Manitoba Insect and Disease Update July 31, 2006 - Manitoba Agriculture, Food and Rural Initiatives

Sunflowers

Insect Concerns on Confection vs. Oilseed Sunflowers

Last week's update discussed controlling insects on sunflower heads. It should be noted that the economics of the seed-feeding insects differs greatly depending on whether it is confection or oilseed sunflowers that are grown. Lygus bugs are a concern on confection sunflowers and the threshold that has been developed (about one lygus bug per nine heads) is for confection sunflowers only. Lygus bug damage is a quality issue in sunflowers, but the effect on yield is not likely great. So lygus bugs would not be an economical concern in oilseed sunflowers. Both seed weevils and banded sunflower moth can decrease yield as well as affect quality, but the quality loss is the biggest concern. So thresholds will be much lower for confection sunflowers than oilseed sunflowers. For sunflower seed weevil the economic threshold is one to two weevils per plant in confection sunflowers and 12 to 14 weevils per head in oilseed sunflowers. However, sunflower seed weevil numbers are very low this year, and it is doubtful that numbers will be above threshold in any sunflower fields in Manitoba, whereas lygus bug numbers have been high in many fields, and insecticide applications for control of lygus bugs continue in confection sunflowers.

Sunflower Wilt

As the cool-season crops are rapidly maturing, attention shifts to the warm-season crops that still have a ways to go to mature. Over the last two weeks our summer students have visited a number of fields of sunflowers, some of which have noticeable levels of wilt caused by Sclerotinia and otherwise known as basal stalk rot. A field near Oak Bluff had 2% of the plant infested and another west of Carey had 8% of stems affected.

These infections are reflective of the concentration of resting bodies in the soil and are related to past white mold incidence in the preceding crops. This type of infection does not depend on wet soil – in fact, no spores are involved in the infection process. The probability of spread is minimal although plants that are in close proximity to infected stems may also succumb before the end of the season. Dry as it has been, we have also been amazed to see some trace incidence of head rot. Because such infections can only occur from ascospores, there must be some apothecia lurking somewhere, either under irrigated crops or in fields that have cashed in on the thunderstorm lottery.

Soybeans

Soybean Aphids - Economic Threshold vs. Economic Injury Level

Because of the potentially quick rate of increase in aphid numbers, in many crops the economic thresholds for aphids (the point where control is recommended to prevent economic injury) is often lower than the economic injury level (the level of aphids that can cause enough injury to equal the costs of controlling them). The economic injury level for aphids on soybeans is actually about 1,000 aphids per plant on average. If natural enemies are not present at levels high enough to provide good regulation of aphid numbers, aphid numbers can increase quite rapidly. Since there is often some lag time between when a field is scouted and when an insecticide can be applied, economic thresholds are often set lower than the economic injury level so aphid numbers do not have time to go above the economic injury level before an insecticide is applied. In soybeans, the economic threshold for soybean aphid has been set at 250 aphids per plant and the population is increasing. The reason “and the population is increasing” has been included in the economic threshold is because 250 is not the damaging level of aphids, but means the field is in danger of reaching damaging levels and needs to be watched carefully. If natural enemy populations progress to the point where they are able to keep the soybean aphids at or under 250 aphids per plant, than the net loss to the farmer is greater from purchasing and applying the insecticide than the damage aphids would have caused. So when scouting for soybean aphids, keep track of whether the population seem to be growing, or are stabilizing below a level where control would be economical. Our IPM assistants for the summer have been informing me that predator populations are quite high in some of the soybean fields they have been in recently.

Canola

Bertha Armyworm

Control of bertha armyworm continues on canola in the Northwest. Some considerations when deciding on whether control is practical or even possible for bertha armyworm include the following.

Economic Threshold: A loss of 0.058 Bu/acre for each larva/m² can be expected. Multiplying 0.058 X average number of larvae/m² X expected seed value (dollars/acre) will determine the economic loss (in dollars/acre) due to the larvae. If we are optimistic and assume a grower will get $7.00 per bushel for their canola, and can control bertha armyworm for $10 per acre, the economic threshold would be 25 larvae/m².

If we assume a control cost of $12 per acre and that the grower anticipates $6 per bushel for the canola, the economic threshold would be 34/m². See Table 1 on the Bertha Armyworm Factsheet webpage (MAFRI) for a set of calculated thresholds for various spraying costs and values of canola.

Preharvest Interval: Preharvest interval is the number of days that must pass between the last application of an insecticide and the cutting of the crop. The following is a list of the insecticides registered for control of bertha armyworm in canola and their preharvest intervals:

The shortest preharvest interval is seven days. So make sure canola is scouted and control decisions made in time to meet these preharvest intervals. If an infestation is discovered within a week of swathing, insecticides can not be applied (unless swathing is delayed). There are serious marketing and trade consequences should unacceptable pesticide residues be detected in crops, so growers and agronomists are encouraged to be aware of and follow preharvest intervals.

When to Apply an Insecticide: Many of the bertha armyworm larvae will be hiding under debris on the ground during the day, and come up to feed in the evening. If bertha armyworm are above the economic threshold, and pod feeding is beginning, and there is more than a week until swathing, the ideal time to be spraying would be as late in the day as possible, or early morning.

White Butterflies on Your Crops?

Some have been calling regarding what all the white butterflies are that are quite numerous in some of the fields. These are cabbage butterflies. They are not pests of any field crops, but will often be present in fields, often in big numbers, as they look for nectar. The larvae (referred to as imported cabbageworms) may occasionally be found on canola leaves, but do not cause enough damage to be considered economical pests of canola. They can be serious pests of vegetable crops in the cabbage family, however.