As more and more fields enter the bloom stage the Bollworm complex will become the pest that need to be monitored. The general scenario is to find live worms but no damage squares OR find damage squares and no live worms. This indicates that the technology is working where there is live worms and damage squares means the technology is overwhelmed.
The economic threshold is 6% damaged squares with live worms present in Bt cotton.
We need to once again caution about using pyrethroids even with a combination of aphicide to control bollworms. This is not because they will not do the job but it is due to the likely aphid infestation that can later occur. Pyrethroids are just too harsh on beneficial arthropods to be viable.
It is not the aphids in the field at the time of application one has to worry about – it is the subsequent aphids that move into the field to recolonize it. Adult aphids are always on the move.
A control spray is warranted in Bt cotton when the bollworm population exceeds the economic threshold of 6% square damage plus live worms present. Then the chemical choice becomes critical. Pyrethroid insecticide resistance has been noted in most areas of the Cotton Belt.
A broad spectrum insecticide can kill the targeted pest. Secondary pests can become a problem due to the destruction of beneficial arthropods which normally keep the secondary pests in check. The cost of one insecticide product versus another may be a factor when choosing which chemical to use. However, the potential consequences may far outstrip the initial savings one might encounter.
If a bollworm control spray event needs to occur, two options are possible. One is with a far cheaper product and one may be with a more expensive product. The broad spectrum insecticide may be initially cheaper, but destroy the beneficial population. Then the field has no biological “friendlies” to assist in holding back secondary pest populations.
In the long run the more expensive product may be a better choice if it is less harsh on beneficial arthropods. This retains the biological “friendlies” which are then available to reduce the potential of secondary pest outbreaks.
The gamble is with the absence of beneficial insects, some of the secondary pests may need to be controlled with insecticides. One can see that the costs can add up as noted in the slide below. Loss of beneficial arthropods can cascade into an aphid flare up which would then require one or possibly two applications to control.
The next possible pest could become spider mites, which again will require more product and application for control.
Spider mites often attack cotton when insecticides have removed beneficial arthropod populations which normally keep this pest in check. Infestations are generally aided by hot, dry weather. In most cases, infestations will be localized in a field.
Spider mites damage cotton by feeding on the plant juices and the foliage will turn a reddish or yellowish color under a heavy infestation. Mites are small in size and are generally found on the underside of the leaves. A close inspection is necessary to determine if mites are present.
Cotton aphids are small, soft-bodied insects commonly referred to as “plant lice”. Aphids occasionally occur on cotton in such high numbers that control measures should be implemented. Build ups are localized and usually occur after the use of insecticides that are harsh on beneficial arthropods, including pyrethroid types.
The insects are found on the underside of leaves and along the terminal stem, causing misshapen leaves with a downward curl and stunted plants. The insect damages cotton directly by sucking juices from the plant and indirectly by secreting honeydew.
The honeydew is sticky and can lower the grade of lint. Sticky cotton may result in significant problems during the spinning process at mills. A sooty mold can develop on the aphid honeydew and discolor the lint.