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Staying One Step Ahead

Wednesday, April 1, 2009
filed under: Insects

If you’re talking commercial real estate, it’s about location, location. If you’re focused on controlling seed insects in your sunflower field, it’s all about timing, timing — and timing.

That focus on timing begins with scouting. “Sunflower pests are not distributed evenly throughout a field, and fields should be checked in several locations,” point out North Dakota State University extension entomologist Janet Knodel and USDA-ARS research entomologist Larry Charlet. “Some insect pests, such as banded sunflower moth, are concentrated in areas of a field or are more abundant near the edge of a field than in the middle.”

Writing in Sunflower Production (Ext. Bulletin 25-Revised), published by NDSU, Knodel and Charlet recommend monitoring at least five sites per 40-acre field in order to collect sufficient information on the nature and extent of an insect infestation. “Sampling sites should be at least 75 feet in from the field margin to determine whether an entire field or [only] a portion . . . requires treatment,” they note. “In most cases, 20 plants per sampling site should be examined in [a] Z or X pattern.”

For the banded sunflower moth, entomologists suggest sampling in the late bud stage (R3). Banded moth adults typically appear during the mid-July to mid-August, period, with the larvae present in heads from mid-July to mid-September.

Red sunflower seed weevil adults commonly appear in late June to early July, while the sunflower (head) moth — a migratory insect — normally shows up in the Dakotas in early to mid-July.

Timely, astute scouting must, of course, be followed by timely insecticide treatments if and when economic thresholds are met. That means being proactive, as inclement weather and/or heavy demands on aerial applicators’ schedules can — and sometimes do — delay treatment past the optimum control period.

“Because of those types of difficulties, we advise growers to go on the early side for spraying,” Knodel says. For the seed weevil and the banded sunflower moth, “we typically recommend treatment at early flowering — R5.1, when you just see that yellow rim of pollen.”

The treatment window is actually a little broader for the banded moth. “With the seed weevil, the female chews a hole in the hull and lays its egg inside. So [the insecticide treatment] needs to be preventative and targeted at the adult, not the larva,” Knodel advises. “Once that egg is laid inside the seed, it’s protected from the insecticide.

“For the banded sunflower moth, you have a little more leeway,” the NDSU entomologist explains, “because the eggs are laid on the outer bracts of the developing head. Then the larva hatches and has to crawl around to the front of the head, where it feeds on pollen for a week to 10 days before going into the seed.”

While one well-timed insecticide treatment will normally control seed weevil and banded moth in oil-type sunflower, two treatments are often necessary in confection fields. That’s because of the low-tolerance industry standards for seed damage in the confections.

Here’s a recap of economic thresholds and treatment recommendations for the sunflower (head) moth, the banded sunflower moth and the red sunflower seed weevil, as outlined in NDSU’s 2009 Field Crop Insect Management Guide, which is edited by Knodel.

Sunflower Moth

The grayish-tan moth moves into sunflower fields in early bloom, depositing its eggs on the face of the flower. Damage is similar to that caused by the banded sunflower moth. Insecticide should be applied in the early flowering (R5.1-R5.3) stage.

Treatment should be considered when one to two moths are found per every five plants inspected.

Red Seed Weevil

This insect begins to emerge in early July and continues until mid-August, with peak emergence in late July. Start counting adult seed weevils when the yellow ray petals are just beginning to show. Counts should continue until the economic threshold level has been reached, or until most plants have reached 70% pollen shed.

Use the X pattern when sampling. Begin counting at least 75 feet into the field to avoid field margin effects. Count the number of weevils on five plants at each site, for a total of 25 plants. The ideal plant stage for treatment is when most of the individual plants are at 40% pollen shed. However, NDSU recommends considering treatment when three out of 10 plants are just beginning to shed pollen.

The economic threshold for oil-type sunflower can be calculated by using the following formula:

Threshold (Weevils/Head) Equals (=)

Cost of Insecticide Treatment

Divided By

(Market Price x 21.5) (0.000022 x Plant Population + 0.18)

For confection sunflower or the hulling market, treatment is recommended when one seed weevil is found per plant.

Banded Sunflower Moth

The banded moth begins emerging from the soil around mid-July in North Dakota, with peak activity normally during late July/early August. The moths fly from last year’s fields to current ones, congregating around field margins. They move into sunflower fields during the bud stage — preferring mid-bud.

Two different sampling procedures can be used to estimate field damage potential from the banded moth — one being daytime adult moth sampling and the other being egg sampling.

Adult: Sampling sites should be at least 75 to 100 feet from field margins. Use the X pattern, counting moths on 20 plants per site to obtain a total number of moths per 100 plants. Do sampling in the late bud stage (R3). If treatment is warranted, application should take place at the R5.1 sunflower growth stage (10% of head area having disk flowers that are flowering or have completed flowering).

Adult moths remain sedentary during midday hours, resting on upper or lower leaf surfaces. When sampling during the day, the decision whether to treat is based on a comparison of the mean number of adult moths in the field to the EIL for moths. (EIL — economic injury level — indicates the number of moths per head that will, if not controlled, result in seed damage with a value equal to the cost of treatment.) Here is the formula:

EIL (Moths/100 Plants) Equals (=)

Per-Acre Treatment Cost / Market Price

Divided By

Plant Population

Multiplied By

582.9 - 0.7

As an example, if the treatment cost is $8.00 per acre, the market price is $0.09/lb and the plant population is 20,000, an infestation of 1.9 moths per 100 plants will result in sufficient larvae to destroy seeds in the sunflower head equal to the $8.00 treatment cost, based on that market price and plant population.

Egg: Banded moth eggs can be counted accurately using a low-power magnifier, e.g., a head-mounted 3.5x magnifier to leave both hands free for manipulating the bud being observed. Egg counts should be made when most of the plants are at the R3 stage; however, buds should be selected randomly to avoid bias. Sampling steps include:

1. Divide each side of the field into two sections.

2. Sample the center of each section at 20 feet into the field from its edge.

3. Randomly select five buds.

4. From each bud, randomly select six bracts from the outer whorl and count the eggs on each bract.

5. Average the egg counts from the five buds, then map the average count from each site to a diagram of the field.

To calculate the economic injury level (EIL), use the following formula:

EIL =

Treatment Cost ($)

Divided By

Market Price x Plant Population x 0.00078

Insecticide treatment for banded moth is directed at the insect’s larval stage (which does the actual damage). The best plant stage at which to treat is R5.1 (pollen shed just beginning). This is when most of the moth eggs have hatched and larvae are present, but before seeds are forming. Larvae thus begin by feeding on the disk flowers, which leaves them exposed and susceptible to the insecticide.