Effect of maize–legume intercropping on fall armyworm Spodoptera frugiperda (J. E. Smith) incidence

Introduction

Maize (Zea mays L.) is one of the most versatile cereal crops cultivated across diverse agroecologies and is a major source of food, feed and industrial raw material worldwide. Its productivity, however, is persistently threatened by insect pests, among which the fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), has emerged as one of the most destructive invasive pests since its detection on the Indian subcontinent. The polyphagous and highly mobile nature of this pest, combined with its preference for the whorl stage of maize, allows it to cause extensive defoliation, whorl damage and yield loss if left unmanaged. Reliance on synthetic insecticides for fall armyworm management raises concerns of residue build-up, resurgence, development of resistance and adverse effects on natural enemies and the environment. Consequently, ecologically compatible tactics that lower pest pressure without relying solely on chemical inputs have gained importance within integrated pest management programmes. Cereal-legume intercropping is one such cultural practice: legumes intercropped with maize can modify the microclimate, alter host-plant apparency and volatile cues used by ovipositing moths, support natural enemies, and improve soil fertility and crop vigour, all of which may contribute to reduced pest colonisation and damage. Beyond its direct pest-suppressive role, intercropping is known to influence the morphological development of the companion cereal crop, and traits such as plant height, foliage density and leaf area index have frequently been linked with the extent of insect infestation, since a more vigorous canopy can both dilute apparency to pests and tolerate a given level of feeding better than a sparse one. However, quantitative evidence linking fall armyworm incidence with specific morphological attributes of maize under different legume intercropping geometries remains limited, particularly under the agro-climatic conditions of central India. Keeping this background in view, the present study was carried out with the objectives of (i) evaluating the effect of maize intercropped with green gram, black gram and cowpea in 1:1 and 2:2 row proportions on the incidence of fall armyworm, (ii) examining the correlation between larval population and plant height, number of leaves and leaf area index of maize, and (iii) assessing the consequent effect of these intercropping systems on maize grain equivalent yield.

Materials and Methods

Experimental site and season

The field experiment was conducted during the kharif season of 2022 at the Research-cum-Instructional Farm of Indira Gandhi Krishi Vishwavidyalaya (IGKV), Raipur, Chhattisgarh, India, under natural, unprotected field conditions that permitted uniform and adequate fall armyworm pressure across treatments.

Experimental details

Treatment details

Seven treatments, involving maize intercropped with three legumes at two row proportions and an unintercropped control, were laid out as follows:

Observations recorded

Fall armyworm larval counts and morphological observations were recorded at 10-day intervals through the crop growth cycle, with five randomly selected and tagged plants per plot used for all observations, in order to relate pest incidence with the growth pattern of the crop.

Larval population

The number of fall armyworm larvae present in the whorls and on the foliage of five randomly tagged plants per plot was counted and expressed as the mean number of larvae per five plants.

Plant height of the five tagged plants in each plot was measured 45 days after sowing (DAS), from the ground level to the tip of the fully unfurled leaf prior to tasselling, or to the tip of the tassel thereafter, using a meter scale. The mean of five plants was recorded as the plant height for that plot.

Number of leaves per plant

The fully expanded, green leaves on each of the five tagged plants were counted at 45 DAS and averaged to obtain the mean number of leaves per plant.

Leaf area index (LAI)

Leaf area index, defined as the ratio of total green leaf area to the ground area occupied by the plant (Watson, 1947) [2] , was computed from the length and width of individual leaves on the five tagged plants and averaged across plants and replications

Leaf damage and grain equivalent yield

Per cent leaf damage caused by fall armyworm feeding was recorded on the tagged plants, and maize grain yield from each intercropping system was converted to a maize grain equivalent yield (q ha⁻¹) to allow comparison of system productivity on a common basis.

Statistical analysis

Data were subjected to analysis of variance as appropriate for a randomized block design, and standard error of mean (SEm±) and critical difference (CD at 5% probability) were computed following Gomez and Gomez (1984) [1] . Simple Pearson correlation coefficients were worked out between larval population and each morphological trait, and simple linear regression equations were fitted to describe the relationship of larval incidence with plant height, number of leaves and leaf area index.

Results and Discussion

Effect of intercropping systems on fall armyworm larval population

The mean number of fall armyworm larvae recorded per five plants differed significantly among the seven treatments (Table 3, Fig. 1). The lowest larval population, 0.90 larvae per five plants, was recorded in the Maize + Cowpea (1:1) system, statistically superior to all other treatments. This was followed by Maize + Green gram (1:1) with The consistently lower larval incidence under legume intercropping, and particularly under maize-cowpea combinations, is in line with the general premise that companion legumes disrupt the visual and olfactory hostfinding cues used by oviposition fall armyworm moths, while also creating a more favourable microhabitat for natural enemies. The superiority of the 1:1 ratio over the 2:2 ratio across all three legumes suggests that a higher relative proportion and more even spatial distribution of the legume component strengthens this disruptive effect compared with wider blocks of maize rows in the 2:2 arrangement.

Effect of intercropping on morphological traits of maize and their correlation with larval population

Growth parameters recorded at 45 DAS, namely plant height, number of leaves per plant and leaf area index, varied among the intercropping systems and showed a consistent inverse relationship with fall armyworm larval population (Table 4, Fig. 2, Fig. 3).

Plant height

Plant height at 45 DAS ranged from 79.80 cm in the sole maize control to a maximum of 86.90 cm in Maize + Cowpea (1:1). A significant negative correlation was observed between plant height and larval population (r = -0.903), and the fitted regression, Y = -0.36X + 32.36 (where Y is larval population and X is plant height in cm), indicated that each additional centimetre of plant height was associated with a decline in larval numbers, consistent with taller, more vigorously growing plants being less attractive or less suitable for larval establishment and survival.

Number of leaves per plant

The number of leaves per plant was highest in Maize + Cowpea (1:1) (6.43 leaves) and lowest in sole maize (5.03 leaves).

Leaf number showed a significant negative correlation with larval population (r = -0.891), with the regression Y = -1.76X + 12.49 indicating a comparable decline in larval numbers with greater foliage density, likely reflecting both a dilution effect on oviposition moths and improved plant tolerance to feeding damage.

Leaf area index

Leaf area index followed the same pattern, ranging from 1.03 in sole maize to 1.76 in Maize + Cowpea (1:1), and was significantly and negatively correlated with larval population (r = -0.913; Y = -3.41X + 7.14), the strongest of the three associations recorded in this study. This suggests that overall canopy development, integrating both plant height and leafiness, was the trait most closely linked with reduced fall armyworm incidence among the intercropping systems tested. Taken together, the strongly negative correlations for all three traits (Table 4, Fig. 2) point to a consistent pattern in which intercropping systems that promoted better maize growth also experienced lower fall armyworm pressure, whether through improved plant vigour and tolerance, or through the concurrent microclimatic and cue-masking effects of the legume component itself.

References

1. Gomez KA, Gomez AA. Statistical procedures for agricultural research. 2nd ed. New York: John Wiley & Sons. 1984.
2. Watson DJ. Comparative physiological studies on the growth of field crops: I. Variation in net assimilation rate and leaf area between species and varieties, and within and between years. Annals of Botany. 1947;11(41):41-76.
3. Sharanabasappa, Kalleshwaraswamy CM, Asokan R, Swamy HM, Maruthi MS, Pavithra HB, et al. First report of the fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), an alien invasive pest on maize in India. Pest Management in Horticultural Ecosystems. 2018;24(1):23-29.
4. Prasanna BM, Huesing JE, Eddy R, Peschke VM, editors. Fall armyworm in Africa: a guide for integrated pest management. Mexico City: CIMMYT. 2018.
5. Altieri MA. The ecological role of biodiversity in agroecosystems. Agriculture, Ecosystems & Environment. 1999;74(1-3):19-31.
6. Andow DA. Vegetational diversity and arthropod population response. Annual Review of Entomology. 1991;36(1):561-586.