Fern sawfly

Systematic position: The fern sawfly (Blasticotoma filiceti) is a member of the order Hymenoptera and the family Blasticotomidae. It is a highly specialized insect that has evolved to develop within the stems of specific fern species, making it an interesting but occasionally problematic pest for fern enthusiasts.

Host plants and damage symptoms: The primary host is the bracken fern (Pteridium aquilinum). The damage is caused by the larvae, which bore into the petioles (leaf stalks). A distinct symptom of an infestation is the production of a frothy, foam-like substance that surrounds the larva, effectively masking its presence and protecting it from desiccation and natural enemies.

Biology and life cycle: Adult sawflies emerge in the spring. After mating, the female deposits eggs into the tissues of the fern stalk. Upon hatching, the larva bores into the stalk, where it feeds on internal tissues. Throughout its development, the larva secretes the characteristic foam that accumulates at the entry hole. Pupation typically takes place either within the soil or at the base of the plant once the larval feeding stage is complete.

Economic impact: The internal feeding of the larvae disrupts the vascular tissue of the fronds. This leads to the wilting, twisting, and premature browning of the leaves. In garden settings or commercial fern nurseries, the aesthetic damage can be significant, potentially leading to the loss of plant vigor if infestations remain unchecked over several seasons.

Control measures: Management of the fern sawfly relies on integrated pest management practices rather than heavy chemical reliance:

• Regular inspection of fern fronds for signs of foam or abnormal stunting.
• Mechanical removal and destruction of infested stems to interrupt the life cycle.
• Sanitation practices, such as removing and composting fallen fronds in the autumn, can reduce the number of overwintering individuals.
• Use of systemic insecticides is reserved for severe infestations where mechanical control is insufficient.