Increasing crop field size does not consistently exacerbate insect pest problems

Abstract

Increasing diversity on farms can enhance many key ecosystem services to and from agriculture, and natural control of arthropod pests is often presumed to be among them. The expectation that increasing the size of monocultural crop plantings exacerbates the impact of pests is common throughout the agroecological literature. However, the theoretical basis for this expectation is uncertain; mechanistic mathematical models suggest instead that increasing field size can have positive, negative, neutral, or even nonlinear effects on arthropod pest densities. Here, we report a broad survey of crop field-size effects: across 14 pest species, 5 crops, and 20,000 field years of observations, we quantify the impact of field size on pest densities, pesticide applications, and crop yield. We find no evidence that larger fields cause consistently worse pest impacts. The most common outcome (9 of 14 species) was for pest severity to be independent of field size; larger fields resulted in less severe pest problems for four species, and only one species exhibited the expected trend of larger fields worsening pest severity. Importantly, pest responses to field size strongly correlated with their responses to the fraction of the surrounding landscape planted to the focal crop, suggesting that shared ecological processes produce parallel responses to crop simplification across spatial scales. We conclude that the idea that larger field sizes consistently disrupt natural pest control services is without foundation in either the theoretical or empirical record.

Keywords: agroecology; crop yield; field size; pest density; pesticide use.

Fig. 1.

Influence of field size on (A) the density (circles) and no. of pesticide applications (squares) targeted to control each of 14 surveyed crop pests (shown are mean ± SE for standardized coefficients from linear terms in GAMMs); filled symbols indicate significant effects. Subsequent panels show examples for which field size is positively (B and C), neutrally (D and E), and negatively (F and G) associated with pest severity. Specifically, panels depict (B) the density of the olive fly (B. oleae) in Spanish olive orchards (GAMM, effect of field size, n = 16,207, F = 4.83, P < 0.0001), (C) the no. of pesticide applications targeting B. oleae (effect of field size, n = 9,340, χ² = 11.0, P = 0.0009), (D) the density of the citrus red mite (P. citri) in California citrus groves (GAMM, effect of field size, n = 1,350, F = 2.92, P = 0.09 [not significant, NS]), (E) the no. of pesticide applications targeting P. citri (effect of field size, n = 2,176, F = 0.01, P = 0.92 [NS]), (F) the density of Andean potato weevils (Premnotrypes spp.) in Peruvian potato fields (GAMM, effect of field size, n = 138, F = 7.03, P = 0.0004), and (G) the no. of pesticide applications targeting Premnotrypes and Epitrix sp. (farmers target both beetle pests with the same insecticide applications; effect of field size, n = 138, F = 0.68, P = 0.41 [NS]). Panels B–G show residuals on the y axis, field size (m²) on the x axis, the smooth functions fit by the GAMM, and the 95% CIs (shaded area); each point represents a single field year.

Fig. 2.

Influence of citrus grove size on the density of predatory mites (Euseius spp.; GAMM, effect of field size, n = 335, F = 1.54, P = 0.22 [NS]). Shown are residuals on the y axis, field size (m² x 10⁴) on the x axis, the function fit by the GAMM, and the 95% CI (uncertainty in the slope value; shaded area). Each point represents a single field year.

Fig. 3.

Influence of field size on yield of (A) California cotton (GAMM, effect of field size, n = 1,236, F = 1.50, P = 0.22 [NS]), (B) Peruvian potatoes (effect of field size, n = 125, F = 1.85, P = 0.14 [NS]), and (C) California citrus (effect of field size, n = 1,647, F = 4.49, P = 0.034). Panels show residuals on the y axis, the functions fit by the GAMM, and the 95% CIs (shaded area). Each point represents a single field year.

Fig. 4.

The influence of field size on arthropod density (GAMM coefficient, β_{field size}) covaries with the influence of the same crop planted in the surrounding landscape on arthropod density (GAMM coefficient, β_{same-crop in landscape}). Each point represents one of the 15 surveyed arthropod species (linear model, r = 0.76, df = 13, P = 0.001). Symbols include Ao, Aonidiella aurantii (California red scale); Ba, Bactrocera oleae (olive fruit fly); Co, Coccus pseudomagnoliarum (citricola scale); Ep, Epitrix sp. (flea beetles); Eu, Euseius sp. (predatory mites); Ic, Icerya purchasi (cottony cushion scale); Ja, Jacobiasca sp. (leafhoppers); Lo, Lobesia botrana (European grapevine moth); Ly, Lygus hesperus (western tarnished plant bug); Ma, Marmara gulosa (citrus peelminer); Pa, Panonychus citri (citrus red mite); Pr, Prays oleae (olive moth); Pre, Premnotrypes sp. (Andean potato weevil); Sc, Scirtothrips citri (California citrus thrips); and Scu, Scudderia furcata (fork-tailed bush katydid).