Insects of the Month - Corn Earworm vs European Corn Borer

  Hort Snacks - September 2017
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 Corn Earworm | European Corn Borer

Corn Earworm

Causal Agent: Helicoverpa zea

Crops Affected: corn, tomatoes, peppers, plus a range of other crops

Life Cycle:

  • Consists of 4 life stages, including adult (moth), eggs, larvae (caterpillar) and pupa
  • Typically only a single generation per season in northern climates
  • Adults are tan/yellow-brown-coloured moths which are approximately 3.5-4cm (1.25-1.5in) wide (wings spread out)
    • Several dark markings are visible on the forewings
  • Adults overwinter as pupae in the soil (2-4in depth) in warmer climates and migrate north on wind patterns, arriving typically later in the season
    • Moths are night-flying moths, capable of moving long distances
  • Eggs are laid singly, typically on the corn silks, with larvae hatching within about 3 days (ranging from 2-10 days, depending on temperatures)
    • A female can lay around 1000 eggs (on average)
  • Larvae are striped, yellow/green/brown-coloured caterpillars with distinct stripes running the length of the body and tan heads
    • Larvae can reach up to 3.7cm (1.5in) in length at maturity
    • Larvae are cannibalistic, resulting in typically a single larva per ear
  • Larvae feed almost exclusively around the ear tip, entering through to silk channel to feed on the kernels for approximately 1 month
    • Larvae feed from the tip of the ear/cob downwards as they grow, typically staying within the top 1/3 of the ear
  • Fields with sequential plantings will have the most egg-laying activity on whichever plants have the most fresh silking plants
  • Eggs are difficult to detect, as they have a similar colouration to fresh silk
  • Larvae feed through the silks and into the top 1/3 of the ear, remaining in the ear, fouling it with frass
  • Secondary impact can include infection by moulds or by secondary inspects (e.g. sap beetles)
  • Damage by larvae (and larval presence) is very difficult to detect without opening the ears
  • Black light or pheromone traps can be used to detect the presence and scale of a corn earworm infestation, in order to make control timing decisions
  • Early planting can help to avoid susceptible/attractive stages when the adults arrive or when populations are higher
  • Varieties that tend towards ears that tighter, with the shuck (or outer husk/covering) tightly closed (rather than ears with a looser shuck)
  • Beneficial insects can help to reduce a population but will not provide economic levels of control
  • The application of mineral oils to the silks of each ear can control the pest in organic situations, but is very time consuming and may result in reduced marketability if consumers are repelled by the oil residue
  • Registered chemical controls must be applied regularly while fresh silks are present (to target hatching larvae)
    • Controls may be applied preventatively starting at 10 percent silking through to 90 percent of the silks being wilted down
    • Controls should be centred around the zone where ears are located, and should be of high enough pressure to penetrate to the silks
    • Apply controls in the evening, to prolong activity and increase efficacy
    • Once larvae hatch and enter the ear, they are essentially impossible to control
    Corn earworm larva
    European Corn Borer larva
    Photo by R.L. Croissant,
    Photos by Robert Spencer

European Corn Borer

Causal Agent: Ostrinia nubulalis

Crops Affected: sweet corn, snap bean, potato, pepper, eggplant

Life Cycle:
  • Adults are a 2.5 cm (1 inch) wide, light brown moth with dark wavy bands
  • Larvae are a 3 cm (1+ inch) long grey/tan caterpillar with brown spotty plates
  • Late instar larvae pupate in corn stalks in the spring and adults emerge late June to late July
  • In corn:
    • Adults lay eggs in flat masses near the midrib on the undersides of leaves
    • Eggs hatch within approximately 1 week
    • Borers feed in leaf axils or developing tassel, then bore into the main stalk
    • Larvae stay in the stalk in a flimsy cocoon in an arrested state until spring
  • May move into other crops from corn residues in field or adjacent fields where ECB was present
  • Leaves that emerge from the whorl have a row of small holes, where larvae have fed through them when they were rolled up in the whorl
  • Some midrib breakage may occur as the leaves become bigger
  • Tassels may break off due to larval boring
  • Larvae enter the stalk and developing ears of corn
    • Feeding on ears affects ear development
    • Presence of larvae in the ears or larval feeding on the ears renders them unmarketable for fresh-market sale
    • Boring into stalks results in stalk breakage
  • Most damage to ears and stalks is suggested to be due to 2nd generation strains, which typically are associated with specific regions in Canada (not Prairies)
  • Range of factors influence both population size and infestation
  • Monitor for presence of adults at susceptible stages (depends on generation type and type of crop) – includes monitoring for leaf damage, egg masses, etc.
  • Employ adequate crop rotations to non-host crops
  • Fall ploughing or spring disking can be effective in killing many overwintering larvae
  • Shredding plant residues after harvest can destroy larvae in corn stalks and stubble
  • Natural parasites and predators will kill many borers but will not usually drop populations to below economic levels where ECB is a significant problem
  • Chemical control applications should be directed into the plant whorl (pre-tassel) or the ear area (post-tassel) when damage is first observed and when larvae are abundant
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on August 31, 2017.