Saline soil: bad for crops, good for spider mites (Tetranychus urticae)

Soil salinity is an increasingly severe problem for agricultural systems in North Dakota and in many places throughout the world, yet little is known about how salinity affects plant-herbivore interactions. Salinity in low to moderate levels can decrease crop health and yield, but what about the arthropod pests feeding on those plants? We investigated how soil salinity affects the two spotted spider mite by measuring its fecundity and population growth on corn and soybeans grown in soil across a range of salinity conditions. Our experimentally manipulated soil used two salts relevant to North Dakota, sodium sulfate and magnesium sulfate. We created salinity concentrations found in local fields that range from no salinity to relatively high: EC 0,2,4,6, and 8. We used these concentrations to grow both corn and soybean and performed experiments in the greenhouse. In the first experiment we measured fecundity of 3 adult female mites that were placed onto plants in clip cages for three days. Then we quantified all eggs laid. In the second experiment we measured population growth using 7 adult female mites enclosed onto one plant for 7 days. We then quantified all life stages of mites. We found that both fecundity and population size increased with salinity on each of our two crops. These results stress the importance of managing saline areas within the field, as they not only decrease crop yield, but can also serve as “hot spots” for arthropod crop pests, resulting in further problems for production.