Final answer:
Most flea pesticides do not contain an ingredient, making flea control difficult.
Step-by-step explanation:
The statement that most flea pesticides do not contain an ingredient is true. Flea control is indeed difficult because many flea pesticides lack an ingredient that effectively eliminates fleas. Some pesticides may only kill a small percentage of the fleas, like the shampoo mentioned in the question that only killed 25 percent of the fleas. This difficulty in flea control is due to the adaptability of fleas and the limitations of certain pesticides.