Ian MacRae

Dept of Entomology

Univ. of Minnesota

NW Research & Outreach Center

If They're

Corn Borers,

Why Are

They In My

Potatoes!?

Manitoba Potato Production Days

Brandon, MB. Jan 26-28, 2016

Wingspan = 2cm

Forewing

- straw colored,

wavy line on outer

margin, dot and dash

on fresh specimens

- darker, yellowish

wavy lines more

pronounced,

Hindwing

Similar in both sexes

European Corn Borer (ECB)

• Ostrinia nubilalis

– Probably intro’d from Italy or Hungary in

broom corn, 1st found in N. America in

Boston 1917/19

– 1st found in Manitoba in 1948

– Very wide host range (~200 plant

hosts) but strongly prefers maize

Life history

• First reported in N. Am

1909, intro’d from Europe

• 200+ acceptable hosts

• 1 gen/yr in MB

• Overwinter as last instar

(larval stage) in host

stalks, plant debris,

Life history

• Pupate in the spring and

emerge as adults June

• Moths aggregate in

weedy margins, mate

and females enter field

on warm, humid

evenings to lay eggs (15-

20 on underside)

Life history

• Eggs hatch in 5-7d,

‘blackhead’ stage just

before hatch, these hatch

in 24-48 h

– High mortality at this

stage (important

point…)

Life history

• Larvae disperse, feed on

foliage for a few days (2-

10d, depends on temp)

• Enter stem @ leaf axils,

bore in and start to

excavate tunnels inside

– Entry holes noted by

brownish ‘saw-dust’ (frass)

Life history • In long summers may get

some adults late summer but

any eggs from these won’t

make it through to next year

• Tunneled stems become

weak and collapse

• Entry holes and broken stem

provide entry point for

pathogens…

Life history • Larvae feed / tunnel until fully developed

– 5 larval stages (instars)

– Full developmental time for larvae ~50

days but dependent on temp

• Overwinter in stems of infested plants

• Pupate in spring

Life history • Emerge as adults in late June – mid July

– 500-600 GDD (threshold temp >10°C)

• mate in weedy field margins

• Females enter the field lay eggs on

underside of leaves (15-20/time, total

~500)

– Degree Day models for development of eggs!

Damage to potatoes • newly hatched larvae

feed on leaves few days

before tunneling into

stems typically in the

leaf axil at the node

• Most tunneling in lower

1/3 of main stems, a

single larva can make

multiple tunnels in a

single plant

http://www.plantmanagementnetwork.org

Damage to potatoes

• Tunneling near base

of stems causes

breakage and lodging

& entry points for

stem rot pathogens

• The assoc. with

disease has

traditionally been

largest yield hit*

http://www.plantmanagementnetwork.org

*another important point…

European Corn Borer

Slide courtesy of Vikram Bisht

Slide courtesy of Vikram Bisht

Slide courtesy of Vikram Bisht

ECB injury and Stem rot

Slide courtesy of Vikram Bisht

The impact of the European corn borer on

the potato plant appears to be a function

of:

1. larval numbers

2. phenological stage of the plant at the

time of attack

3. Cultivar

4. presence of other insect pests and

diseases, and other stress factors such

as temperature, water, and fertility.

Varietal influence

• NCSU team examined varietal resistance

(or at least preference)

– In field trials, ECB damage 8X greater

in Atlantic, Superior, & Norland than in

K411-2

– Another field trial, 11x more ECB

damage on Atlantic than on NYL 235-4

Hanzlik, Kennedy, Sanders & Monks, 1997

Varietal influence

• NCSU team examined varietal

resistance (or at least preference)

– In GH choice trials, ECB deposited

more egg masses on Kennebec than

on NYL 235-4 but in the absence of

choice, no difference in egg masses

– In above trial, more ;arvae

established on NYL 235-4 than on

Kennebec

Hanzlik, Kennedy, Sanders & Monks, 1997

Damage elsewhere • ECB doesn’t cause economic damage to

potatoes in many US production areas

– North Carolina - over 50% of potato

stems may be tunneled without

significant loss of tuber yield

– Depending on timing of infestation and

variety, nearly 100% of stems can be

infested without economic yield loss,

particularly in the absence of disease

pathogens.

Damage elsewhere • ECB doesn’t cause economic damage to

potatoes in many US production areas

– Similar results found in New Jersey,

Michigan, Maine, Maryland, and Virginia

also found it difficult to detect any yield

loss in potatoes from ECB tunneling.

– natural ECB infestations rarely cause

economic damage to potatoes,

particularly in the absence of disease

pathogens.

Declining populations • ECB pops declining in eastern and

central United States.

• may involve increased use of Bt corn

Images from Plant Health Progress article:

Decline of European Corn Borer as a Pest of Potatoes

Other insecticide mortality

• At-plant insecticide applications

targeted at Colorado Potato Beetle

effective against ECB

• Applications for other insects may also

contribute to control

• Be aware, many producers in SE U.S.

tend to use these for ECB when not

necessary

– No evidence of presence

Treatment - timing • As larvae continue to tunnel in stem

through growing season, stems

increasingly weaken

– Application most effective against neonates

(Nault & Kennedy)

– Applying against older larvae ineffective

– Some reduced risk insecticides have efficacy

on older eggs, spinosad, novaluron and

indoxacarb all had efficacy if applied to egg

masses 2 days prior to hatch (Boiteau &

Noronha 2007)

Treatment - timing • As larvae continue to tunnel in stem

through growing season, stems

increasingly weaken

• But timing applications needs trapping

(pheromone/blacklight) and scouting

larvae

Hartstack pheromone trap vegedge.umn.edu/moth-data/cew-info

Trap plans available at:

http://www2.ca.uky.edu/entomology/entfacts/ef010.asp

Natural mortality • Large pheromone trap catch doesn’t

necessarily mean large larval population…

• Other mortality sources

– weather

– disease

– Predators

– parasitoids

Weather?

• Heavy precip @ egg hatch can cause

some mortality – Larvae generally don’t survive sub-freezing temps

• Low humidity, low nighttime temps, wind

with heavy rain all cause mortality to

most moth populations’ survival & esp

egg laying

• BUT long term studies don’t show

consistent relationship…

Disease • Some areas a

number of common,

general diseases of

caterpillars

– Beaveria

bassiana, a fungal

pathogen

– Microsporidian,

Nosema pyrausta

https://extension.entm.purdue.edu/pestcrop/2012/issue1/index.html

Predators • Eggs and young

larvae

– Minute Pirate Bugs

– Green lacewing

– Ladybird beetles

Parasitoids

• ~24 spp of exotic

parasitoids in N.

America attack ECB

– Most from ECB

home range

– Trichogramma

ostriniae, minute

wasp parasitizes

eggs

Scouting egg hatch

ipm.ncsu.edu

• So, trap catch and egg masses are not

reliable predictors – look for no. of ECB

infested stems

• Examine leaf axils & petioles for larvae and

entry holes

– 10 stems from 10 random sample locations

in <30Ha

– Earlier plantings at greater risk, start there

– Sequential sampling plans available

Threshold • Well established in corn

– Simple thresholds thru to worksheets

incorporating current market values

• No real damage threshold calculated for potatoes

– Not enough research?

• Some data…

– Est. threshold of 5%-10% infested stems

– Research from P.E.I. indicated that 1.2

larvae/stalk/week reduced yields of Russet

Burbank by a margin of 8-9%

Integrated pest management • IPM holds well here

• Conserve natural enemies (avoid

unneeded insecticide applications!)

– esp. prophylactic applications!

• Use scouting & available thresholds

• Wait till population levels known

– Larvae not egg counts

• Use cultural controls

– chop corn, planting time if poss, etc

limited here

Decreasing pest…

Halo effect • Non Bt corn

fields seeing

decrease in

ECB nos, as

well

• Fewer ECB to

infest colonize

fields

This decrease in insecticide use associated with

decrease in sampled insect populations

Why did it happen?

• Bad for natural enemies?

–Environmental conditions bad for

natural enemies lowers their pops

and pest escapes

–Lack of early control often allows

populations of boring insects a

‘start’

Why did it happen?

• Good year for European Corn

Borers?

–Environment favoring pest insect

reproduction (just the right temp,

humidity, etc) can allow pests to

reproduce faster than natural

enemies can kill them off…

Landscape influence on Halo

effect?

http://www.statcan.gc.ca/pub/96-325-x/2014001/article/11913-eng.htm#a4

Landscape influence on Halo

effect?

http://www.statcan.gc.ca/pub/96-325-x/2014001/article/11913-eng.htm#a4

USDA NWG, 2014

- ~80% field corn is Bt modified

- most sweet corn is not Bt modified

- In US, field & sweet corn raised in proximity

Questions??

THANKS to the

KPPA, the MPPD

committee for

inviting me here

and to DuPont

Canada for

sponsoring the

talk!

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