Is mineral balance beneficial? Gypsum fertilization affects European corn borer development and insect damage to corn

Mineral supplementation can benefit nutrient-deficient crops, but the plant mineral composition can also affect the insect pests that feed upon them.  Gypsum (Ca₂SO₄) is a common fertilizer used to improve soil aggregation and increase calcium and sulfur concentrations.  It is unknown how regular gypsum additions may affect plant-insect interactions.  We conducted a field study in 2013 using 8 randomized corn plots in Wisconsin, organically farmed in a corn-soybean-alfalfa-alfalfa rotation since 2008.  Plots have been fertilized with manure only (standard organic, or STDO) or manure+gypsum (mineral balanced plots, or BAL).  Leaves from corn plants at two growth stages (V7 and blister stage) were collected and analyzed for mineral content.  Plants were infested with European corn borer (ECB) egg masses; the eggs were either covered immediately with a mesh bag (no predation) or exposed for 2 days then covered (predation).  After at 10+ days we destructively sampled the plants and measured dry mass (V7) or proportion of corn kernels damaged (blister stage).

At the V7 stage, BAL plants had higher sulfur concentrations than STDO plants. ECB performance decreased when eggs were exposed to predation, but predation was unaffected by soil fertilization.  Development time and ECB mass decreased at high larval densities in STDO plots, but not in BAL plots.  Plant biomass was unaffected by soil fertilization whether ECBs were present or absent.  At the blister stage, BAL plants had higher sulfur and calcium concentrations than STDO plants. ECB survivorship and development were unaffected by predation or soil fertilization treatments at the plot level, but kernel damage was reduced in BAL plots at high ECB densities.  Path analysis at the individual plant level showed that plant calcium and sulfur concentrations indirectly affected yield loss via their effects on ECB density.  Our results suggest that mineral supplementation may benefit early season insect pests, but may negatively impact late season pests and impart some protection to maturing corn.