Hi I'm Laura Ingwell Specialty Crops Extension
Entomologist with Purdue University and I am here today at the Purdue Student Farm just
off campus to talk to you about seedcorn maggot and onion maggot. So I am standing here in a field that was
transplanted 10 days ago with some onion transplants. Prior to this year the farm has not grown
produce in this field it was full of cover crops and so prior to planting this year there
was a little bit of cultivation and mixing in of that organic matter but we were surprised
to find that within 6 days of transplanting we saw some severe damage. Onion maggot and seedcorn maggot are two closely
related species both belonging to the genus Delia. The adults resemble house flies. In our region, the flies survive the winter
as pupa in the soil. When spring comes and the soil warms and growing
degree days accumulate the flies emerge from the soil and locate suitable host plants to
oviposit their eggs on. Things that are attractive to these flies
are high levels of organic matter in the soils. The flies cue in on the smells associated
with organic decomposition and find those as a suitable place to lay eggs. So some of the factors that contributing to
the infestation that we are seeing in this field include that high organic matter in
the soil which we are aiming for for good crop production and also fluctuations in the
weather that we are experiencing. So right now it is first week of May and we
have seen some days in the 70's and 80's F but more typically it has been sort of a cool,
wet spring with warm days scattered here and there. So in that situation the larvae, the eggs
of these flies, thrive in the cool soils. There are a variety of factors that could
contribute to wilting especially after transplanting since these onions came out of the greenhouse
they didn't have enough time to harden off so we were anticipating a little bit of transplant
shock. But when that wilting was so severe we decided
to investigate to go down a little further. What you want to do is dig around the soil
to investigate the plant and roots for any suspected damage. So they start out as eggs in the the soil
and as those eggs hatch they feed on the roots then they move to the onion tuber. When we have young transplants like this they
are susceptible to this type of maggot feeding and lead to plant death. As plant get bigger later in the season onion
can generally tolerate this type of feeding but you may see some aesthetic damage or deformation
on the bulb but the plants themselves survive. The onion maggot species is fairly specific
in its host range and specializes on Allium so that includes onion and garlic and other
allium crops. However, the seedcorn maggot has a much broader
host range and you can find it feeding on corn and moving into allium and onion crops
where we see a little overlap. In Indiana often here about seedcorn maggot
affecting untreated sweetcorn that is planted and young melon transplants in the field when
we have cool, wet springs can see damage from that. There are a few cultural strategies we can
implement to minimize the amount of damage caused by these maggot, seed feeders. One of those revolves around organic matter,
which is a bit conflicting because we want to increase our organic matter in our fields. So some of the others include monitoring the
development of the insects themselves and delaying planting times so we miss the peak
emergence and flight of the adults in the spring. So we can do this by tracking the development
days over 40F in which the flies are accumulating and developing in the soil and we can delay
our planting or transplanting to miss that. If you have a fairly small farm you could
plant early but use a protective cover over the young transplants to block the flies from
being able to oviposit on the young plants. The flies themselves have 3-4 generations
per year but like I mentioned earlier as the crops get older and bigger they can tolerate
that damage much better than young transplants that can become killed. The most efficacious way to control these
pests through the application of seed treatments so this is applying the insecticide to the
outside coating of the seed and when that seed is directly transplanted into the ground
the chemical moves into the growing tissues therefore when you have a new emerging seed
that is very attractive to the seed feeding maggots it is protected by that insecticide. However that strategy has to be used prior
to any sort of monitoring to see if the flies and the pests are going to be there. So an alternative way to use chemical control
is to use infurrow drenches so this is a similar mode where you put the chemical in the furrow
in the line when you are transplanting and the chemical is systemic and moves into the
plant at that time and protects that young tissue. But again, you usually have to make that determination
at the time of transplant but looking at the accumulation of degree days may help you predict
the abundance or emergence of flies in relation to when you are planting those seedlings. Foliar applications are not efficacious because
this insect is underground in the roots and soil and too it is not efficacious to try
to treat the adults. So really we have to be preemptive in our
management strategies and monitor and think about crop placement and crop timing to minimize
the damage from these pests.
