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Red Sunflower Seed Weevils Emerging (08/06/20)

Time to scout for red sunflower seed weevils (RSSW). IPM scouts are starting to see RSSWs on early blooming sunflowers in the southeast and south central areas of North Dakota. See the past issue #13 of NDSU Extension Crop & Pest Report for scouting and E.T. for RSSW.

Time to scout for red sunflower seed weevils (RSSW). IPM scouts are starting to see RSSWs on early blooming sunflowers in the southeast and south central areas of North Dakota. See the past issue #13 of NDSU Extension Crop & Pest Report for scouting and E.T. for RSSW.

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When is the best timing for insecticide applications? Sunflower plant stage is used to time insecticide treatment. Both bloom and flowering describe the sunflower plant when yellow ray petals are showing and pollen is being shed. It is important to distinguish between the percentage of the field in bloom from the percentage of individual plants in bloom. A field with 50 percent of the plants in bloom indicates that half of the plants are shedding pollen and the other half of the plants are in the bud stage. However, the individual plants in bloom will probably not all be at the same stage of bloom. Some plants may have just started to shed pollen and others may be at the end of pollen shed. A plant in the 40 percent bloom stage would have 40 percent of the head shedding pollen (R5.4). This would be a ring of opened florets comprising about 25 percent of the head radius. The remaining 60 percent of the florets would be unopened.

The ideal plant stage to treat is when most plants in the field are at 40 percent pollen shed (R5.4). However, we recommend that treatment be considered when more than half of the plants in the field are just beginning to show yellow ray petals (R5.0) to 30 percent of the head shedding pollen (R5.3) and the rest of the plants in the field are still in the bud stage. This difference between the ideal plant stage (R5.4) to treat and the earlier plant stage (just beginning pollen shed) is based, in part, on the fact that aerial applicators -- because of a busy schedule or adverse weather -- will not always be available to spray at the ideal stage of sunflower development. Considering treatment at the early bloom stage should allow growers a sufficient cushion of time to have their fields treated. Growers must be aware, however, that if weevil populations are high and/or spraying is done too early, a re-infestation may occur and a second insecticide application may be necessary.

Although insecticides applied to sunflower at the bud stage will kill weevils, treatments at that stage are not economical or effective because (1) seeds have not developed to a stage suitable for oviposition, (2) eggs within the weevil are not mature, and (3) adult weevil emergence is still continuing. Sunflower normally reaches the bud stage in late July at which time only about 30 percent of the weevils in the soil have pupated and emerged. Most weevils emerge from the soil by the first week of August. If growers were to spray bud stage sunflower in mid to late July, a second spray may be necessary as more weevils continue to emerge.

 

Janet J. Knodel

Extension Entomologist

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