Swede Midge (Contarinia nasturtii Keiffer)
Introduction
The Swede midge, an invasive agricultural pest (also known as the cabbage
crowngall fly and cabbage gall midge) was first detected in
New York in 2004 in
Niagara
County. Although the insect is a native of Europe
and southwestern Asia, it is believed the midge was introduced into NY from the
Canadian
province of
Ontario where it was first found on broccoli in
1996. By the end of 2007, the Swede midge had been confirmed in 12 NY counties
(Allegany, Chenango, Franklin, Herkimer, Jefferson, Livingston, Onondaga,
Otsego, Rensselaer, Steuben,
Suffolk, and Yates).
Biology
This species is a small (1.5 - 2 mm), light brown fly that is indistinguishable
from many other midges except by an expert entomologist. Adult midges emerge in
the spring from pupae that have over-wintered in the soil. Adult flies mate soon
after and females search for suitable host plants. Each female can lay about 100
eggs during their one to five day lifespan. The females lay their eggs on the
growing point of young plants. Larvae hatch from the eggs after a few days and
begin to feed in groups on the growing plant tissue. Larvae complete their
development in 7 - 21 days after which they drop to the ground and pupate in the
soil. Adults can emerge within two weeks, restarting the cycle. Depending on
temperature and length of growing season, there can be up to five overlapping
generations of Swede midge per year.
Damage
As they feed, Swede midge larvae produce a secretion that breaks down the
surface of the growing point of the plant and liquefies the cell contents,
resulting the formation of leaf and flower galls and a misshapen growing point.
Damage caused by Swede midge larvae feeding results in distorted growing tips
and may produce multiple (or no) growing tips; young leaves may become swollen
or crumpled and leaf petioles or stems may exhibit brown scarring. Swede midges
feed only on cruciferous vegetable crops, such as cabbage, cauliflower,
broccoli, and Brussels sprouts, frequently causing severe losses. The insect
also damages canola, collard, horseradish, kale, mustard, rutabaga, turnip, and
radish.
Management
Insecticides can be used to kill adults or prevent them from laying viable eggs.
However, controlling larvae is much more difficult because insecticide would
have to enter the plant tissue upon which the larvae are feeding. Currently, the
best way to manage Swede midge damage is to limit the spread of the insect into
new areas. Adults are very weak fliers, so the primary vector of introduction is
believed to be the movement of transplants which may contain eggs or larvae, or
movement of soil which may contain pupae. Repeated working of infested soil can
reduce the number of viable pupae. Also, because adult Swede midges cannot
travel far, crop rotation using noncruciferous plants can help to reduce the
likelihood of spreading an infestation.
Based on information published by J.R. Kikkert and C.A. Hoepting (Cornell
Vegetable Program) and A.M. Shelton (Department of Entomology, NYS Agricultural
Experiment Station,
Cornell
University)