Ecological Archives A021-124-A1

Kevin Gross and Jay A. Rosenheim. 2011. Quantifying secondary pest outbreaks in cotton and their monetary cost with causal-inference statistics. Ecological Applications 21:2770–2780.

Appendix A. Detailed description of methods to calculate the cost of pesticide applications.

We estimated the cost of pesticide applications made after July 1 as follows:

1.  Estimating the cost of the pesticides, as purchased from agricultural chemical suppliers.

Our database documented the applications of 44 different insecticides or acaricides (i.e., distinct active ingredients) to cotton. In many cases, different agricultural chemical companies market the same active ingredient under different trade names, which are sold at very similar prices; we therefore combined all brands of a given active ingredient when estimating cost. We used two sources of information to estimate the cost of the insecticides. First, we obtained list prices for 40 of the most common pesticides from a leading agricultural chemical company that sells to California farmers. The remaining four chemicals (chlorfenpyr, tebufenozide, carbofuran, and amitraz), which were applied to cotton only very rarely, were assigned a cost equal to the average for the 40 pesticides for which data were available ($10.56 per application; see below). Second, because California cotton is typically grown on a large scale, and because customers who purchase large quantities of agricultural chemicals can negotiate substantial price discounts, we solicited from our cooperating growers the actual prices they paid in recent (early 2010) chemical purchases. We obtained contract prices paid for 16 of the 40 pesticides, including most of the commonly used materials; the mean discount realized was 57%. For the remaining chemicals, we applied this mean discount to the list prices to estimate the price likely to be paid by commercial cotton farmers.

2. Estimating the quantities typically used in a single application to cotton.

We reviewed records in our database to obtain recent data (2007–2008) on the amounts of pesticides typically applied in a single application to a cotton crop. Standard rates can vary somewhat, within the legal bounds established by pesticide label requirements, across years, farmers, and pest control advisors. In all cases, we attempted to identify the most common or mid-range application rate. In a few cases, application rates varied as a function of (i) the pest target, (ii) the time of year, or (iii) whether or not the pesticide was being applied in combination with another pesticide. In each case, we adjusted our cost estimates accordingly.

Bringing together our data on pesticide costs and application rates, we estimated the mean chemical cost for a single application per acre of cotton of $10.56 (range: $1.73 – $37.80 across the 44 different pesticides)

3.  Estimating the costs of applying the pesticide to the cotton crop.

Nearly all pesticide applications are made by custom applicators, rather than by the farmers themselves, facilitating estimation of the cost of applying the materials. Most pesticides are applied to cotton by air, with fixed-wing aircraft, at a rate of 10 gallons water (carrier) per acre. The cost of such an application was estimated as $9.00 per acre, as confirmed by cooperating pest control advisors and local custom applicators. Some pesticides were applied by air, but using a higher gallonage (15 gallons/acre; cost = $10.90/acre) or using ground spray rigs ($9.00/acre). Finally, one of the commonest pesticides, aldicarb, is frequently applied during the planting operation; in this case we assigned no additional application cost.

4.  Rules for assigning application costs when multiple agricultural chemicals were applied together (‘tank mixes’).

It is common for farmers to apply several chemicals simultaneously as a ‘tank mix.’ Such mixes may include multiple pesticides, or may include combinations of pesticides and other agricultural chemicals, including plant growth regulators (e.g., mepiquat chloride, which restrains vegetative growth) or defoliants (which must be applied at least once prior to harvest). We used the following rules in assigning application costs to pesticides used for secondary pests when applied as tank mixes: (i) we assigned zero application cost in cases where a plant growth regulator or a defoliant was present in the tank mix; we reasoned that applications of plant growth regulators or defoliants are often obligatory for the farmer, and thus including pesticides in tank mixes with such chemicals entails no additional application cost; (ii) in the remaining cases, if N pesticides were applied together, we assigned 1/ N of the full application cost to each of the N pesticides.

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