Dutch elm disease combat plan weakness

Dutch elm disease, which is caused by the fungus C. ulmi spread by adult scolytid beetles, has already destroyed 70 percent of the elms in the Greenwood Forest. Another naturally ocurring fungus, P. oblonga, kills larvae of the scolytid beetle. Forest rangers plan to introduce P. oblonga into Greenwood Forest in order to save the remaning mature elms.

Which of the following, if true, would cast the most serious doubt on the plan's prospects of success?

A. During last year, the scolytd beetle population in the Greenwood Forest has decreased by 30 percent because of cold-weather conditions.
B. Dutch elm disease cannot be abated by introducing chemical compounds used to arrest the diseases of many other species of tree.
C. Introducing the P. oblonga saved elm trees in neighboring Gatemar and Lavemont forests.
D. For P. oblonga to control scolytd beetle successfully, it must be in a forest prior to the beetle infestation.
E. Greenwood Forest has lost many maple trees because of fungus infection.