Awoman treated her home with a pesticide that kills spiders. the first application killed 78% of the spiders. two months later she applied the pesticide again, but it only killed 45% of the spiders. what would best explain the decrease in the effectiveness of the pesticide?

The effectiveness of the pesticide is not decreased rather the spiders body became resistant to the pesticide. When a pesticide is used on a large population of spiders some of them may survive due to the their distinct genetic makeup. The spiders that were not having this distinct gene died. The individuals that were having distinct genetic makeup survived and reproduce and hence their number increased, their population increased and hence the resistant spiders increased so, next time the application killed only 45% of the population rather 78%.

Evolution of pesticide resistance population of spiders

Explanation:

The pesticide resistance was present in some of the spiders of the total population. Continuous use of pesticide led to natural selection of spiders with pesticide resistance. These spiders were able to survive in presence of pesticide and produced more pesticide resistant progeny. In two months, the most of the spider population had pesticide resistance leading to their reduced rate of killing.

i would think it's dendrites