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ISSUE 14   August 21, 2008

SOYBEAN APHIDS MOVING FIELD TO FIELD AND WHITE DWARFS PRESENT

This past week there has been some calls about soybean aphid populations increasing and ‘white dwarf’ aphids in maturing R4 (full pod) to R5 (beginning seed) soybeans. In mid to late August, soybean aphids typically move around from field to field as soybeans mature. It’s easy to find little white aphids called ‘White Dwarfs’.

White dwarf soybean aphid
'White dwarf" soybean aphid

These are not baby aphids or diseased aphids. Many species of aphids do this in response to change, like hot temperatures, higher humidity, shorter day length, or change in plant nutritional quality. Be sure not to confuse them with the caste skins of aphids (see photograph) as they molt to the next growth stage.

Caste skin of soybean aphid
Caste skin of soybean aphid.
Source: J. Gavloski, Manitoba Agriculture

These are living, feeding and reproducing soybean aphids and should be included in your total plant population counts when assessing economic levels of soybean aphids. Research from University of Minnesota indicates that white dwarf aphids (those typically on lower leaves) live half as long and their lifetime fecundity is reduced 70% compared to the ‘mountain dew’ colored soybean aphids (source: D. Ragsdale). Thus if the population is only comprised of white dwarf aphids, the economic injury level (population of aphids that cause a yield loss) would probably not be reached in 7 days. An economic threshold (population of aphids that triggers an insecticide) has not been established for plants with primarily white dwarfs, however, it is obviously higher than 250 aphids per plant.

The other thing to keep in mind is that we are now past the critical day length (mid-August) and once soybean aphid experiences temperatures below 45F, the next generation will be sexual. Winged gynoparae will fly to buckthorn and leave soybean fields.

Remember, to continue scouting soybean fields for aphids until R6 or full seed! No insecticide sprays are recommended in R6 or later R7 (beginning maturity) as there is little or no economic return from treating.

 

2008 SUNFLOWER SURVEY

The National Sunflower Association is conducting and sponsoring the 2008 sunflower survey for this cropping season again. This survey assess the major pest problems including insect pests, birds, diseases, weeds and agronomic problems in sunflowers grown in the Great Plains (ND, SD, MN, KS, CO, Manitoba). Two new insect pests will be surveyed - sunflower bud moth (Suleima helianthana (Riley)), and sunflower head maggot (Neotephritis finalis (Loew)). Many sunflower producers, seed companies, crop consultants, ... are concerned that these two insect pests are becoming more common and economically important causing an unknown yield loss. Larvae of both insects tunnel in the heads causing distorted heads (see photographs). Surveyors will be measuring the percent of plants that have distorted heads (or tunneling in the heads). In the past, entomologists claimed that injury caused by these insects are not economical; however, the recent increase in the percentage of head injured has many concerned. As a result, this survey data will help assess the impact of these insect pests in sunflower.

sunflower bud moth damage   Sunflower bud moth damage
Sunflower bud moth damage
 
sunflower head maggot damage   sunflower head maggot damage
Sunflower head maggot damage

 

INSECTICIDE SPRAY TIMING FOR CONTROL SUNFLOWER HEAD INSECTS

Field reports indicate that populations of banded sunflower moth and red sunflower seed weevil are finally starting to decline. There has been some confusion with the timing of insecticides for these two insect pests. Insecticide spraying is targeted at the adult red sunflower seed weevil to prevent egg laying and at the early instar larvae of banded sunflower moth (after egg hatch).

The best time to treat is when more than half of the plants in a field are beginning to show yellow ray petals (R5.1) to 30% of the head shedding pollen (R5.3) and the rest of the plants in the field are still in the late bud stage (R4). Although insecticides applied to sunflower at the bud stage will kill seed 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. If spraying is done too early, weevils can re-infest a field requiring a second treatment. At R5.1, the young larvae of banded sunflower moth are typically beginning to feed on the disk flowers, are exposed on the head and are susceptible to the insecticide treatment. Application at an earlier growth stage may be warranted if monitoring reveals earlier than normal egg-laying activity for banded sunflower moth. Banded sunflower moth and red sunflower seed weevil can both be controlled with a well-timed spray, usually near 10% of the head shedding pollen or R5.1 in oilseed sunflowers. Confection sunflowers usually require two insecticide applications to control sunflower head insects: one near R5.1 and another one week later due to the industry standards for very low insect damage. Sunflower fields should always be scouted for both insect pests through R5.7.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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