that we cannot extrapolate results even in related species.

In conclusion, we found that rectangular, lure-free, small-sized blue sticky cards resulted in much higher adult captures than rectangular, lure-free, small-sized yellow sticky cards, but a combination of shape, size, and orientation has not been tested for D. platura. In future studies, it is recommended to test different attributes of the sticky cards in combination with color to evaluate which other attributes can further increase catches of D. platura in the field.