FROM AROUND THE STATE
ISSUE 15 AUGUST 12, 1999
Scout for Red Seed Weevils in Sunflowers
Reports of red seed weevils in early seeded sunflowers are common.
Weevil populations appear
heavier in 1999 than 1998. Many confectionary sunflower fields are above the economic threshold
of 1 weevil per head. For oilseed sunflower fields, an economic threshold of 8-9 weevils per head
is recommended by NDSU. Scouting should begin when more than half of the field is showing yellow
ray petals (R5.0) to 30% of the head shedding pollen (R5.3) and continue until 70% pollen shed (R5.7).
When sampling, choose four different sites within 75 feet of the edge of the field. This will avoid any
sampling bias -- "edge effect." Count the number of weevil for three plants per sites for a total of 12
plants sampled. Spraying the sunflower head with a light spray of commercial mosquito repellent
containing DEET (diethyl toluamidae) will cause the weevils to move out of any hiding spot and make
counting easier. Later seeded sunflower fields will probably escape the heavier pressures since the
weevils are concentrating in the earlier seeded sunflower fields.
When is the Best Time for an Insecticide Treatment?
The ideal plant stage to treat is when most plants in the field are
at 40% pollen shed (R5.4) which is
described as a ring of opened florets comprising about 25% of the head radius. However, NDSU
recommends treating when more than half of the plants in the field are just beginning to show yellow ray
petals (R5.0) to 30% of the head shedding pollen (R5.3). The reason for treating at an earlier plant
growth stage is because adverse weather conditions or busy aerial applicators may delay the actual
application. Timing is very critical for successful control of red sunflower seed weevil. If spraying is
done too early, weevils can reinfest a field requiring a second treatment.
Crucifer Flea Beetle on Canola
The new generation of the crucifer flea beetles is emerging now.
Most canola fields are blooming
to pod development and these beetles should not represent a problem to the crop. Certain areas,
like McLean and Ward Counties, are reporting extremely high populations. Flea beetles are feeding
on the green tissue of the upper seed pods which can cause small seeds and shattering during harvest.
No economic threshold exist for flea beetle during the later plant stages of canola, since most the canola
yield is from the lower pods. Flea beetles usually move into the stubble regrowth after the canola is
swathed. Some complaints are also coming in from home gardeners where flea beetles are damaging
the crucifer plants (i.e., cabbage, broccoli). After feeding, beetles move into shelterbelts to overwinter.
Janet J. Knodel
Area Extension Specialist Crop Protection
North Central Research and Extension Center