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ISSUE 12  JULY 22, 1999

 

APHIDS INCREASING RAPIDLY IN CANOLA

    Aphid populations are increasing rapidly in canola fields. Late planted canola in the south central
counties of North Dakota appear to have the worst problem at the moment. Joe Gross, Kidder County
Extension Agent, and many others have observed that the mid to late-May planting dates have the
highest numbers of aphids.

    Aphids that infest canola are the Green peach aphid, Cabbage aphid, and the Turnip aphid. The
aphid colonies cluster at the tops of the stems causing wilting and cessation of flowering. In most
cases, individual or small groups of plants are infested in maturing fields and the damage is rarely
significant since the bulk of pod formation has been completed. In those cases, spraying is not
economical. Several kinds of beneficial insects, like the ladybird beetle and lacewing, feed primarily
on aphids. Their populations increase as aphid populations increase, usually in numbers sufficient
to control the aphids.

    The big concern, however, is in later planted fields. Green peach aphid are very difficult to control
because they tolerate most of the insecticides currently available. Insecticides labeled in canola are:
methyl parathion, 6,3-ethyl-methyl parathion, endosulfan (Thiodan and Endocide), and most recently
bifenthrin (Capture). Methyl parathion has been used in Georgia on winter seeded canola to control
aphids; endosulfan has not been a strong product on them in local potatoes. If treatments are made,
labels require that honeybees be protected by limiting treatments to evening hours, 3 hours before
sunset, a time when bees should no longer be foraging in the fields.

    There are no established thresholds for this problem. Researchers from Georgia indicate that aphids
can cause a 10 to 30% yield reduction when infestations reach populations averaging 1 to 2 aphids per
leaf. This threshold guideline does not fit circumstances observed in our fields.

    If aphid control is attempted, I would suggest using the methyl parathion at this time. Follow-up
treatments with a field scouting to evaluate success and to determine if/when aphid populations increase
again. Treatments will eliminate the abundant numbers of predators that are moving in to feed on the
aphids. Without the predators and the natural control they offer, aphid populations could increase rapidly
after an insecticide is applied.

    These treatments will be risky from the standpoint that population decrease of the aphids may not reach
levels we desire because of the poor control expected for green peach aphid; with few predators, the aphid
population could rebound quickly without any natural controls in the field; and, impact on yield is not
well understood.

 

ASTER YELLOWS IN FLAX

    Aster yellows, a mycoplasma-like organism, can be transmitted to flax, canola, sunflower, carrots,
potatoes, and some weeds by the six-spotted leafhopper (a.k.a., aster leafhopper). Current concerns
have been raised about transmission and symptoms appearing in flax. The disease occurs annually, but
usually at levels below economic concern. Late seeded flax is at greater risk to infection. Many flax
researchers have commented that infections often are concentrated on field margins, but in smaller
research plots the infections are throughout due to alleyways and more plot edges.

    Unfortunately, there isn’t a recommendation for controlling the leafhopper to prevent the transmission
of the organism. Chemical control has not proven to be effective at preventing the appearance of aster
yellows. In literature available from the Flax Council of Canada, there is reference to a major epidemic
that occurred in 1957 which resulted in severe yield losses. Beyond that, information is sketchy.

    Symptoms of aster yellows in flax include the yellowing of the top part of the plant, malformation of
flowers, and stunted growth. The flowers appear as small, yellowish green leaves. Diseased flowers are
sterile and produce no seed. The severity of the disease depends on the growth stage at infection and the
number of insects that are carrying the organism.

    Management of aster yellows in flax is accomplished through early seeding to avoid the migrating
leafhoppers. Normally, infected leafhoppers arrive in the region in late June. This year, leafhoppers were
abundant by late May. As their populations increase and aster yellow infected plants become available,
more of the population becomes infected and capable of vectoring the organism. Unfortunately, chemical
control has not been recommended in the past because of lack of success in effecting the outcome at
season’s end.

 

EUROPEAN CORN BORER UPDATE

    Moth captures continue at many of the blacklight trapping sites around the state. Numbers have generally
been low. Egg masses were found on about 10% of plants in the Dickinson area by Jan Knodel, NDSU
Plant Protection Specialist. Some growers have indicated there intentions to spray some irrigated corn in
the east central counties. In those cases, larvae are being found near threshold levels of 3/4 larvae per stalk.

    Watch corn carefully for the next two weeks to determine infestation levels developing in fields. Most
undetected infestations have occurred because scouting was stopped one week too soon. Generally, egg
masses and larvae are at low levels. If egg masses are detected on 20 to 25% of the plants scouted,
significant infestations can develop. I believe we are still in some phase of emergence of our single-generation
type of corn borer. Emergence cages in several locations in the Red river Valley continue to have moths
emerge from last years stalks.

 

 SUNFLOWER INSECT UPDATE

    Sunflower midge have been emerging over the past two weeks. Dr. Gary Brewer, NDSU Entomologist,
reports that populations appear to be smaller than last year. Some infestations could result in deformed heads,
but widespread problems are not anticipated at this time.

    Banded sunflower moth is active in the area and beginning to lay eggs on the bracts of developing buds.
Check grassy areas along field margins to detect whether BSM are active in your area. The eggs are laid on
buds and the larvae will move into the heads later to feed on developing seeds. Treating early in the flowering
stage has proven the most effective at controlling and preventing losses from this caterpillar. If scouting for
BSM at dusk, a reasonable treatment threshold would be one moth per two plants. If attempting to scout
during the daytime, finding about 4 moths per 100 plants sampled would indicate the potential for economic
infestations of larvae to develop.

    Some reports of Sunflower bud moth activity have come from around eastern North Dakota. The first
generation of this caterpillar pest often feeds at the base of the leaf axils. In the second generation, the larvae
are feeding behind the bud or flower head. The entrance hole formed by the larvae is easily seen due to the
black frass that plugs the hole. No treatment thresholds have been determined for this insect.

    No reports of Red seed weevil at this point. Populations of this pest have been low throughout the 90's.
Last year, isolated fields in the Red River Valley and southeast North Dakota required treatment, but these
fields were the exception rather than the rule. Treatment thresholds for RSW are going to be about 9 to 10
weevils per head in oilseed and 1 per head in confection flowers.

Phillip Glogoza
Extension Entomologist


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