Nitrogen fertilizer decreases survival and reproduction of locusts by increasing plant protein to carbohydrate ratio
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. 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. For this, we measured p:c in millet plants (Pennisetum glaucum) that received two level of fertilizer (high and moderate) and a control. We found that fertilization increased p:c ratio in a concentration dependent fashion. When we caged locusts on those plants, we found that eggs laid by locusts that ate plants from the high fertilization treatment were smaller. We counted the number of locusts alive over the course of two weeks and showed that fewer females survived on fertilized plants than on control plants. Furthermore, with a laboratory choice experiment we showed that female locusts prefer unfertilized plants to plants with a high p:c. We hypothesize that this pattern will apply broadly to species that have extensive carbohydrate needs, such as long-distance migrators.