Grain Yield is Not Impacted by Early Defoliation of Maize: Implications for Fall Armyworm Action Thresholds
Carlos A. Blanco, Gerardo Hernandez, Giseli Valentini, Maribel Portilla, Craig A. Abel, Paul Williams, Urbano Nava-Camberos, William D. Hutchison, and Galen P. Dively
Abstract. The fall armyworm, Spodoptera frugiperda (J.E. Smith), now is arguably the most important global insect pest of maize, Zea mays L., in the world. Maize growers in the Americas have battled the pest for centuries, and control recommendations have been adapted for Africa and Asia, based on contrasting results of the impact on yield when the pest infests young maize plants. Important control decision-making tools such as action thresholds, or economic thresholds, are not completely developed to control fall armyworms, and insecticide applications are still recommended at low levels of infestation on young plants. To further assess the damage-yield relationship for fall armyworm, we manually removed 0, 33, and 66% of foliage when maize had 1-2 (V1-V2), and 3-4 (V3-V4) fully developed leaves. The amount of defoliation did not reduce maize yield potential when compared with nondefoliated plants, regardless of the defoliation timing: V1-V2 or V3-V4. Fertilizing defoliated plants significantly yielded more grain than non-fertilized plants, and these obvious results showed that smallholder maize growers that can afford investing in either fertilizer or insecticide will benefit more from the former. Our results add to the number of reports that indicate young maize plants can compensate for large amounts of defoliation without reducing yields.
Introduction
Defoliation of maize, Zea mays L., by abiotic and biotic factors during early developmental stages produced contrasting impacts on grain yield. Researchers evaluated natural herbivory and tried different techniques to mimic gastropod, arthropod, and hail damage, the most frequent cause of defoliation in young maize
plants. Hail damages maize, and when it occurs during early developmental stages, yields have not been negatively or consistently impacted (Klein and Shapiro 2011, Battaglia et al. 2019, Thomason and Battaglia 2020). Among numerous arthropod pests of maize, the black cutworm, Agrotis ipsilon Hufnagel; beet armyworm Spodoptera exigua (Hübner); and fall armyworm, Spodoptera frugiperda (J. E. Smith); are the most common defoliators of early-stage maize in the Americas, with several control tactics developed for each pest (Purdue University 2009, Blanco et al. 2016, Capinera 2020, Ostlie and Potter 2021).
There is wide discrepancy among studies designed to quantify herbivory and yield impact of fall armyworms during early-stage vegetative maize (V1- V6, Abendroth et al. 2011, ISU 2022). Several studies demonstrated substantial variability in yield impact, including 12-100% potential yield reduction when the pest feeds on early stages of maize (Cruz and Turpin 1983, Willink et al. 1993, Dal Pogetto et al. 2012, Sunil Kumar et al. 2020, Deshmukh et al. 2020). By contrast, others found that fall armyworm does not cause significant or negative effect on yield (Morrill and Greene 1974, Andrews 1988, Marenco et al. 1992, Thomason and Battaglia 2020, Babendreier et al. 2020). Crookston and Hicks (1978) reported as much as 100% defoliation of V1-V4 maize might even increase grain yield potential, suggesting a compensatory response (Pedigo et al. 2021). Recently, Overton et al. (2021) reviewed seven published articles regarding infestation and negative effects by fall armyworm on maize yield. Unfortunately, they concluded the relationship was unclear between fall armyworm herbivory and yield loss during early plant growth stages.
Protecting young, vegetative maize by applying insecticides against infestation by fall armyworms produced variable results. Several applications of insecticides at early vegetative maize stages resulted in doubled grain yield (Deshmukh et al. 2020), but other studies showed no significant increase in grain yield (Morrill and Greene 1974, Lima et al. 2010, Sunil Kumar et al. 2020). Knowing when to control fall armyworms requires understanding its potential impact at different stages of maize development. Unfortunately, according to Prasanna et al. (2018), ‘in practice, true economic thresholds and economic injury levels have not been determined for most crops. Instead, nominal thresholds (or action thresholds), are calculated based on expert opinion and experience coupled with accurate field scouting assessments.’
Growers around the world apply insecticides to control fall armyworm larvae in early, vegetative (V1-V4) maize development (Blanco et al. 2010, 2014, 2016; ICAR- IIMR 2019, Chimweta et al. 2020). This creates a significant economic burden to growers and potentially negative impacts on the environment. In Mexico alone control of fall armyworms on maize, generally treated at V1-V4, with one to three applications amounts 3,200 tons of insecticidal active ingredient per year (Blanco et al. 2010). The recent global invasion of fall armyworm is likely to increase the use of insecticide, production costs, and environmental impacts, because government aid has already supported increased insecticide use in some regions of Africa (Hruska 2019). A substantial share of the 56 million hectares of maize grown in Asia might also be treated for fall armyworms (Yang et al. 2021). Because most maize-growing areas of the world are now under pressure by fall armyworms (FAO 2022), almost 200 million hectares (FAOSTAT 2021) could be affected by the pest. Consequently, many hectares of maize during vegetative stages of development are treated with insecticide because the practice continues to be recommended.
Discrepancy between the high cost and putative effect of insecticide on greater maize yields also requires better assessment. The impact of early defoliation in corn continues to be reinforced around the world indicating the action threshold for fall armyworms during early whorl stage is 20% (range 10-30%) of plants infested with larvae, or defoliated seedlings. Insecticide application is recommended above this action threshold. Crop consultants in some instances choose to spray maize with 5 10% damaged plants (ICAR-IIMR 2019), while other recommendations endorse to 40% damaged plants, without specifying the crop development stage (du Plessis et al. 2020).
Maize is increasing in importance throughout the world. A decade ago, about 73% of the crop-producing area was in the developing world (Prasanna 2011). A fundamental first step to meet proposed agro-ecological alternatives to control the pest (Harrison et al. 2019) is to thoroughly evaluate the validity of recommending insecticide applications against fall armyworm in early stages of maize. In this study, we researched maize defoliation with two specific objectives. We simulated fall armyworm herbivory during V1-V4 growth stages and then compared the effect of fertilization and defoliation on maize yield to evaluate the degree to which such application could assist plants in compensating for potential detrimental effect of foliage removal.
Materials and Methods
A field corn hybrid (P0506AM, P0) and four sequential generations of replanted P0506AM progeny for 5 consecutive years (F1 to F4), harvested in 2020, were planted at 74,165 seeds per hectare in plots of eight (0.75 m centers) rows, 44 m long on 12 May, 21 June, and 8 July 2021 (replicates) at the University of Maryland Research Experiment Station, Beltsville, MD. Plots were managed using current agronomic practices, divided lengthwise into two subplots, half receiving side-dress application of 45 kg/ha of nitrogen at planting, and 112 kg of N and 22 kg of sulfur per hectare. Weeds were controlled with a preemergence, tank mix application of glyphosate, atrazine, pyroxasulfone, and mesotrione, immediately after planting. Insect pests were scarce during the experiment; therefore, no insecticides were applied. The experimental field received 7.6, 12.4, 8.9, 22.3, 12.7, and 11.7 cm of monthly precipitation between May and October. Irrigation was not provided.
At V1-V2 developmental stages, 33% of foliage of all fertilized and non- fertilized plants in a single row per plot was removed with scissors by cutting 33% of foliage. In another row, 66% of the foliage was removed by cutting the leaf area necessary for 66% defoliation. At V3-V4, 33 and 66% of the foliage was removed in additional rows. The two central rows of each plot were the nontreated check, where foliage was not removed, while rows 2, 3, 6, and 7 were assigned at random for defoliation. At harvest (~20% grain moisture), a final plant population per subplot (fertilized or non-fertilized) was counted, and 40 ears from each subplot were removed by hand during mid-October to early November, weighed, and grain weight calculated per hectare.
Grain weight per hectare was, as expected, significantly greater in fertilized than non-fertilized plots (p = 0.0009). Therefore, the two treatments were analyzed separately using the same procedure. Because the research studied the effect of defoliation, a one-way ANOVA compared the effect of three amounts of defoliation (0, 33, and 66%) on yield. To seek further reduction of variance associated with the stage of development at which defoliation occurred, and the variety of maize used -- and possibly finding interactions of effects-- a two-way independent ANOVA for factor pairs defoliation-development stage and defoliation-variety, followed by Tukey's test was planned. For the pair defoliation-stage, only two levels of defoliation, 33, and 66% were considered. R software was used for the tests and exploratory analysis.
Fig. 1. Fertilized and non-fertilized maize yield of five generations under three artificial defoliation rates at V1-V2 and V3-V4 developmental stages.
Results and Discussion
The uncertainty surrounding the putative yield detriment by infestations of fall armyworms in young maize plants creates confusion among farmers that spray fields without solid knowledge of the beneficial effect of their investment in insecticide (Babendreier et al. 2020). Lack of basic knowledge has produced indiscriminate insecticide use with dubious results (Blanco et al. 2010, Harrison et al. 2019). Our experimental design addressed large amounts of defoliation of maize plants, with results indicating that 33 or 66% defoliation during V1-V2 and V3-V4 in plots of fertilized (F(2,72) = 0.011, P = 0.98) or non-fertilized (F(2,72) = 0.55, P = 0.95) maize did not reduce maize yields (Fig. 1). We used standard information contained in boxplots: lower and upper lines corresponded to first and third quartiles with the median in between, maximum, and minimum values of the set at the extreme of the whiskers, and outliers appear as open circles.
The developmental stage at which defoliation occurred (V1-V2 and V3-V4) did not significantly affect maize yield (F(1,56) = 0.043, P = 0.83), or non-fertilized plots (F(1,56) = 0.000, P = 0.99) (Fig. 2). Interactions between defoliation percentage and developmental stage were also non-significant in fertilized (F(1,56) = 0.071, P = 0.79) and non-fertilized plots (F(1,56) = 0.039, P = 0.84).
Previous reports of early-stage (<V4) defoliation produced similar results (Brown and Mohamed 1972, Mahmoodi et al. 2008, Lima et al. 2010, Klein and Shapiro 2011, Battaglia et al. 2019, Thomason and Battaglia 2020), or only 15% yield reduction (Hanway 1969). However, artificial defoliation might raise the question of whether cutting leaves once would have the same effect on yield as gradual herbivory by fall armyworms that eventually cause 66% foliage loss in a few days Accumulation of foliage loss during gradual herbivory under more natural conditions amounts to less area loss over time than instant foliage loss. Furthermore, because the apical meristem of maize is below or at ground level before it reaches V6 (Fortin et al. 1994), maize can compensate for foliar damage before it reaches the whorl stage, which indicates leaf herbivory might produce a similar response on maize than artificial defoliation. By contrast, some results showed greater yields in maize plants protected not exclusively from fall armyworm using multiple sequential applications of insecticide (Dal Pogetto et al. 2012, Babendreier et al. 2020, Deshmukh et al. 2020). The studies indicated yield increased to 100%, including use of multiple sprays when plants reached ≥V6, a developmental stage sensitive to yield decrease because, at this time, the apical meristem is at the whorl level. Because insecticides used in the experiments control multiple pests, their effect might have masked control of insects that do not produce apparent damage caused by fall armyworm on foliage (e.g., corn rootworms, thrips, aphids) but also have potential to reduce yield.
Fig. 2. Combined effect of three defoliation rates at two developmental stages of five maize generations.
The cost of multiple insecticide applications might exceed the economic losses produced by pests. The current international maize grain price is ~$150 per ton, which is close to the cost of three insecticide applications (per hectare basis). Early season pests including fall armyworms are sprayed multiple times and late-season pests require additional control (Blanco et al. 2014, 2016). Hruska (2019) calculated that a smallholder maize grower, the most prevalent producer around the world (Prasanna 2011), should not spend more than US$8.00 per hectare to make insecticidal control economically rational. Because average corn yield in the world is 5.6 tons per hectare and its price in 2019-2020 was $140 (USDA FAS 2021), $100 spent on insecticide applications should produce at least 700 extra kilos to make the cost of fall armyworm control economically feasible for growers. Unfortunately, many maize growers might not be familiar with control of fall armyworms or proper use of insecticides and might not have financial resources to invest $100/ha for insect control (Jones-García and Krishna 2021). Recent invasion by fall armyworms in Ghana cost $52/ha for control (Kwasi Bannor and Oppong-Kyeremeh 2022).
Smallholders around the world might not have access or purchasing power to invest in hybrid seed or be able to invest an additional $200 in fertilizer per hectare. In Mexico, maize yields for smallholders under dryland conditions using open pollinated varieties average 1,200 kg/ha, while commercial growers able to invest in hybrid seed, fertilizer, and irrigated fields might harvest 18,000 kg/ha (Blanco unpublished). Our results showed that 0, 33, and 66% defoliated hybrid and four non- hybrid field maize consecutive generations with fertilizer produced significantly different yields (F(4,70) = 45.54, P < 0.0001) (Fig. 3). However, interaction between maize generations and defoliation percentage was not significant (F(8,60) = 0.626, P = 0.75).
Fig. 3. Yield of five fertilized maize generations with a combined effect of 0, 33, and 66% defoliation at V1-V2 and V3-V4 developmental stages.
Therefore, if the smallholder grower would have $200 to invest either in hybrid seed, fertilizer, or insecticide, (s)he would obtain 2 tons more by planting hybrid seed without fertilizer (9 tons per hectare) (Fig. 4) or fertilizing non-hybrid seed and obtain a similar yield (8.8 tons/ha) (Fig. 3). The difference between hybrid and subsequent generations when plots did not receive fertilization is less pronounced (Fig. 4). Still, it indicates that a $200 investment in hybrid seed, rather than $5 per hectare investment in open-pollinated varieties or saved seed, would produce higher yields and more profitable return on investment (and still statistically significant (F(4,60) = 12.58, P < 0.0001), without significant interaction between variety and defoliation (F(8,60) = 0.516, P = 0.84).
Yield loss produced by fall armyworm in Africa has been estimated at ~11% (Baudron et al. 2019), while in the Americas a wide range of yield impact has been reported. Appropriate recommendations, such as described by Prasanna et al. (2018), should be followed, including that fall armyworms should not be controlled until after the V6 developmental stage. Unfortunately, insecticide treatments at low fall armyworm infestation is commonly recommended (Durocher-Granger et al. 2018, Bessin 2019) without specifying the appropriate maize development stage, while a threshold of 20% of plants infested with fall armyworms continues to be recommended (Kumar et al. 2020). As noted by Overton et al. (2021), additional research is critically needed to better define the yield-loss relationship for fall armyworm and vegetative maize, so reliable action or economic thresholds can be developed (see Nault and Shelton 2010, Pedigo et al. 2021). Because of the range of maize hybrids and open-pollinated varieties grown globally, action thresholds that are hybrid or variety specific might be needed.
Fig. 4. Yield of five non-fertilized maize generations with a combined effect of 0, 33,and 66% leaf defoliation at V1-V2 and V3-V4 developmental stages.
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