|  EUROPEAN
CORN BORER (on potato)
Introduced
into North America, the European corn borer (ECB) probably came
in "broom" corn from Hungary or Italy around 1909. It
was first identified in Massachusetts in 1917 from where it spread
west to Nebraska in 1944 as a two-year generation ('bivoltine')
strain. Corn is its preferred or primary host but it can infest
some 200 plants including dry bean, soybean, and Solanaceous crops
such as potato, pepper and tomato.
ECB
(Ostrinia nubilalis) is a major pest on corn. As a result, much
of the North American corn acreage is planted with Bt-engineered
hybrids. Bt-corn is highly effective in controlling ECB, 99% kill
as opposed to 75-80% using insecticides. One might assume that this
success in corn would lessen the amount of ECB in potato. This may
not be the case as recent observations in Nebraska suggest that
ECB may be spending more time in potato fields. The increased use
of imidacloprid may also play an important role because this active
ingredient does not affect ECB larvae (see section on control).
Dry summers may also increase the incidence of ECB in potato fields
as well (see section on population dynamics).
Description
ECB
undergo complete metamorphosis: adult, egg, larva, and pupa.
Adult
(not damaging) -- ECB are triangular shaped when at rest. They are
about three-quarter inch long along their axis. Wings from a distance
appear straw colored but male and female vary somewhat in their
coloration. Females appear creamy to pale yellow to light brown
and have a stout body. Male bodies are smaller and more slender,
and the wings are darker than females. The adult wing-span is about
one inch and, at rest, they are about a half inch wide. The outer
third of the wings are marked by two dark serrated lines running
across the wing.
 Egg
-- An egg of the ECB is about half the size of the head of a pin.
Color is white or creamy when laid and change to pale yellow and
then darken with a "black head" just before hatching.
Eggs are deposited on the underside of leaves in masses of 20 to
30 and are covered with a waxy film. The eggs in the mass overlap
giving a fish-scale type of appearance. An egg mass is one-eighth
to three-sixteenth of an inch long. A female adult lays 200 to 500
eggs during her 14-21 days at a rate of 24 to 40 per day. The eggs
hatch in three to 12 days depending on temperature.
Larva
(damaging stage) -- The larva is light gray to faint pink with a
brown to black head, and have small brown to black spots along the
sides on each segment and a faint stripe along its back. The body
is capsule-shaped and segmented with legs. They grow to about one
inch. Larvae are usually found burrowed into potato stems.
 Pupa
-- Pupae are smooth, reddish-brown, cylindrical and about a half
inch long. They are found in a chamber burrowed into the stem.
Life
Cycle
The
strain of ECB in Nebraska is mostly bivoltine, undergoes two generation
cycles but in the Panhandle the univoltine strain (single generation)
also occurs. The overwintering form is the full-grown larva found
usually in corn stalk residue. In Spring, the larva develops and
pupates as air temperatures rise above 50 F. The pupal stage lasts
two weeks at the end of which the adult emerges. The process is
slower at cooler temperatures and accelerates at warmer ones. The
adult stage lasts between two and three weeks during which eggs
are laid in the evening. Egg masses hatch after about seven days
depending on temperature. A few days after being laid, the black
head of the developing larva can be seen ("black head"
stage). Upon emerging, larvae grow for about a month during which
they feed on foliage and bore into the stem. After the month, they
will bore and settle inside a stem and pupate. The total life cycle
takes eight to ten weeks on the average. Across Nebraska, the first
generation or "brood" of adults appears in late May to
mid June and damage to potato plants appears toward the end of June.
The second-generation adults appear toward the end of July with
larval damage appearing in August. The larvae of this generation
overwinter. The first generation of the bivoltine strain is normally
considered the problem but the second generation can cause storage
damage by introducing rot pathogens if they are present in the field.
Adults of the univoltine strain appear in early July and their activity
is between the two generations of the bivoltine (2-generational)
population.
Population
Dynamics
ECB
populations are lower in dry summers because of their need for water,
after extremely cold winters due to freezing and when heavy rains
occur during hatching since the larvae drown.
Once
moths emerge, chemical treatment should begin seven to 10 days later
depending on temperature. Treatments must be applied before ECB
larvae tunnel into the stem. The young larvae feeding on the top
of the canopy and tunneling more than once may be killed along with
adults but the older larvae tunnel deep into the stem and stay there.
Scouting for the second generation on potato can be done using traps
as with the first but also looking for egg masses on the underside
of potato leaflets.
A
Possible Key to Population Movement into Potato: Since ECB prefer
corn and do not care for potato very much, why do they come into
potato fields at all? The key is irrigation and the potato's dense
canopy ability to hold moisture. The female ECB has a voracious
thirst and requires a daily recharge of moisture. Therefore, it
will leave dry corn fields at night searching for wet areas such
as potato fields. Once there, the female may decide to lay eggs,
foregoing its favorite host for a good water supply.
Scouting
and Field Detection | Monitoring
European Corn Borer Moth Flight
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ECB
activity can be detected by monitoring adults using black-light
or pheromone traps. Black-light traps are monitored by land
grant universities’ Cooperative Extension Service and often reported
on Entomology Department’s web-sites. Sex pheromone traps are specific
for the ECB male population. Refer to insert by Dr. Gary Hein, UNL
- Extension Entomologist. Additionally, moths may be "flushed
up" by walking through the field. As moth activity increases,
potato stems should be examined for entry holes made by newly hatched
larvae.
If
the number of corn borer infested stems exceeds 20 per 100 stems
sampled, an insecticide application may be warranted. In general,
the economic threshold is reported as 15% of plant stems infested.
If corn borer pressure is high, a second application may be required
7 to 10 days after the first application, depending on temperature.
Rarely are more than 2 applications needed. As long as the initial
insecticide application is made at the beginning of the infestation
episode, even if some young larvae have penetrated the plant, the
overall infestation will be reduced to a level below that at which
time the application was made. Young larvae can move around making
several holes for a week or so before tunneling deep into the stem
and staying there.
While
scouting a field, the first symptom that catches attention is a
leaf-roll appearance at the top of the plant. Upon approaching the
plant, I look to see if the whole plant has this appearance or just
the leaves of a particular stem. If only the leaves of one stem
show this symptom, I look for the hole that marks a ECB tunnel.
At the tunnel entrance, I will find the excrement of the ECB termed
'frass.' It is tan-colored and grainy like fine sand. Splitting
the stem will often uncover the larva responsible. On occasion the
pupa will be found. Since young larva, early instar, may make several
tunnels, it may not be present. Young larvae tend to feed mostly
in the upper canopy and leave their frass along the leaflet axil.
Older larvae, later instar, bore deep into the branch base.
Damage
to Potato
 There
are several types of damage that the ECB can do. The larvae burrow
into the stem eating out the pith and, in the process, also eat
the vascular tissue thereby disrupting nutrient flow and resulting
in reduced vigor and wilt. A more subtle and, possibly, the major
damage is that ECB tunnels have been associated with the introduction
of bacterial and fungal pathogens into the plant. The key one is
the bacteria Erwinia carotovora the causal agent of black-leg on
vines and tuber soft-rot. ECB has been reported to be a vector of
E. carotovora and to promote the spread of black-leg in potato fields.
Yield
losses due to ECB have been assumed due to the wilting effects of
older larvae's burrows. However, published reports from North Carolina
concluded that there was no direct correlation between ECB damage
and yield of the cultivars Atlantic and Pungo. Yield loss associated
with ECB infestation and injury has not been observed often. Yield
losses, when observed on occasion, occurred with early-season, pre-bloom,
infestation (first generation) and not with mid or late season,
after bloom, infestation. When losses were reported, early-maturing
cultivars showed over 20% stem infestation and later-season cultivars
needed a 50% infestation. Current research on the cultivars Atlantic,
Snowden and Superior support these observations (Dr. Brian Nault,
personal communications).
Quality
losses of tubers were reported as vascular discoloration and as
soft rot in storage due to E. carotovora.
Control
Broad-spectrum
foliar insecticides are effective in killing adult ECB. Chemical
treatment should begin about a week after peak moth flight in corn
and a second application made a week to 10 days later. [The peak
moth flight is monitored by black-light traps and the peak reading
in the sex-pheromone traps occur about a week later which is when
treatment begins.] Chemical effectiveness on larvae may be spotty
because of the short time that larvae repeatedly tunnel. After adults
are first detected in black-light traps, start by examining stems
for small entry holes of the young, newly-hatched larvae, first
instar. If 15% of the plants have stems with entry holes, treatment
is recommended. In-furrow systemic insecticides will NOT seriously
affect ECB in potato.
Biological
control by predators introduced from Europe is spotty. Ladybird
beetles are predators of ECB eggs as well as Col. potato beetle
eggs and its population should be encouraged.
Quick
Review
Appearance:
- Adult
-- creamy to pale yellow to light brown; triangular shaped moth;
3/4 inch long
- Egg
-- white to creamy changes to pale yellow to dark
- Larva
-- light gray to faint pink with dark head
- Pupa
-- reddish-brown and cylindrical, 1/2 inch long
Life
Cycle:
- Overwinters
as larva in corn stalk residue
- Adult
females move into potato field in search of water
Damage:
- Adult
-- none
- Larva
-- burrows into stem disrupting nutrient flow
Management:
- Monitoring
traps
- Scouting
for adults
- Biological
-- lady beetles feed on eggs, control spotty
- Foliar
insecticides against adults; little control of larva
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