Economic damage caused by insect pests is over – AgFax | Panda Anku

As we near the end of August, one question that has been asked more than any other lately is at what stage do I stop worrying about bugs in cotton? The main insect activity I notice at this stage involves Lygus and stink bugs.

Stop sampling and treatment for Lygus beetles when cotton accumulates 350 DD60 (degree-days 60) over five nodes over white flower (NAWF). The window of cotton boll development susceptibility to lygus bug damage is significantly smaller at ~8 days after anthesis, when the carpel wall has become sufficiently thick and mature that probing activity is less deleterious.

Lygus beetles will continue to feed on squares and small capsules in the plant ends beyond 5NAWF+350 DD60, but these fruit structures do not contribute significantly to yield (not enough to offset the cost of an insecticide application). The same applies to capsule worms. Fields that have reached 5NAWF+350 DD60 are no longer vulnerable to small (≤ 1/4 inch) larvae.

This time frame may extend for larger worms, but if you don’t see any bollworm activity at this point, it’s very unlikely that they’ll show up from anywhere and reach harmful levels.

Stink bugs, on the other hand, take a little longer to track down. They prefer larger seed pods (about quarter size diameter) and may need to be treated up to 450 DD60 after 5NAWF. Once the cotton reaches 450 DD60 beyond 5NAWF, sampling and treatment for stink bugs may no longer be necessary as the bolls produced after this point do not fully mature or contribute significantly to the crop yield.

Now it is possible that this value will shift slightly due to factors such as shading, variety, irrigation, presence of pathogens (e.g. boll rot), but it should not change to the extent that we need to continue scouting and spraying until mid/end of September or October. According to the recommended threshold we need to reach 50% boll damage in the 8th week of flowering to trigger a spray and the 8th week of flowering would not go past August.

We have had cases in the past of stink bug damage (hard curls) being found in harvestable capsules in October – this scenario is most likely due to the earlier infestation going undetected.


Cotton insects

As I refer to previous issues of this newsletter, we will see that on July 28th (Vol. 27 – No. 11) our average cotton plant, based on the fields of the IPM scouting program, indicated that it was 5.5 Knot over the White Flower (NAWF). The following week, on August 4th, the newsletter (volume 27 – #12) indicated that 60% of all fields were <5 NAWF that week.

Therefore, much of our cotton reached the physiological limit (5 NAWF) on or about July 31 to August 1. Based on my data as a Levelland National Weather Service observer for August 1-25, we accumulated 449 heat units.

By August 26, a majority of the cotton acres in our scouting program area from Ropesville to Morton and north near Littlefield will have sufficient heater units to meet the criteria of 5 NAWF + 450 DD60 heater units and from economic insect damage to be protected. Cotton aphids are an exception to this.

I haven’t seen cotton aphids in 18 days. So this isn’t the “all clear, everyone off cotton” signal. However, the signal has been given to be very cautious about spending funds to control an insect situation that does not return in the final yield.

Cotton nematodes

Now is an excellent time to collect soil samples for cotton root-knot nematodes. Soil sampling is important to determine populations of plant-parasitic nematodes that can reduce yield. Nematode samples collected before harvest can provide the best estimate of nematode populations.

When collecting soil samples, several factors need to be considered such as sampling method(s), sample preparation and handling, and field conditions.

Several methods can be used to collect soil samples. Samples should be taken in a random, crisscross, or zigzag pattern (Fig. 1). A total of three mixed samples (each from 1/3 of the field) are to be taken from each field. Additional samples may be required when dealing with different soil types in a field.

A composite sample consists of 15 to 20 soil cores collected with a soil probe or narrow bladed scoop from a depth of 12 inches. Samples should be taken within a 4 inch radius of the taproot as it is important that root fragments are present in the sample. The soil kernels should be placed in a bucket and mixed thoroughly, taking care to break up any clods of soil. A 1 quart aliquot should be placed in a sealed plastic bag.

Nematode samples must be kept cool (not frozen) and out of direct sunlight. In addition to sample collection and handling, field conditions at the time of sample collection can affect test results. When taking samples, attention should be paid to the amount of soil moisture.

Samples should not be taken when soil moisture is too wet or too dry. Specimens should be sent to a qualified laboratory capable of performing microscopic evaluation to determine populations.

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