What characteristic of corn need nematodes makes them difficult to control?

The correct characteristic of nematodes that makes them difficult to control is that they are resistant to chemicals. This resistance complicates management efforts because many pest control methods rely on chemical interventions. Nematodes can have mechanisms that allow them to survive exposure to various pesticides, reducing the effectiveness of these chemical treatments.

Nematodes are primarily root-feeding parasites that exert significant damage by disrupting the plant's root system, which is essential for water and nutrient uptake. Their ability to thrive even when chemical controls are applied means that alternative control strategies must be considered, such as crop rotation, biological control, or resistant crop varieties.

While the other options may contribute to challenges in pest management, they do not specifically address the inherent resistance to chemicals that nematodes exhibit. This resistance is a core factor in their difficulty of control in agricultural settings.