Taxonomic position: Phylum: Arthropoda; Class: Insecta; Order: Lepidoptera; Family: Noctuidae
Description: The Spanish common name of this insect of the Lepidoptera order is bicho quemador (“burning bug”), because its branching hairs produce a highly itchy skin rash, which feels like a burning of the skin. H. nigricans is a species native to Argentina, and every year it causes serious damage to the foliage of fruit trees and forests. Its attacks are aimed at a great number of tree and fruit tree species. Severe defoliation can occur, even though the damage is not very serious. The skin irritation caused by the specimens ―specially during the harvest― is very annoying.
The female individuals lay their eggs on the branches, in overlapping layers consisting of up to 900 eggs. The eggs are protected by a hemispherical yellow cocoon which the female weaves with hairs from its abdomen and silk. In the spring, after they are born, the caterpillars move about branches, trunks and leaves. They usually feed on buds and leaves. The fully developed larva is about 40 millimeters long, it is black with yellowish areas, and it has highly developed setiferous tubers, which bear urticating hairs. The specimens start pupation by the end of January. The adult butterflies emerge by the beginning of March.
There are seven larval instars. The behaviour of the specimens during each instar will differ.
First larval instar:
Specimens are about 2,5 millimeters long, bright yellow. For the first two or three days they remain clustered near the laid eggs. They later start to look for a nearing branch with tender leaves so as to feed on them. They move from one place to the other in groups, as in a procession. This is why in Europe they are known as “processioning” insects. After the seventh or eight day, they feed on the leaf parenchyma; the veins, however, are left untouched. Molting occurs here; the specimens leave the shed skin glued to the leaves. This instar may last 20-25 days.
Second larval instar:
Specimens cluster together on the underside of the leaves. During the first and second days, they do not feed. Then they move on to a new leaf, on which they feed, without touching the veins. This instar lasts six or seven days.
Third larval instar:
Specimens cluster together, in this case protected with silk hairs. They always host on the underside of the leaves, on which they feed without touching the veins. This instar lasts seven to eight days.
Fourth larval instar:
The specimens remain clustered on the underside of the leaves. However, the damage is more serious and visible at this stage. Entire branches suffer total defoliation, the silk threads knotted around them. This normally happens in December, when temperatures are high, and it usually takes five or six days. By the end of this instar, the specimens go down, in columns, to the main trunk; the most developed larvae lead the procession. As they move, they leave behind silky, sticky threads, with which they can cling better to the plant. They gather in a group of about 10-15 centimeters wide, or even more. They can be found from the base of the plant upwards, up to the area where the first leaf projects. This gathering may last four or five days. The procession then goes up, and molted skins are left behind.
Fifth larval instar:
During this instar, the specimens still have gregarious behavior. They remain grouped on the underside of the leaves of terminal branches. On account of the weight of the group of larvae, the branches bend downwards. These branches suffer total defoliation. The specimens are highly fidgety and tend to drop immediately to the ground when disturbed. At the moment of molting, they go downwards, in a procession, and end up gathering in a group of about 500 to 600 larvae; the gathering may have a diameter of 15 to 25 centimeters. As in the before mentioned instars, mortality rates are really low because there are almost no natural enemies. This instar lasts five to six days.
Sixth larval instar:
From this moment onwards, the larvae no longer behave in groups, but, in isolation, start to disperse. They move on to other trees, where they will live on their own. Mortality rate is high at this stage. At the end of this stage, they will build a sort of case with silk hairs; molting will occur inside this cocoon. This stage lasts about seven or eight days.
Seventh larval instar:
The larvae cannot move about very much. They spend part of the day within the protecting cocoon. It is common to see more than one larva within a same cocoon. In this way, they are sheltered from the harsh environment and natural enemies, which would kill them in great numbers. When they reach their maximum size, the larvae can reach a length of 40 millimeters. Their integument is velvety, light-chestnut in color with greenish hues and dark-colored tubercles, from which branching hairs and setae protrude. This instar takes place during the second half of December and the first half of January.
The adult specimen is a medium-sized moth, black and with a layer of silky, golden hairs on the rear section of the abdomen.
They overwinter as pupae. Depending on the latitude, there may be one to three generations per year.
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Importing countries may, if required, contact the National Directorate of Plant Protection for an official technical report on the phytosanitary status of the crops. The Directorate shall prepare such report in accordance with the information contained in this database.