OF THE THESIS OF
Leonard B. Coop
for the degree of Doctor of Philosophy in Entomology
presented on April 16 1987,
Oregon State University
Title: Management of Variegated Cutworm in Peppermint
A pest management program for variegated cutworm (VC), Peridroma saucia
(Hübner), in Oregon peppermint was developed based on studies of pheromone trapping,
sampling methods, and economic thresholds.
Pheromone traps effectively trapped VC
males and were used to reflect development and oviposition trends. Trap height was
linearly correlated to moth catch (P < 0.001); the largest catch occurred at a height
of 80 cm.
Male moths caught from mid-May through June, the number of egg masses collected on
pheromone traps, and estimates of peppermint canopy height were used to estimate third and
fourth instar larval densities by regression analysis (r² = 0.64). A discriminant
analysis based on similar independent variables correctly placed 16 out of 18 fields into
two threshold density classes by a validation procedure.
Parasitism rates of variegated cutworm
and peppermint leaf consumption rates of parasitized and unparasitized larvae were
measured. Instars 4 to 6 consumed an average of 184 cm², equivalent to 888 mg (dry
weight) of peppermint foliage. Consumption by VC larvae parasitized by Meteorus
communis (Cresson) was reduced by 93%. Parasitism rates averaged 35.1% for instars 2
to 4 and 5.4% for instar 5.
Addition of peppermint mainstem and lateral leaves, rates of leaf senescence, leaf
specific oil yields, VC larval development, feeding behavior, feeding injury, and
parasitism rates were all simulated by a computer model to determine economic threshold
values. Significant injury occurred when fifth and sixth instar larvae were present in
early August just prior to harvest. Fields harvested later in August had higher thresholds
because of increased time for regrowth following cutworm injury. Economic threshold values
calculated from this study ranged from 1.7 to 3.0 times higher than the previously used
threshold of 0.9 larvae per 1000 cm². Larval damage units (LDUs) were used to express
individual instar damage potential (kg/ha oil per cutworm) at various times in the growing
Sweepnet samples (n = 10, 180° sweeps) were most efficient for sampling VC instars 2 to
4. Ground search (GS) samples (1000 cm² for 10 minutes) were more efficient for instars 5
and 6. Sweepnet sample means were regressed against GS sample means for each VC instar.
Efficiency of GS sampling for each instar was determined by vacuuming and searching the
soil surface sampled. Slope values from sampling method regressions were used with GS
recovery efficiency percentages to derive approximate economic threshold (ET) estimates
for instars 2 to 4 using the sweepnet method. Sample size requirements and sequential
sampling plans for each sampling method also were developed.