Variegated Cutworm with Entomopathogenic Nematodes
Note: 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unpublished work and should not be used as final or finished results.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.
Research Progress Report - 1993
Ralph Berry, Joyce Takeyasu, and Mark Morris
Entomopathogenic nematodes are effective in controlling many soil insect pests; however,
they do not have the same degree of success against foliar pests (Begley 1990). Adverse
environmental conditions, i.e. lack of adequate moisture, temperature extremes and
exposure to UV light, make the foliar environmental detrimental to nematodes. As a result,
entomopathogenic nematodes often fail to control foliar pests to an acceptable level
despite performing well in initial susceptibility tests in the laboratory.
Variegated cutworm larvae are susceptible to S. carpocapsae in the laboratory,
but it was unknown whether it would be as susceptible in the field. Good control of sixth
instars can be expected since they burrow in the soil preparing to pupate. Unfortunately,
by this time, crop damage has already occurred. Control of variegated cutworm should
target earlier instars but it is not clear if early instars are resistant to the nematodes
by virtue of a foliar habit. In fact, a field population would have a mixture of
developmental stages present at any given time and the question of whether to apply the
nematodes as a foliar or ground application arises. To determine the effectiveness of
variegated cutworm control with entompathogenic nematodes, two experiments were conducted
in a peppermint field near Eugene, OR.
Materials and Methods
Experiment 1: A completely randomized design experiment with eight replicates was
conducted using bottomless 5 gallon buckets ringed with Tanglefoot. On June 21, 1993, each
bucket was infested with 10 variegated cutworm larvae (late 3rd to early 5th instars) and
the foliage cut back below the top of the bucket. On June 25, 1993, eight buckets chosen
at random were treated with a 3 billion IJs/acre rate of S. carpocapsae either at the
beginning of the irrigation set (ground application of nematodes) or at the end of the
irrigation set (foliar application of nematodes). The remaining eight buckets were left
untreated. A CO2 backpack sprayer with a single nozzle boom was used to apply the
nematodes. Each bucket was then fitted with a sleeve made of "no-see-um" netting
to confine the peppermint inside the buckets. The experiment was evaluated on July 7, 1993
by examining the foliage and sifting the soil to a depth of 2 inches, recording the number
of variegated cutworm larvae recovered.
Experiment 2: An experiment was conducted under field conditions on July 16, 1993. Six
irrigation lines covering approximately 21/2 acres were randomly assigned to receive a 2
billion IJs/acre rate with either 1/8 or 1 inch of irrigation. Pre-treatment ground
searches for cutworms and loopers were taken along the irrigation lines prior to nematode
application. The predominant species present in the field was the variegated cutworm. On
July 29, 1993, ground searches were again taken. For consistency, the same person did all
the sampling. The number of larvae found on the two sampling dates were compared using the
Results and Discussion
Excellent control of variegated cutworms was achieved in the bucket trial. Both the foliar
and ground application of nematodes resulted in 94.5% reduction. On the other hand, the
field trial yielded only 43.9% and 45.8% reductions respectively for foliar and ground
nematode applications. It did not seem to matter whether the nematodes were applied to the
foliage or ground.
The results between the two experiments may be
explained by the use of different nematode rates. A higher rate of 3 billion IJs/acre was
used in the bucket trial while a 2 billion IJs/acre rate was used in the field trial.
However, another thing to consider is whether the buckets created a favorable environment
for nematode survival. In addition, whether the developmental stages used in the buckets
represented the field population and whether all developmental stages are uniformly
susceptible to the nematodes needs to be considered.
Table 2: Evaluation of foliar and ground applications of S. carpocapsae against
variegated cutworm larvae.
mean no. ¹ ( SEM)
VC larvae in 0.6 ft2 % reduction
9.1 ( 0.5) a
0.5 ( 0.3)
mean no. ( SEM)
larvae in one
¹Means followed by the same letter are not significantly different. Separation of means
by FPLSD, p = 0.05.
²1 ft² Of soil surface was examined for larvae of cutworms and loopers.