Resource Guide for Organic Insect and Disease ... - Cornell University

ORGANIC INSECT AND DISEASE CONTROL FOR
Allium Crops
INTRODUCTION
Onions, garlic, leeks, shallots, scallions, and chives all belong to the genus Allium, as do about
20 other species of food crops used around the world. The edible portion of the plant is the
leaf in some species (e.g., scallions, chives, leeks), the bulb in others (e.g., onion, garlic), or both.
A bulb is a reproductive organ consisting of swollen leaf bases on a very shortened stem. Diseases
and insects attack the root, leaf, or the bulb, reducing the yield and marketability of the
crop or, if infested at the seedling stage, destroying it entirely. To a greater or lesser extent, all
species in the genus generally suffer from the same pests and diseases.
I. INSECT CONTROL__________________________________
ONION MAGGOT (Delia antiqua)
The onion maggot is the larva of a fly that is slightly smaller than the common house fly but
with longer legs. Each female can lay several hundred eggs at the base of onion plants. The
eggs hatch in 2-3 days, and the maggots crawl down into the soil and feed on the roots and
basal plate of the onion (Photo 1.1). Onion seedlings cannot tolerate the feeding and are
usually killed. A maggot may kill several adjacent seedlings before it completes its development.
The first sign of the problem is wilting seedlings. Larger onions that are attacked by the
second or third generation of this pest may survive but may be misshapen, invaded by disease
organisms at the wound site, or both, resulting in rotting onions. Yellow onions are much more
attractive to the fly than red onions; leeks, garlic, and shallots are less attractive than onions.
In the Northeast, there are typically three generations per year. In the spring (May in the
Northeast), adult flies emerge from overwintering pupae and search for onions. Flights can be
predicted by following accumulated base 40° F degree days. Peak flight of the first generation
is at 700 DD at a base of 40° F; peak for the second is 1960 DD at a base of 40° F, and peak for
the third is 3240 DD at a base of 40° F. In some states, DD are calculated for specific locations;
these should be used to help predict management strategies. In New York, Massachusetts, and
New Jersey, for example, see: http://newa.cornell.edu/index.php?page=onion-maggot.
Another method of monitoring flights is to bait (with onions) an inverted screen cone trap
mounted on wire legs. Its usefulness is greatest for detecting population levels in the early
season. If flights are detected before onions are planted, then floating row covers or other
management strategies are warranted. Observing the flies in the field is possible too. They are
common in the morning, especially on damaged onions, which are more attractive to the flies.
There is little an organic producer can do once the eggs are laid, which limits the usefulness of
monitoring after egg-laying has occurred.
Cultural Control:
1. Encouraging natural enemies by diversifying habitat and food sources and refraining
from the use of broad-spectrum pesticides will help lower onion maggot populations;
however, these measures will not normally provide sufficient control and must be
supplemented by other strategies. There is a parasitic wasp that attacks maggots and
a fungal pathogen that infects adults but these are generally not useful in preventing
early season injury. Important predators of eggs, larvae, and pupae include many
species of rove beetles (Coleoptera: Staphylinidae) and ground beetles (Coleoptera
Carabidae). Maintaining untilled refuge strips may help sustain predator populations.
4 Organic Resource Guide