The Udea ferrugalis, commonly referred to as the rusty dot pearl moth, is an insect belonging to the order Lepidoptera, family Crambidae, subfamily Pyraustinae. This pest earned its name due to its habit of devouring vegetable leaves. This lepidopteran species is widespread across the European continent and is quite common in our country as well. Let’s explore how to recognize its presence in your garden, the plants most vulnerable to its attacks, the damage it causes, and the most effective biological defense strategies.
Description of the Rusty Dot Pearl Moth
As an adult, Udea ferrugalis is a small butterfly with a wingspan of 18-20 mm. Its forewings are reddish-brown, adorned with numerous scattered black dots, dark circular spots, numerous stripes, and an outer submarginal band in ochre. The head is pointed, with pronounced, dark, and highly visible eyes.
The eggs are discoidal in shape and are laid in tight, overlapping clusters composed of approximately 20 elements.
The larvae of the rusty dot pearl moth differ depending on their developmental stage. First-instar larvae are creamy-white with black heads. Fully developed larvae are green, with a darker dorsal line flanked by two bluish longitudinal lines. Finally, mature larvae are entirely yellowish with two small black spots on the sides of the prothoracic shield. At full development, they measure about 10 mm in length.
The final stage of Udea ferrugalis is the pupa, which is reddish-brown with a darker caudal end.
Plants Attacked by the Rusty Dot Pearl Moth
The rusty dot pearl moth is a common pest in vegetable gardens, as it is a highly polyphagous species capable of attacking numerous vegetable and industrial crops. Among vegetables, the most affected are celery and radicchio, but other susceptible crops include lettuce, parsley, Swiss chard, artichoke, cabbage, cucumber, zucchini, strawberry, eggplant, bell pepper, spinach, peas, and beans. In intensive crops, damage is also reported on soybeans, alfalfa, and tobacco.
Damage to Plants
The larval stages of Udea ferrugalis are responsible for plant damage as they feed on the leaves. In the early stages of feeding activity, the larvae limit themselves to causing erosions on the underside of the leaves, respecting the upper epidermis. Later on, they consume larger or smaller parts of the leaf blade, sparing only the central veins. If the infestation is severe, leaf erosions can be significant, resulting in crop loss. On celery, the damage caused by the rusty dot pearl moth is even more severe because the larvae descend into the heart of the plant to feed on the fleshy leaf ribs. The feeding damage by the moth also indirectly opens the door to bacterial or fungal infections and rot. In open-field celery cultivation, production losses can reach high percentages, up to 50% of the crop.
Life Cycle of Udea ferrugalis
The adult rusty dot pearl moth appears in fields in late spring, usually in May. It is a nocturnal moth that rests on the leaves of host plants during the day. Adults mate within a few days of emergence, with females laying eggs on the undersides of leaves. The incubation period lasts about 2-3 weeks. Newly hatched larvae begin to disperse among the vegetation and feed. By late June, the larvae reach maturity and pupate, creating a silken case specifically prepared by the larvae within a folded leaf blade. New adults emerge about a week later.
Following the first generation of Udea ferrugalis, another generation occurs, with active larvae from mid-August onward. These larvae cause the most significant damage, especially to crops like celery, which are nearing harvest. In total, up to 4 generations of the rusty dot pearl moth can follow. Larvae from the last generation reach maturity at the end of October and overwinter in the soil within earthen cells.
Natural Enemies of the Rusty Dot Pearl Moth
The natural enemies of Udea ferrugalis include parasitoids such as Angita fenestralis, Exochus sp. (ichneumon wasps), and Cerosamia senilis (tachinid fly).
Preventing Udea ferrugalis
To prevent infestations of the rusty dot pearl moth in previously affected fields and gardens, it’s a good practice to perform shallow soil cultivation in early winter to expose overwintering larvae to the cold.
Eliminating the Rusty Dot Pearl Moth from Plants
The most effective organic product for controlling Udea ferrugalis larvae (and lepidopteran pests in general) is bacillus thuringiensis var. kurstaki, which is available for purchase in specialized stores. Bacillus thuringiensis acts through ingestion, so it’s important to thoroughly wet the vegetation. It’s also advisable to apply the treatment in the evening. For celery, treatment can be more challenging because the larvae penetrate into the leaf ribs, making them harder to reach. Therefore, visual monitoring of the crop and immediate intervention upon the first appearance of larvae are crucial.