Volume
2 Issue
5
June 21,
2005
Aphids in Small Grains - Timing IS Everything!!
 Over
the past week there have been reports of aphids in small
grains. Mostly these seem to be Bird-Cherry Oat aphids,
the most common aphid we get in small grains in this
region. Not being of hearty Norwegian or Scandinavian
blood (well, actually not having blood in the mammalian
sense at all) small grain aphids don’t make it through
winter in MN and ND. The aphid populations infesting small
grains in this region re-establish every year from
individuals immigrating up from states west of the
Mississippi.
In
the summer, all grain aphids are female, reproduction is
parthenogenic (non-sexual reproduction, basically cloning)
and their offspring are born alive, which is unusual for
insects. Adult aphids are either winged or non-winged;
non-winged adult aphids will never develop wings (immature
aphids, called nymphs, also lack wings). The ‘job’ of a
winged aphid is to disperse and be colony founders, the
‘job’ of a non-winged aphid is to feed and reproduce,
they’re colony builders. The daughters of winged aphids
will always develop into non-winged, colony builders.
When
a winged aphid arrives in this region, it lands in a wheat
field, gives birth to a daughter who, in turn, will feed
and have more daughters (most of these will also never
develop wings) establishing a colony on plants. The
population will spread out from these infestation points
and eventually spread across a field. If the population
gets heavy enough, more winged forms will develop.
There are
4 main species that infest wheat, barley and oats:
Bird-Cherry Oat aphid, English grain aphid, corn leaf
aphid and greenbug. These 4 species can, if present long
enough and in high enough numbers, cause significant
decreases in yield. They can also vector viral diseases of
wheat including Barley Yellow Dwarf Virus (BYDV).
Aphids damage plants by sucking sap and
it’s a long term process unlike foliage feeders, like
grasshoppers, that simply remove plant material by chewing
it up. Yield loss from aphid damage depends both on the
number of aphids on a plant and the length of time they’ve
been feeding. Entomologists express this as Aphid Days
(AD) (1 aphid on a plant for 2 days is 2 AD, 2 aphids on a
plant for 2 days is 4 AD, 10 aphids on a plant for 2 days
is 20 AD, etc). But, AD is a cumulative measure, in our
previous example, 10 aphids on a plant for 2 days is 20
AD, but those 10 aphids didn’t appear overnight, the
colony had to develop, so there was some accumulation of
AD already. Basically, Aphid Days are just a way to
express both population pressure & time, the two factors
contributing to yield loss from aphids. At average wheat
prices and average application costs, wheat will suffer
yield losses equal to the cost of application roughly
around 300 AD.
Aphids generally immigrate into this region around or
slightly before tillering. This means that aphid
populations in small grains are developing by flag leaf
stage. So the best time to scout for aphids is now, at or
slightly before flag leaf. This provides enough time to
assess aphid populations and determine if a treatment is
warranted. Treatment is recommended when 80% of tillers
have aphids present (this will equate to about 12-15
aphids per tiller). It is important to sample across an
entire field as small grain aphids in this region readily
colonize anywhere in a field. Waiting until heading to
apply insecticide with fungicides targeted against FHB may
allow aphids to raise to levels where they cause yield
loss, if treatable populations are present at flag, it is
best to treat then. Data indicates that treating
post-heading does not usually provide an economic return.
Applying insecticides in the absence of insects has
obvious economic drawbacks…..
Ian MacRae
U of MN Extension Entomologist
|
