Associational Susceptibility: Effects of Cropping Pattern and Fertilizer on Malawian Bean Fly Levels

Abstract

Subsistence farmers with little access to costly pesticides must rely on cultural practices to enhance biological control, reduce crop apparency or suitability to pests, or increase crop tolerance to pests. Farmers in Central Malawi traditionally plant beans (Phaseolus vulgaris L.) with the first rains and apply fertilizer to bean only when it is grown in dicultures with maize (Zea mays L.). I used a factorial experiment with a key pest of beans to assess the relative effects of fertilizer (predicted to increase pest levels) and intercropping (predicted to decrease pest densities). Stem‐boring bean fly (Diptera: Agromyzidae) densities and their parasitism rates were compared with respect to cropping pattern, fertilizer application, sowing date, and Vaccinium diversity of beans. Together these factors address the effects of current practices (intercropping, mixed varieties) on bean fly infestation rates and those now encouraged for increased production (relay cropping, fertilized monoculture). In addition, associational resistance in this system is tested in the context of actual farming practices. In contrast to many other studies of specialist herbivores, neither the densities of bean flies [Ophiomyia phaseoli (Tryon) and O. spencerella (Greathead)] nor their rates of parasitism were changed significantly by diversifying the field with non‐host plants (bean‐maize dicultures). However, the two fly species were differentially susceptible to plant quality and parasitism, with fertilized bean plants harboring more bean fly puparia than did unfertilized plants regardless of cropping pattern. Thus, the indirect effect of fertilizing intercrops resulted in elevated pest densities. Differential plant quality has not been included in most studies of intercropping and pest incidence, yet reflects actual farming practices. Bean fly densities tended to increase in plants sown later in the wet season, suggesting that relay cropping of beans will exacerbate bean fly damage. In this study, greater yields were achieved in mixed variety bean treatments, but standard fertilizer applications did not contribute to increased bean production.

“Filling the maize basket” supports crop diversity and quality of household diet in Malawi

“…It is encouraging that we observed compatibility of costly government agricultural subsidy programs with crop diversification and dietary diversification. Specifically, the link to crop diversification may then promote environmental benefits such as pest control services and reduced fertilizer requirements (Letourneau 1995;Snapp et al 2010). However, to support gains in dietary diversity, there is evidence of the need for complementary investments in education, particularly of female heads of households, and improvement in opportunities for women to earn income.…”

“…In Malawi, maize MVs have also been promoted in conjunction with the recommendation that MVs be grown as sole crops (Letourneau 1995), although on-farm trials have shown MVs to be fully compatible with intercrop production practices (Snapp et al 2010). To the best of our knowledge, the impact of MVadoption on farm and field level diversity, and consequences for household diet, has not been the subject of previous studies.…”

Diversity and composition of phytophagous insect guilds on Brassicaceae

“…Several studies indicate that attack of plants by insects corresponds with the content in nutrients (e.g. McClure 1991; Rossi and Strong 1991;Letourneau 1995). For a broad comparative study it is very dicult to get reliable measures of the nitrogen status in each plant species.…”

The scale of landscape fragmentation affects herbivore response to vegetation heterogeneity

“…The importance of scale, which is increasingly discussed in the ecology literature (Murphy 1989;Wiens 1989;Wiens and Milne 1989;Rose and Leggett 1990;Doak et al 1992;Molumby 1995;Underwood and Chapman 1996; see review by Levin 1992), may ®nd its most practical application in studies of cropping patterns and their in¯uence on phytophagous insects. While there have been many experiments that examine the impacts of vegetation diversity on phytophagous insects (Bach 1980a(Bach ,b, 1986(Bach , 1988Andow 1983;Letourneau 1987Letourneau , 1995Perfecto 1992; see reviews by Risch et al 1983;Andow 1990Andow , 1991, none have explicitly addressed the consequences of the scale at which the diversity is deployed. I report here one of the ®rst ®eld experiments designed to examine how scale in¯uences the response of insect populations to particular aspects of vegetation patterning (see also Marino and Landis 1996).…”