The local agricultural official gave the fruit growers of the District 10 Farmers' Cooperative a new pesticide that they applied for a period of three years to their pear orchards in place of the pesticide they had formerly applied. During those three years, the proportion of pears lost to insects was significantly less than it had been during the previous three-year period. On the basis of these results, the official concluded that the new pesticide was more effective than the old pesticide, at least in the short term, in limiting the loss of certain fruit to insects. Each of the following, if true, weakens the official's argument EXCEPT: ['Some of the species of insects in District 10 that infest pear trees are water breeders, and the reservoirs and marshlands in this district have been shrinking rapidly over the past three years.', 'The amount of fruit that an orchard can potentially produce depends in part on how many mature trees it contains, and the number of mature pear trees in District 10 has declined steadily over the past eight years.', 'Over the past five years, one of the several species of birds that typically prey on the insects that feed on pears has gradually shifted its migratory patterns, spending more and more months each year in the region that contains District 10.', "During the past five years, the farmers of the District 10 Farmers' Cooperative have been gradually implementing a variety of insect-abatement programs, and some of these programs have proven successful."]