Abstract

Sudden increase in numbers ("outbreak") of the southern chinch bug, Blissus insularis, associated with severe turf damage, seems to follow heavy fertilization of St. Augustinegrass, Stenotaphrum secundatum.  The objective of this study was to test the relation between fertilization practices and southern chinch bug outbreak.  In one field experiment, fertilization factors were rate (0, 2.5, 5.0, and 10.0 g N m^-2 month^-1) and source of N (ammonium nitrate; isobutylidene diurea [IBDU]; and Milorganite^R). In the early stages of outbreak, fertilization practice was associated with chinch bug population.  Nitrogen sources differed (P < 0.05) in their effect; chinch bug densities were increased relative to nonfertilized plots by 65%, 28%, and 0% for ammonium nitrate, IBDU, and Milorganite^R, respectively.  N application rates of 5.0 and 10.0 g N m^-2 month^-1 increased chinch bug densities 45% and 49%, respectively, compared with nonfertilized plots (P < 0.05).  In a second field experiment, involving primarily ammonium nitrate fertilization, split plots of multiple genotypes showed 9% higher canopy damage at high (14.7 g N m^-2 yr^-1), compared with low (4.8 g N m^-2 yr^-1) fertilization rates.  During outbreak months 1 through 4, the progressive increase in chinch bug damage was parallel in high and low fertilization split plots, but damage under high fertilization was greater due to earlier damage initiation.   Plots in both experiments were small enough (0.9 x 0.9 m and 4.6 x 6.7 m) that chinch bug densities must have been confounded by compensatory migration of chinch bugs across adjoining plots.  While early population regulation effects (e.g., reproduction, eggs per female per week; and frequency of successful instar development) may explain the response of chinch bugs to high fertilization with readily available N, in such small-sized plot areas preference may be a plausible mechanism.