Which stage of oystershell scale is effectively treated with horticultural oils?

• A. Crawlers.
• B. Adults.
• C. Eggs.
• D. Chrysalis.

Answer: (A)

Horticultural oils kill scale insects by smothering them with a coating, so the stage that is most easily reached is the one that is exposed and not yet protected by a hard cover. The crawler stage of oystershell scale is the mobile, recently hatched nymph that has not yet developed the full protective waxy shell. Because crawlers are on the plant surface and not shielded, the oil film can coat them and block respiration, making this stage the most susceptible.

Eggs remain tucked under the female’s protective cover, so oil generally can’t reach them effectively. Adults are protected by a rigid armored shell that prevents the oil from reaching the insect inside. Chrysalis isn’t a life stage of oystershell scale, so it’s not relevant here. Thus, the crawler stage is the best target for horticultural oils.