Should you over-spray Bollgard cotton?

Although starting out very cautiously with a 2 percent adoption rate in 1996 and 1997, North Carolina's cotton producers have turned to Bollgard technology in big way, with about 70 percent of our acreage planted to Bollgard cotton for the past three years, admittedly helped along by the pull of stacked (BG plus Roundup Ready) varieties.

With an average up-front investment of $20 per acre in the Bollgard technology fee in this area, the bill for a significant portion of the insect control costs is due well before the first bollworm takes to a cotton field.

With this early investment in insect control and the anticipated good control of bollworms, it's tempting to some producers to scout this technology minimally, and in some cases even jettison their consultant, particularly if a grower plans to make an automatic pyrethroid “over-spray”.

In Bollgard cotton, an over-spray refers to an often automatic foliar application of insecticide to control low (or hard to define) mixtures of bollworms, plant bugs and stink bugs. They're always out there, right?

Support for this approach (aside from being able to sleep better at night) often takes the form of comparing untreated and treated Bollgard plots or whole fields, and noting the lower boll damage and higher yields in the latter.

Even in our own replicated tests in North Carolina, we have measured lint yield increases of up to 100 pounds, or more, with a single well timed pyrethroid spray on Bollgard cotton. It's hard to argue with the ease and apparent effectiveness of such a “program”.

In North Carolina, all consultants participate in an annual survey of insecticide use on conventional and Bollgard cotton. If one looks at the average number of late season applications for bollworms and bug pests recommended by consultants on Bollgard cotton, that number is very close to one.

However, the range of applications used by consultants reveals that some of their acreage has also received zero, two, and, rarely, three treatments. This average of one application additionally says nothing about the rate, timing, or insecticide selected.

So while a significant proportion of this consulted Bollgard acreage receives a single application, a scouting based call is often better timed and the rate more often tied to the levels and variety of pests than is the case with unscouted, or minimally-scouted, over-sprays, which have also become all too common here and elsewhere.

Our project's annual fall damaged boll survey, primarily compares bollworm and bug damage to bolls in conventional vs. Bollgard cotton fields (approximately 150 per year) under grower conditions in various parts of the state. Although the surveyed cotton fields are selected randomly, after the survey completion dates, in some cases, we have been able to determine which cotton fields were managed by consultants and which fields were managed by other means (producers, industry field reps offering a scouting service, etc.).

Although some fields not managed by consultants were clean, in both Bollgard and in conventional cotton fields, the average levels of damaged bolls were typically 2 to 4-fold less in the cotton fields managed by consultants.

Thus, in a number of cases, if overall damaged boll levels (stink bug plus bollworm) in a consultant's field were in the 2 percent to 3 percent range, boll damage in a cotton field not managed by a consultant was often in the 4 percent to 12 percent range. At an average of approximately 12 pounds of lint lost per damaged boll (the range was 8.8 to 18.9 pounds of lint lost per bollworm damaged boll in threshold tests conducted here in the mid-1980s), the consultant's spray decisions would have gained the producer 24 to 108 pounds of lint in the above example.

Although these lint differences were closer to the 24 to 50 pound end of the range in the Bollgard fields (and stink bugs, which also figure into this damage sometimes do not damage the whole boll), the value of carefully scouting and responding quickly to threshold levels of bugs and caterpillars is apparent.

Paying a $20 up front technology fee for July and August caterpillars is a big investment, especially with profits margins so tight. Once the decision to plant Bollgard cotton has been made, however, late season insect management decisions should focus on the best economic return for your insecticide dollar from that point on.

A single automatic pyrethroid over-spray on Bt cotton is one way to “cookbook” late season insect control. However, to get the best possible return, one must know stage of the fruit, the bug or caterpillar species present and its stage of development, its location on the plant, if single or multiple-pest thresholds have been met, if a tank-mix might be appropriate (brown stink bugs, armyworms, etc.), what rates are needed, and how many, if any, applications might be required.

Fortunately, inspecting Bollgard cotton thoroughly and responding to appropriate thresholds quickly often results in returns well in excess of the costs of scouting and sound advice.

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