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ProCrop 2001 |
Pest Management of the Sunflower Seed Weevils
There are two species of seed weevils that are economically important in sunflowers in North Dakota:
Red sunflower seed weevil = RSSW (Smicronyx fulvus LeConte)
Gray Sunflower seed weevil = GSSW (Smicronyx sordidus LeConte)
Biology
The gray sunflower seed weevil is not as serious of a pest as the RSSW because of its biology. For egg laying, the GSSW lays it eggs during the earlier in the mid bud stage and has a lower fecundity rate (egg laying) than the RWWS. The treatment strategies used to control the RWWS will not work for the GSSW. In oilseed sunflower fields, the populations of GSSW are usually too low to justify the cost of control. However, in confection sunflower fields with the lower tolerance for insect damage and seed contamination, treatments may be necessary in the early bud stage to prevent egg laying. The larvae of the GSSW eat the entire seed which results in a hollow, brittle seeds. These seeds are often removed during threshing or seed cleaning.
The reddish-brown RSSW adults begin to emerge in early to mid-July and continue until mid-August. Newly emerged adults feed on the bracts, sunflower buds and pollen. The female RWWS must feed on pollen to initiate fertile egg laying. Usually one egg per seed is deposited and hatches into a cream-colored, legless, C-shaped larvae in about a week. The larvae consume the meat of the seed and develop until mid-August through September. Then, the larvae drop to the ground and move down into the soil (about 6 inches) for overwintering. The following mid to late June the larvae pupate for about one week. In North Dakota there is one generation a year.
Damage
The sunflower seeds are damaged by the larval feeding which lower the seed weight and the oil content. It is recommended that the larvae be allowed to exit the seed before any bin treatment. If a field is heavily infested, harvest should be delayed until most of the larvae have dropped out the seed.
Pest Management
For cultural control strategies, early to mid-May planting days will help reduce RSSW damage. However, early plantings encourage the development of other insect pests of sunflowers like sunflower stem weevil, banded sunflower moth, sunflower beetle, to name a few. Spring or fall moldboard tillage is also effective in reducing the overwintering larval populations and adult emergence in the spring.
Trap cropping consists of a field margin planted to an early blooming sunflower that surrounds the remaining field area. The earlier planted sunflowers, trap crop, concentrate the weevils in a smaller area around the edge of a field (usually about 10% of the total field area). The trap crop will attract RSSWs first, because RSSWs prefer fields shedding pollen and adult weevils must find their field from previous year's fields. The trap crop can then be treated to control weevils before they infest the rest of the field. This technique reduces the cost of insecticides and the time required for control. To be successful, it is recommended that the trap crop be planted at least 7-10 days earlier than the rest of the field.
To successfully control RSSWs with chemicals, one must answer three pest management questions:
1) What is the correct economic threshold level to justify the cost of control?
2) How many weevils are actually present in a field? Must scout field.
3) When should an insecticide be properly applied?
Economic Thresholds
For oilseed sunflowers, the economic threshold depends on the following variables:
the cost of the insecticide treatment per acre;
the market price of sunflower in dollars
the plant population per acre
Please refer to Table 1 on page 7 in the "Sunflower Seed Weevil Management" (1997). In 1998, an economic threshold of 6-7 weevil per head was used, for example.
The economic threshold for confection sunflowers is much lower due to the industry's standards to keep seed damage below 3-4%. Thus, only one weevil per head is recommended as the economic threshold for RSSW on confection sunflowers.
Scouting
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 (diehyl toluamidae) will cause the weevils to move out of any hiding spot and make counting easier.
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 RSSW. If spraying is done too early, weevils can re-infest a field requiring a second treatment.
Additional References
Berglund, D.R. 1994. Sunflower Production. NDSU Ext Serv., Ext. Bull 25,
98 pp.
Glogoza, P.A. 1993. Sunlfower and Canola Insects of North Dakota. NDSU
Ext. Serv., Ext. Entomol., 2 pp.
McBride, D.K., D.D. Kopp, and C.Y. Oseto. 1990. Insect Pest Management for
Sunflower. NDSU Ext. Serv., Ext. Bull. 28, 28 pp.
Oseto, C. 1981, Key to the Larvae Attacking Sunflowers in North Dakota.
NDSU Ext. Serv., Ext. Entomol., Circ E-708, 3 pp.
Peng, C., C.J. Brewer, L.D. Charlet, and P.A. Glogoza. 1997. Sunflower
Seed Weevil Management. NDSU Ext. Serv., Ext. Entomol., Circ. E-707, 4 pp.
Weinzieri, R., D.D. Kopp, and C. Oseto. 1991. Key to the Adults of Insects
Commonly Attacking Sunflowers in North Dakota. NDSU Ext. Serv., Ext. Entomol., Circ.
E-707, 4 pp.
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