Two-spotted slug moth

The two-spotted slug moth (Parasa bimaculata) is a member of the Limacodidae family. Adult moths are easily identified by their distinct green to brownish forewings, which typically feature two prominent dark spots, giving the species its common name.

The larvae, known as slug caterpillars, possess a very unique morphology. They are flattened and lack visible true legs, appearing more like small slugs as they glide across leaf surfaces using rhythmic contractions of their ventral side.

A crucial identifying feature is the presence of urticating (stinging) hairs or spines on the tubercles along the body. These hairs can cause severe irritation, itching, and inflammation upon contact with human skin.

Adult moths are nocturnal and are rarely seen during the day. They prefer to remain stationary and hidden within dense foliage or within crevices in tree bark, blending in perfectly with their surroundings.

The life cycle undergoes complete metamorphosis, including egg, larva, pupa (in a hard, parchment-like cocoon), and adult stages. Overwintering occurs in the pupal stage, usually within the protective cocoon attached to twigs or branches.

The larvae are polyphagous and feed on a wide range of woody plants, including various fruit trees like apple, pear, plum, and cherry, as well as several ornamental tree species.

Early-instar larvae typically skeletonize the leaves, consuming the soft tissue and leaving only the leaf veins intact. This early stage of feeding can be easily overlooked if the infestation is light.

As the larvae mature, they consume the entire leaf lamina, often leaving only the midrib. Severe infestations can lead to widespread defoliation, significantly stressing the host tree.

Chronic loss of foliage reduces the tree's photosynthetic capacity, which directly results in reduced fruit yield, poor fruit quality, and weakened overall tree health, making it more susceptible to other pests.

Furthermore, the damaged leaf tissues serve as entry points for secondary infections, including fungal and bacterial pathogens, which can exacerbate the impact of the pest on the orchard.

Adult moth activity typically peaks during the summer months, usually between June and August. This is the period when mating occurs, and females lay eggs on the undersides of host plant leaves.

Larval emergence generally occurs in mid-summer. Consequently, the period from mid-July through August is the most critical time for monitoring and observing feeding damage on tree foliage.

The larvae are most active and visible during the warmer parts of the late summer. During this time, they feed intensely to build up energy reserves required for pupation.

As autumn approaches and temperatures drop, the larvae build their characteristic hard cocoons. This process is generally completed by the arrival of the colder season, ensuring the insect survives the winter.

Gardeners and orchard managers should plan regular inspections of their trees throughout the summer, focusing on the undersides of leaves and branches to identify early signs of infestation.

The most common sign of an infestation is the presence of leaves with "skeletonized" patches or large, irregular holes where tissue has been removed. Larval frass (droppings) may also be present on lower leaves.

The presence of the unique, colorful, slug-like caterpillars is the most definitive sign. Their unusual shape and prominent, brightly colored, spiky tubercles make them distinct from other common lepidopteran pests.

Hard, rounded, or oval cocoons found on branches or in bark fissures during the winter and early spring are clear indicators that the pest is overwintering on the site.

Accidental skin contact while pruning or inspecting trees is an unfortunate but common way of detecting the larvae, as the resulting stinging sensation usually draws immediate attention to the presence of the insect.

• Skeletonized or irregularly eaten foliage.
• Unique slug-shaped, spiny caterpillars on leaves.
• Hard cocoons attached to tree twigs and branches.
• Stinging skin reaction upon touching the foliage.

Cultural control involves the manual removal and destruction of cocoons during the dormant season (late autumn to early spring). This is an effective way to reduce the population before the next generation emerges.

Chemical control using insecticides is effective when applied during the peak feeding phase of the larvae in mid-summer. Targeted applications should be timed to hit early larval stages for maximum efficacy.

Biological control methods, such as utilizing products containing Bacillus thuringiensis, provide an environmentally friendly alternative that targets the larvae while preserving beneficial insect populations.

Always prioritize safety: wear gloves and long-sleeved clothing when working near affected trees to prevent direct contact with the stinging hairs of the larvae.

Encourage biodiversity in the garden. By maintaining habitats for predatory insects, birds, and other natural enemies, you can naturally suppress the population of many garden pests, including the slug moth.