A few fields of Tennessee Valley cotton were planted the week of April 23. These fields began squaring during the week of June 4 and tarnished plant bugs (TPB) were being collected in sweep net samples in these squaring cotton fields that same week.
Tarnished Plant Bugs
A few TPB sprays were initiated the week of June 11 in the oldest cotton fields. There will be many more fields treated for the first time for TPB’s during the week of June 18 as more migratory TPB’s move into cotton and square retention starts to decline in some fields.
Fields planted prior to May 10 are the fields that are most likely to be at risk to plant bug damage at this time. The only way a grower can determine if a plant bug spray is needed is to scout fields closely, monitor plant bug num-bers with a sweepnet and assess square retention.
Square retention should not be allowed to drop below 80%. Remember that square retention can drop from 85% to 70% or less in a few days.
Optimum timing for plant bug sprays will vary from field to field and some fields will possibly not require a TPB treatment during the year. Residual activity of TPB insecticides cannot be expected to last more than seven days on migratory adults and five days is a better estimate.
Commonly used insecticides in the Tennessee Valley for TPB control are acephate (Orthene), dicrotophos (Bidrin), sulfoxaflor (Transform), and neonicotinoids (Centric, Admire Pro and Belay). Thiamethoxam (Centric) has provided better control of TPB’s than the other two neonicotinoids mentioned in the past. Pyrethroids are also used in areas where they have continued to provide satisfactory control to date.
The EPA granted Transform a Section 18 Emergency Use Exemption in 29 north Alabama counties in 2018. Bidrin is not labeled for use in cotton between formation of the first pinhead square and first bloom so we can’t use it for these initial applications.
The insect growth regulator Diamond is also used to reduce the number of immature TPB’s and is usually applied during the first two weeks of bloom. Levels of control achieved with these five products in comparison to untreated plots at Belle Mina in 2017 at 4 and 13 days after application are shown in the chart on page two.
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The TPB population counted in the 2017 test consisted of more than 90% immatures at both 4 and 13 days after treatment. Per cent control ranged from 46% in the Brigade (bifentrin) treatment to 91% in the Orthene treatment at four days after application (DAA). Diamond and Transform provided the best control at 13 DAA.
It is important to rotate chemistries to help slow the development of resistance to these insecticides. North Alabama cotton growers in past years sometimes applied a neonicotinoid insecticide or Orthene for the first TPB application. Both of these products will significantly reduce numbers of beneficial insects.
Orthene or Transform in past years were often used for the second TPB application. Transform is generally viewed as being “softer” on beneficials and after the problems encountered in 2018 with bollworms in dual Bt-gene cotton farmers may want to conserve as many beneficials as possible with their second TPB application which may be needed in some fields.
In the oldest cotton fields that were squaring the week of June 4 some TPB eggs were likely deposited. TPB eggs will begin hatching one to two weeks after deposition and immatures will develop into adults in two to three weeks. Thus the first adult plant bugs derived from cotton could be present in older fields as early as the week of June 25.
However most cotton fields were not planted until the first half of May and significant numbers of adult plant bugs that are derived from eggs deposited in cotton will not be present until around mid-July.
Since rains have been sufficient to keep wild hosts green and relatively lush, expect movement of adults from wild hosts to extend into early July.
Bollworms
Corn began silking in the earliest planted fields the week of June 4 and bollworm moths likely deposited eggs in some of these silks. The bollworm life cycle requires about 30 days so the first adult bollworm moths derived from corn will emerge from the soil about the 4th of July.
Last year bollworms required treatment in dual Bt-gene cotton as early as the third week of July in the Tennessee Valley. This was at least a week earlier than normal. Since corn was planted over an extended period this year expect bollworm moths derived from corn to keep laying eggs in cotton fields for a few weeks.
During the seven day period ending June 15 the bollworm moth trap at Belle Mina collected 92 bollworm moths. This was 32 more moths than we collected last year for this same period.
Growers should keep in mind the time frames for TPB and bollworm populations to increase in their fields and make insecticide applications in a timely manner to prevent these pests from gaining a “foothold” in their crop and reducing yields.
