The cotton aphid is pale to dark green in cool seasons and yellow in hot, dry summers. Generally distributed throughout temperate, subtropic, and tropic zones, the cotton aphid occurs in all cotton- producing areas of the world. These insects congregate on lower leaf surfaces and on terminal buds, extracting plant sap. If weather is cool during the spring, populations of natural enemies will be slow in building up, and heavy infestations of aphids may result. When this occurs, leaves begin to curl and pucker; seedling plants become stunted and may die. Most aphid damage is of this type. If honeydew resulting from late- season aphid infestations falls onto open cotton, it can act as a growing medium for sooty mold. Cotton stained by this black fungus is reduced in quality and brings a low price for the grower. Entomologists studied the aphids to determine weather conditions that may result in increased aphid density on cotton plants. The following data were reported in Statistics and Data Analysis ( Peck, Olson, and Devore, 2005) and come from an extensive study as reported in the article €œ Estimation of the Economic Threshold of ­Infestation for Cotton Aphid€ [ Mesopotamia Journal of Agriculture (1982): 10, 71€“ 75]. In the following table,
y = infestation rate (aphids/ 100 leaves)
x1 = mean temperature (° C)
x2 = mean relative humidity

a. Fit the model y = β0 + β1x1 + β2x2 + ɛ to the aphid data.
b. Use residual plots, tests of hypotheses, and other diagnostic statistics to identify possible additional terms to add to the model fit in part (a).

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