What naturally occurring compounds give bitter-tasting squashes and cucumbers their defense mechanism against pests?

Cucurbitacins

The compounds responsible for the naturally occurring bitterness found in certain squashes and cucumbers, serving as a defense mechanism against pests, are termed cucurbitacins. In modern agriculture, especially with standard grocery store or garden cucumbers, these compounds are typically bred down to such low concentrations that the resulting fruit tastes sweet and palatable. However, if a cucumber exhibits excessive bitterness, often concentrated near the stem end, it signals a higher concentration of cucurbitacins. Although exceedingly high levels could potentially lead to digestive discomfort, the generally safe amounts found in common varieties are even associated in preliminary studies with potential anti-cancer properties.