Red And Gray Sunflower Seed Weevils

Anitha Chirumamilla

Red and gray sunflower seed weevils colonize sunflowers after the banded sunflower moth. The adult weevils are seen on blooming sunflowers as they are attracted to pollen as a food source. However, weevils can be found on unopened sunflower buds (R4 stage), hiding under the bracts.

Figure 1. (Above) Red sunflower seed weevil. Photo: Kirk Mundal, North Dakota State University

Red sunflower seed weevil, Smicronyx fulvus (Coleoptera: Curculionidae) is reddish brown in color, 1/8 to ¼ of an inch long and has a long snout (Figure 1). Adults are found on sunflower heads feeding on pollen because females need the carbohydrate and protein rich food source before laying eggs. Females place their eggs inside the developing seed and the larva develops inside the seed feeding on the kernel (Figure 2). Because of this internal oviposition, the seed with larva inside looks perfectly healthy from the outside as the oviposition scars heal with the development of the seed. Only part of the kernel is consumed by the larvae and frass is always present in the seed. At maturity, the larva chews a round exit hole on lateral side, approximately ½ the distance from broad end of the seed and drops to the ground. Depending on the timing of harvest, larvae may remain in the seed resulting in heating and moisture problems in seed storage. Larvae overwinter at a depth of 6 inches in the soil. Larvae pupate the following summer, usually in early July, and adults emerge through August.

Figure 2. (Above) Seed damage by red sunflower seed weevil. Photo: Kansas State University

Figure 3. (Above) Gray sunflower seed weevil. Photo: Anitha Chirumamilla, South Dakota State University

Gray sunflower seed weevil Smicronyx sordidus (Coleoptera: Curculionidae) resembles red seed weevil except for the gray body color and large size (1/8 inch) (Figures 3 and 4). Eggs are attached to the disc flowers and the larvae feeds on kernels and develop inside the seed. Unlike red seed weevil, seed damaged by gray sunflower seed weevil (GSSW) is readily visible and seeds swollen by the weevil damage protrude above the surrounding undamaged seeds. Entire kernel is consumed before the larva exits the seed and drops to the ground. The rest of the life cycle is similar to that of RSSW.

Figure 4. (Above) Mating gray sunflower seed weevils.
Photo: Anitha Chirumamilla, SDSU

Sampling

Adult seed weevils are sampled on blooming sunflower heads (R1 stage- first row of disc flowers open). Avoid field margins and walk in an ‘X’ pattern choosing 5 plants at each point (end of each arm and the point of intersection of ‘X’, total of 25 plants). Use a mosquito repellent (DEET) spray on the face of sunflower heads to force the weevils out and take the counts. As per the guidelines of the NDSU Extension, the economic threshold for red sunflower seed weevil (RSSW) in oil seed sunflowers is calculated using the following formula:

• ET for RSSW = Cost of insecticide / [(Market price x 21.5) x (0.000022 x plant population + 0.18)]
• Example: When the insecticide cost is $8 per acre, market price of sunflower seed is $0.19 per pound, and at 18,000 plants/acre the threshold is 4.5 weevils/head.

However, the threshold for confection sunflowers is much lower and spraying is recommended at 1-2 weevils /head.

Note: There is no established economic threshold for gray sunflower seed weevil as it is not considered a serious pest in sunflowers. However, threshold level for RSSW can be used for making insecticide applications.

Management

Cultural practices such as early planting dates to avoid the peak infestation and tillage to control the overwintering populations are recommended, but have their limitations in practice. Screening sunflower germplasm for resistance to seed weevils is a work in progress and once accomplished will be a very valuable tool in managing these insects. Insecticide treatment is the only practical solution that is currently available to tackle these insects. Timing of insecticide treatment is very crucial in achieving sufficient control of seed weevils. The ideal time for spraying is when the sunflowers are at 40% pollen shed. However, blooming sunflowers attract many other insects, most importantly pollinators and natural enemies. Thus, timing the insecticide treatments when pollinators are least active is highly recommended to avoid non-target effects of insecticides on pollinators.

Source : SDSU