What could lead to the necessity for additional pesticide applications?

The necessity for additional pesticide applications can arise from a situation where the initial treatment was insufficient or there is a sudden increase in pest populations. Insufficient treatments might not fully eradicate the pest problem, allowing populations to persist and potentially grow. Furthermore, environmental factors, seasonal changes, or sudden pest invasions can lead to spikes in pest numbers that exceed the control capabilities of an earlier application. Therefore, monitoring pest populations and assessing the effectiveness of initial treatments are vital to determine whether further applications are necessary.

The other options may also represent scenarios that can influence pest management strategies, but they do not specifically address the combination of underperformance of the initial treatment and an unexpected rise in pest numbers as directly contributing factors for the need of additional pesticide applications.