Nitrogen fertilizer decreases survival and reproduction of female locusts by increasing plant protein to carbohydrate ratio

Abstract

1. Nitrogen limitation theory predicts that terrestrial plants should benefit from nitrogen inputs and that herbivores should benefit from subsequent higher plant protein contents. While this pattern has generally been supported, some herbivorous insects have shown preference and higher performance on low protein (p), high carbohydrate (c) diets as juveniles. 2. However, little is known about the effects on reproduction in adults. Using nitrogen fertilizer, we demonstrate that high plant p:c has negative effects on Senegalese locust (Orthoptera: Oedaeleus senegalensis) reproduction and survival in an agroecological setting. 3. For this, we measured p:c in millet plants Pennisetum glaucum that received two levels of fertilizer (high and moderate) and a control, then we caged locusts on these plants for 2 weeks. In the laboratory, we gave locusts the choice between untreated millet leaves and leaves that received one of the two fertilization treatment. 4. We found that fertilization increased p:c ratio in a concentration-dependent fashion. We counted the number of locusts alive over the course of 2 weeks and showed that fewer females survived on fertilized plants than on control plants. Females that ate plants from the high fertilization treatment laid lighter eggs. Finally, we showed that female locusts prefer unfertilized plants to plants with a high p:c. 5. We hypothesize that this pattern will apply broadly to species that have extensive carbohydrate needs, such as long-distance migrators. Because many ecological studies focus primarily on nitrogen or protein, and fail to consider carbohydrates, this study has important implications for how ecologists consider nutrient limitation of primary consumers in ecosystems globally.

Generational variation in nutrient regulation for an outbreaking herbivore

“…We also found that G3 locusts preferred millet seeds which are extremely carbohydrate biased and nutrient dense. This result is in accordance with the existing literature on O. senegalensis that describes it as being a pest of seedling millet and soft seeds (Boys 1978) and as preferring carbohydrate biased artificial diets (Le Gall et al 2020a, b) and plants ( Le Gall et al 2020c). While the weed Paspalum scrobiculatum was not readily preferred in the choice experiment to millet, previous work (Toure et al 2013, Word et al 2019, Le Gall et al 2020a) and our current survey data, all showed that at the beginning of the rainy season locusts are more abundant in fallow fields containing mainly weeds of many species than in millet fields.…”

A Review of the Biology, Ecology, and Management of the South American Locust, Schistocerca cancellata (Serville, 1838), and Future Prospects

“…Further studies on Oedaleus spp. in Senegal and China corroborate the hypothesis that high carbohydrate diets support locust migration [32,33]. Different factors influencing or determining insect outbreaks have been hypothesized including physiological [39] or top-down mechanisms [40].…”

Plant carbohydrate content limits performance and lipid accumulation of an outbreaking herbivore

“…Also, some studies have measured whole plants, while we measured the leaves, which are the primary food of locusts in the wild. Other recent studies of locust nutritional ecology have also found that the p : c intake targets for locusts tended to be lower than the p : c of available plants [18,21,35].…”