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Variegated Cutworm



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[Variegated Cutworm]

[Insect Management]


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 season.

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.