Spatial Analysis of Native and Sterile Pink Bollworm in the Arizona PBW Eradication Program area, June 25 – July 22, 2006
By David Bartels, Ph.D., USDA, APHIS, PPQ, CPHST, Pest Detection, Diagnostics & Management Laboratory, Moore Air Base, Bldg. 6414, 22675 N. Moorefield Rd., Edinburg, TX 78541-9398, and Michelle Walters, Ph.D., USDA APHIS, PPQ, CPHST, Decision Support and Pest Management Systems Laboratory, 3645 E. Wier Avenue, Phoenix, AZ 85040
The pink bollworm (PBW) moth (Pectinophora gossypiella (Saunders) feeds almost exclusively on cotton plants (Gossypium spp.) and can cause significant economic damage to the crop. PBW is often the key pest of cotton in the Southwestern United States and Northwestern Mexico. Currently, Arizona is in the process of eradication PBW from certain areas using a combination of transgenic cotton, mating disruption with pheromones, and sterile insect release. The objective of the analysis was to document the presence of native and sterile PBW populations in the cotton production areas based on trap counts.
Materials and Methods
Native and sterile adult PBW were monitored in the Arizona PBW Eradication Program cotton fields using standard Delta traps baited with PBW sex pheromone. Traps were checked weekly by program personnel and numbers of male moths (native and sterile) were recorded. For each field, we were presented with the number of traps per field and the total number of moths counted each week from June 25th – July 22nd, 2006. The number of traps per field varied based on the size of the field, and the type of cotton being grown. The number of traps ranged from 0 to 14. A field may have no trap located in it because there is one central trap that covers several small fields. The average number of moths per trap was calculated for each cotton field by dividing the total capture by the number of traps. To create a point for the trap captures, the center of each cotton field containing a trap was calculated using its geographic boundary.
To present the trapping data as a predicted surface of PBW numbers, geostatistics was used to calculate a predicted value for areas between the known values of each field. Ordinary Kriging using a spherical model was applied to trap counts for each week to develop a predictive surface model surrounding the cotton fields. Kriging is a technique for the optimal interpolation of points across the spatial domain. Kriging handles spatial autocorrelation and it is not sensitive to preferential (uneven) sampling in specific areas, such as the distribution of cotton field in the eradication program. Kriging constructs a weighted moving average equation that estimates the value of a spatially distributed variable from adjacent values while considering the interdependence. This equation minimizes the effect of the relatively high variance of the sample values by including knowledge of the covariance between the estimated point and other sample points within the range. Kriging results in a smoothing effect with high original values being underestimated and low values being overestimated. There is also less variation than in the original points. Kriging is a best linear unbiased estimator because it minimizes the variance of the estimation errors.
The Kriged surfaces were generated from a total of 3,476 center points from the cotton fields. We used a 2 kilometer range of influence from the center point of the field, so that each field’s data is affected only by other fields within approximately 1 kilometer of the outside border of the field. The 1 km limit reflects the perceived day to day movement of PBW adults in green cotton fields and limits the undue mathematical influence of a “hot” field on a large area. In addition to limiting the area of influence, we truncated trap values to 100 moths/field/week as this indicates a “hot” field biologically and also, because a weighted average is used in Kriging, capping the high values limits undue graphical influence of a single field. A trap with greater than 100 PBW moths also represents a unreliable count as the efficiency of the trap declines quickly once the trap fills with moths.
The maps presented depict the native and sterile male PBW populations in the Arizona PBW Eradication Program based solely on trap captures. Additional factors such as cotton type, soil conditions or crop phenology are not included in the model. The analysis indicates that the sterile PBW adult population is clearly more abundant during each week with many areas have populations of greater than 50 moths per trap. At the same time, the native populations is mainly around 1-5 moths per trap with only 3 - 5 “hot spots” of numbers greater than 25 showing up across the program area during the 4 weeks. The sterile PBW population is also more consistent from week to week and more widely distributed than the native population. The daily releases of sterile PBW from the program aircraft have kept the sterile population levels stable during the 4 weeks of analysis.