Abstract

1
Cutworm Control on Pacific Northwest Mint Holly
Ferguson, Ronald Wight, Timothy Waters, Douglas
Walsh Prosser, Washington
Objectives Test candidate insecticides for
efficacy on pest caterpillars Develop
recommendations for application of acephate
through center-pivot irrigation system
Study spotted cutworm biology and behavior
Abstract A complex of lepidopteran pests infests
mint fields in the Pacific Northwest. Identified
pest species include cabbage looper
(Trichoplusia ni), alfalfa looper (Autographa
californica), and variegated cutworm (Peridroma
saucia). However, our research has determined
that early-spring feeding by spotted cutworm
(Amathes c-nigrum) has the greatest negative
impact on mint oil yields. A. c-nigrum
overwinter as 2nd through 5th instar larvae in
plant debris and soil directly beneath dormant
mint. Feeding commences as the mint plants break
dormancy, typically in mid-March. Our research
indicates that treatment is necessary when A.
c-nigrum populations exceed a density greater
than 1.5 larvae per ½ m2. Since 1999 we have
monitored mating flights of A. c-nigrum and have
determined that there are two complete
generations per year. It is only the
overwintering generation that causes economic
damage to mint crops in the Pacific Northwest
east of the Cascade range. Insecticides have
been field-evaluated over the past several years
to determine their potential for controlling
early-spring A. c-nigrum on peppermint.
Traditional insecticides including acephate,
chlorpyriphos, and lambda-cyhalothrin have proven
effective at controlling A. c-nigrum, whereas
newer insect growth regulators and stomach
poisons are not effective. We speculate that the
IGRs and stomach poisons are not effective under
PNW cold spring conditions.
Applications were made at 40 gallons per acre
Amathes c-nigrum trap catches from 3 locations in
south-central Washington State. Two complete
generations were observed annually.
Introduction A complex of lepidopteran pests
infest mint fields in the Pacific Northwest.
Identified pest species include cabbage looper
Trichoplusia ni, alfalfa looper Autographica
californica, and variegated cutworm Peridroma
saucia. However our research has determined that
early-spring feeding by spotted cutworm, Amathes
c-nigrum, has the greatest negative impact on
mint oil yields. A. c-nigrum overwinter as 2nd
though 5th instar larvae in plant debris and soil
directly beneath dormant mint. Feeding commences
as the mint plants break dormancy, typically in
mid-March. Our research indicates that treatment
is necessary when A. c-nigrum populations exceed
a density greater than 1.5 larva per ½ m2. Since
1999 we have monitored mating flights of A.
c-nigrum and have determined that there are 2
complete generations per year. It is only the
overwintering generation that causes economic
damage to mint crops in the Pacific Northwest
east of the Cascade range.
Moths per trap

• Summary
• Good control of cutworms was obtained using
  chlorpyrifos, acephate, l-cyhalothrin, spinosad,
  and thiamethoxam.
• Comparable cutworm control was obtained with
  chemigated acephate and indoxacarb (2002).
• From pheromone trap data, two generations of
  spotted cutworms were observed annually the 2nd
  overwintering generation causes damage the
  following spring.

Acknowledgments Washington Mint Commission Mint
Industry Research Council Washington State
Commission on Pesticide Registration