Effect of maize-legume intercropping on the incidence of fall armyworm, Spodoptera frugiperda (J. E. Smith), and associated yield traits of maize

Introduction

Maize (Zea mays L.) is a globally vital cereal, and in India it anchors the food-feed-fodderfuel value chain, so any biotic stress that suppresses its yield has direct consequences for farm income and food security. Since its first detection in Africa in 2016 and subsequent spread to the Indian subcontinent in 2018, the fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), has become one of the most damaging pests of maize, feeding cryptically within the whorl and on the tassel and developing cob. Its high fecundity, broad host range and documented resistance to several insecticide groups make it difficult to manage through chemical control alone. Habitat management through legume intercropping is a long-standing, sustainable tool of integrated pest management: companion legumes can disrupt the host-finding behaviour of ovipositing moths, impede larval movement between plants, support natural enemies, and improve maize vigour, while also fixing nitrogen and diversifying farm income. However, field evidence quantifying how the row geometry and identity of the companion legume jointly influence fall armyworm incidence, and how this incidence relates to maize yieldattributing traits, remains limited for the Chhattisgarh plains of central India. The present study was therefore undertaken with the following objectives: • To evaluate the effect of maize intercropped with mung bean, black gram and cowpea, in 1:1 and 2:2 row proportions, on fall armyworm larval population and percent leaf damage. • To assess the relationship of larval incidence with cob length, cob girth, test weight and plot yield through correlation and regression analysis. • To determine the maize grain equivalent yield of each intercropping system and identify the most remunerative and pest-suppressive combination.

Treatment Details

The experiment comprised seven treatments combining maize with three companion legumes at two row arrangements, along with an unintercropped control, as summarised in Table 1.

Observations Recorded 2.4.1 Fall armyworm larval population

Larval counts were recorded at ten-day intervals throughout the crop cycle from five randomly tagged plants per plot and expressed as mean number of larvae per five plants.

Percent leaf damage

Leaf damage was scored visually during the vegetative stage by counting healthy and damaged leaves on tagged plants at ten-day intervals from the date of first infestation until harvest, and computed as:

Percent leaf damage = (Number of damaged leaves per plant ÷ Total number of leaves per plant) × 100

Yield-attributing traits

Cob length (cm): measured from the base of the lowermost primary rachis to the cob tip on five tagged plants and averaged. Cob girth (cm): measured at the basal, middle and apical positions of five selected cobs using vernier callipers and averaged. Test weight (g): weight of 100 randomly drawn kernels per treatment. Plot yield (kg): grain yield harvested from the net plot area.

Maize grain equivalent yield

To enable comparison of productivity across intercropping systems with differing intercrop components, the yield of each system was converted to maize grain equivalent yield (MGEY, q/ha) using the prevailing market prices of maize and the respective legume, following the standard conversion procedure.

Statistical Analysis

Data were subjected to analysis of variance appropriate to a randomized block design (Gomez and Gomez, 1984). Simple correlation and linear regression were used to examine the relationship between larval population and the recorded yield-attributing traits, and a Pearson correlation matrix was additionally computed across all treatment means to visualise inter-trait associations as a heat map.

Results and Discussion

Effect of Intercropping on Fall Armyworm Larval Population

The mean larval population differed significantly among treatments (Table 2, Fig. 1). The regular Maize + Cowpea (1:1) system, T 3 , recorded the lowest larval density (0.90 larvae per five plants), followed by Maize + Mung bean (1:1) (1.43) and the paired Maize + Cowpea (2:2) arrangement (2.13). The highest larval load was recorded in sole maize (3.86 larvae per five plants), representing more than a four-fold increase over the best-performing intercrop. Among the three legumes, cowpea consistently outperformed black gram and mung bean at both row geometries, and the closer 1:1 arrangement was more suppressive than the 2:2 arrangement for every legume tested, indicating that proximity of the companion crop to the maize row, and not merely its presence, governs the strength of pest disruption.

Effect of Intercropping on percent Leaf Damage

Leaf damage followed the same trend as larval density (Table 2, Fig. 2). Minimum leaf damage was recorded under regular Maize + Cowpea (28.00%), statistically at par with paired Maize + Cowpea (28.30%), and significantly lower than Maize + Black gram (29.00-32.00%) and Maize + Mung bean (30.80-33.00%). Sole maize suffered the highest leaf damage (42.06%), roughly 1.5 times that of the best intercrop. This pattern corroborates the hypothesis that a diversified plant canopy interferes with adult oviposition behaviour and constrains larval dispersal between host plants, thereby lowering both colonisation and subsequent feeding injury.

Correlation between Larval Incidence and Yield-Attributing Traits

Fall armyworm larval population showed a consistently negative association with every yield-attributing trait examined (Table 3, Fig. 4). The relationship was strongest and statistically significant with cob girth (r = -0.970), followed by test weight (r = -0.792) and plot yield (r = -0.672); the association with cob length was negative but non-significant (r = -0.698). Regression of larval population on cob girth (Fig. 6) indicated that each one-centimetre increment in cob girth was associated with a decline of approximately 2.77 larvae per five plants, underscoring cob girth as the single most sensitive morphological indicator of pest pressure in this trial. Leaf damage mirrored the larval population pattern, correlating positively with larval density (r = 0.814) and negatively with maize grain equivalent yield (r = -0.826), while yield traits were strongly and positively intercorrelated, confirming that healthier, less-damaged plants channel more assimilate into cob development.

Maize Grain Equivalent Yield

Total system productivity, expressed as maize grain equivalent yield, was highest under regular Maize + Cowpea (1:1) (54.30 q/ha), followed by paired Maize + Cowpea (2:2) (52.36 q/ha) and paired Maize + Black gram (2:2) (49.53 q/ha) (Table 4, Fig. 3). Sole maize was the least productive system (41.56 q/ha). The superiority of the cowpea-based systems is attributable to the combination of favourable cowpea market price and its comparatively high pod yield, in addition to the pest-suppression benefit that translated into higher maize grain yield per se. These results reaffirm that intercropping decisions in fall-armywormprone environments should weigh both direct economic return and crop-protection value rather than pest management alone.

Conclusion

Maize-legume intercropping, particularly with cowpea maintained in a regular 1:1 row ratio, significantly suppressed fall armyworm incidence, reducing larval population by more than 75 percent and leaf damage by 33 percent relative to sole maize, while simultaneously improving cob girth, test weight and plot yield. Cob girth emerged as the trait most tightly and negatively linked to larval density, offering a practical, non-destructive proxy for field-level assessment of intercropping efficacy. Economically, the same regular Maize + Cowpea system delivered the highest maize grain equivalent yield (54.30 q/ha), 30.6 percent above sole maize. These findings support the integration of close-row legume intercropping, with cowpea as the preferred companion, into locally recommended integrated pest management packages for fall armyworm in maize.

Acknowledgement

AcknowledgementsThe authors gratefully acknowledge the Department of Entomology, Indira Gandhi Krishi Vishwavidyalaya, Raipur, for their valuable suggestion, unending assistance and moral support while on the course of the investigation.

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