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Mint Root Borer Economic Injury
Level
Research Progress Report - 1990Note: this information is considered unpublished work and should not be used
as final or finished results. It has been included in IPMP 3.0 because it may not be
available from other sources, and in some cases may include information that may not reach
final publication.
The purpose of this study was to determine what density of MRB larvae may require
treatment in the fall to prevent significant economic injury. Different aged fields were
selected fall 1989 (newly planted, 2 years old, 4 years old, 12 years old). First instar
larvae were released inside cages (9 ft) at each location at densities of 0, 0.5, 1.1,
1.7, 2.2, and 2.8 larvae/ft². Plots were harvested fall l989 and 1990 to determine fresh
hay weight and oil yield in the 2, 4, and 12year old fields only (plots were lost in the
newly planted field). Results showed that the oil and fresh hay weights were significantly
correlated with MRB density in 2- and 4-year old fields. Oil yield in the 12-year old
field was not significantly correlated with MRB density. Based on these data, the
estimated EIL for MRB ranged from 0.7 MRB/ft² in the 2year old field to 1.4 MRB/ft² in
the 4-year old field. This estimated EIL was calculated using the formula T/P/b where: T
is the cost of one application of Lorsban ($30/acre), P is the value of oil ($15/lb) and b
is the regression coefficient. It is evident that additional replicated studies must be
conducted to refine this estimate under different growing conditions and in different aged
fields.
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