Erannis jacobsoni

Name:   Erannis jacobsoni
Pest Authorities:  Diakonoff
Taxonomic Position:  Insecta: Lepidoptera: Geometridae
Sub-specific Taxon:  
Pest Type:   Insect
Common Name(s):
   Jacobson's spanworm (English)
   Hybernia jacobsoni (Diakonoff)
Numerical Score:  6
Relative Risk Rating:  High Risk
Uncertainty:   Very Uncertain
Uncertainty in this assessment results from: The main area of uncertainty concerns the ability of Erannis jacobsoni to adapt to North American species of larch.

Establishment Potential Is Moderate Risk
The relevant criteria chosen for this organism are:  
  • Suitable climatic conditions and suitable host material coincide with ports of entry or major destinations.
  • Organism has high inoculum potential or high likelihood of reproducing after entry.
Justification: Suitable climatic conditions for Erannis jacobsoni coincide with ports of entry in the northern U.S. and Canada. The only known host of this insect is larch, Larix spp. In North America, eastern larch, Larix laricina, occurs in the vicinity of major ports of entry in eastern Canada. This insect is expected to have a high likelihood of reproducing after entry.

Spread Potential Is High Risk
The relevant criteria chosen for this organism are:  
  • Organism is capable of dispersing more than several km per year through its own movement or by abiotic factors (such as wind, water or vectors).
  • Organism has demonstrated the ability for redistribution through human-assisted transport.
  • Organism has a high reproductive potential
  • Potential hosts have contiguous distribution.
  • Newly established populations may go undetected for many years due to cryptic nature, concealed activity, slow development of damage symptoms, or misdiagnosis.
  • Eradication techniques are unknown, infeasible, or expected to be ineffective.
Justification: Adult females of Erannis jacobsoni are wingless and incapable of flight. Therefore, the major agent of dispersal is ballooning of early instar larvae by air currents. This insect could be spread over long distances by egg masses on logs destined for export. E. jacobsoni has a high reproductive potential and eastern larch, Larix laricina, has a more or less continuous distribution across the boreal forests of North America. The two other larches native to North America, L. occidentalis and L. lyallii, have more localized distributions. This insect could escape detection, especially in remote forest areas and eradication is expected to be difficult.

Economic Potential Is Moderate Risk
The relevant criteria chosen for this organism are:  
  • Organism attacks hosts or products with significant commercial value (such as for timber, pulp, or wood products.
  • Organism directly causes tree mortality or predisposes host to mortality by other organisms.
  • Organism may cause loss of markets (domestic or foreign) due to presence and quarantine significant status.
Justification: Erannis jacobsoni is capable of developing into periodic outbreaks over extensive areas of larch forests in Russia. These outbreaks result in growth loss and loss of seed crops. Moreover, successive defoliation could make trees susceptible to bark beetle and/or woodborer attack. Similar damage could ocur to larch in North America.

Environmental Potential Is High Risk
The relevant criteria chosen for this organism are:  
  • Organism is expected to cause significant direct environmental effects, such as extensive ecological disruption or large scale reduction of biodiversity.
  • Organism may attack host with small native range.
  • Introduction of the organism would likely result in control/eradication programs that may have potential adverse environmental affects.
Justification: Defoliation caused by Erannis jacobsoni can lead to the death of large areas of larch forests, either directly or by leaving the forest susceptible to subsequent attack by other forest pests such as bark beetles and woodborers. Defoliated forests are also predisposed to more severe wildfires, which will have adverse impacts on other ecosystem components. Control or eradication would involve use of chemical or biological pesticides with potential adverse effects on non-target organisms.

Erannis jacobsoni feeds exclusively on larch, Larix spp. Within its natural range Larix gmelinii (= Larix dahurica) and Larix sibirica (= Larix sukaczevii) are known hosts.

     Russia (east of southern Siberia, Transbaïkalia, southern portions of Siberia and the Russian Far East) and Mongolia.
Erannis jacobsoni requires one year to complete a generation (Pleshanov 1982) and is one of several species of Erannis known to be defoliators of forests in the northern hemisphere. In North America, the indigenous linden looper, E. tiliaria (Harris), is a defoliator of broadleaf forests in eastern North America (Drooz 1985).

Adults usually appear in the middle of September and remain until the middle of October. The maximum activity occurs at the end of September. Females have no wings and do not fly. The migratory capacity of Erannis jacobsoni is, therefore, limited. This results in epidemic populations appearing on the same trees for many consecutive years.

Females lay eggs under bark scales on the trunks and in cracks in the bark of branches. Eggs overwinter and first instar larvae appear at the end of May and the beginning of June. They feed on needles, and peak defoliation occurs in the middle of July. Populations may reach a density of 6000 larvae per tree. Larvae molt four times before making cocoons in the soil and pupating at the end of July.

Outbreaks of Erannis jacobsoni are characterized by cycles of a slow build-up of population numbers over several years, reaching a peak and followed by a population collapse. These outbreaks occur with a periodicity of about 6-7 years (Amsheev 1996) and usually last 3-4 years. They are often preceded by periods of drought and can occur in conjunction with outbreaks of other forest defoliatiors including Orgyia antiqua, Lymantria dispar (Lepidoptera: Lymantridae) and/or Dendrolimus sibiricus (Lepidoptera: Lasiocampidae).

Erannis jacobsoni prefers dry conditions and areas with a continental climate. Complete defoliation over 3-4 consecutive years often leads to the death of forests. Furthermore, outbreaks of E. jacobsoni are also very often followed by outbreaks of bark beetles and woodborers, particularly, Ips subelongatus (Coleoptera: Scolytidae), Tetropium gracilicorne (Coleoptera: Cerambycidae), Melanophila guttulata (Coleoptera: Buprestidae) and Sirex ermak (Hymenoptera: Siricidae) (Epova and Pleshanov 1995). These insects are able to kill trees, which are already heavily stressed by E. jacobsoni.

Natural enemies of Erannis jacobsoni include the egg parasitoid Telenomus mayri (Hymenoptera: Scelionidae), the pupal parasitoids Cratichneumon pachymerus, Cratichneumon nigritarius (Hymenoptera: Ichneumonidae), Ceromasia rubrifrons (Diptera: Tachinidae), and the larval parasitoids Rhogas rossicus (Hymenoptera: Braconidae) and Phryxe vulgaris(Diptera: Tachinidae). Other parasitoids, predators and diseases may also play an important role in population regulation (Boldaruev 1972, Kondakov 1979, Pleshanov 1982, Amsheev 1996).

Economic Impact:    Erannis jacobsoni develops into outbreaks in Russia that occur over large areas (thousands of hectares) and usually cause major reductions in tree growth and seed production. Particularly serious damage occurred at the end of the 1950s. Young trees may be killed rather quickly and 100% defoliation of large trees over 3-4 years often causes extensive tree mortality. This is especially true if E. jacobsoni occurs in association with other defoliators (Pleshanov and Vassilieva 1974, Pleshanov and Issaev 1981, Pleshanov 1982, Boldaruev 1972, Kondakov 1979, Amsheev 1996).

Environmental Impact:   Defoliation caused by Erannis jacobsoni can lead to the death of large areas of larch forests, either directly or by leaving the forest susceptible to subsequent attack by other forest pests, and/or by predisposing the forest to wildfires of increased intensity with consequent impacts on other elements of the environment (Pleshanov 1982) .

Some forest ecologists, on the other hand, argue that defoliating insects are nutrient recyclers in forests (Mattson and Addy 1975). Larval feeding results in a transfer of nutrients from the foliage to the soil. Moreover, defoliators are instrumental in setting the stage for natural succession. Crown thinning caused by defoliation allows more sunlight to reach the forest floor and encourages growth of understory plants. Stand openings created by heavy feeding favors the growth of successive generations of trees. Despite the fact that native forest defoliators may have devastating economic impacts, they may be beneficial in ecological terms because they contribute to greater diversity of plant species and, therefore, increased ecosystem stability.

Control:    In Russia, direct control of outbreaks is done via aerial applications of chemical and bacterial insecticides (Pleshanov 1982).

Symptoms:    Symptoms include defoliation of larch and presence of insect life stages.

Morphology:    The larvae are typical inchworms. Two forms of larvae are known: a normal and dark-colored form. The difference between the two forms appears from the second instar onward. It has been suggested that melanism occurs as a consequence of food deficit or of high population density (group effect). Dark-colored caterpillars have a brown-red head capsule with some light spots. Along the dorsal surface of the body, there are two brown dotted lines, with four similar sub-dorsal lines below them. Along each side of the body between the 1st and 6th segments, there is a yellow stripe, which is broken by a stigmal spot. The ventral side of the body is more or less light with four brown dotted lines. Sometimes dark lines join each other and, in these cases, the caterpillars become almost completely dark. There exist many transitional color variations between normal and dark forms. In addition to the typical and dark forms, there also exists a more genetically stable dark form, named Erannis jacobsoni ab. nigra Pleshanov. The existence of dark forms is characteristic of many species of the genus Erannis.

Females have no wings. Dark females have an abdomen completely covered with dark scales, which form only dark spots on the white abdomen of typical females. The front wings of males are uniformly brown with a darker transversal band, which is difficult to distinguish in the case of the dark form. The rear wings are cream colored, and do not have the well-developed dark, transverse band and dark middle spot.

Testing Methods for Identification:    Examination of adults by a taxonomist with expertise in the family Geometridae is required for positive identification. Adults and larvae have sufficient characteristics to permit entomologists to make field identifications at least to genus.

Erannis jacobsoni females are wingless and incapable of flight. Therefore, tree to tree dispersal is primarily by ballooning of early instar larvae by air currents.

Eggs may be easily transported on wood containing bark because they are carefully hidden under scales and cracks and remain in place for long periods (9 months between September and June). All stages of the life cycle can be transported on plants moving in trade, particularly nursery stock.

Amsheev, R. M. 1996. Ecology of the most important phytophagous insects in Buryatia and their control. Doctorate thesis in biological science, Irkutsk, 102 pp. (in Russian).
Boldaruev, V. O. 1972. Jakobson's spanworm - Erannis jacobsoni Djak. (Lepidoptera, Geometridae) in forests of Buryatia. In: Main Pests of Trees and Shrubs of Transbaïkalia. Trudy Buryatskogo Instituta Estestvennykh Nauk 7: 3 - 19 (in Russian).
Boldaruev, V. O. 1972. Erannis jacobsoni Djac. (Lepidiptera, Geometridae) - a serious pest of larch in Transbaïkalia. Entomologicheskoe Obozrenie 51: 47 - 58 (in Russian).
Drooz, A.T. 1985. Insects of eastern forests. USDA Forest Service, Miscellaneous Publication 1426, 608 pp.
Epova V. I.; A. S. Pleshanov. 1995. The forest regions injured by phyllophagous insects in Asian Russia. Novosibirsk: Nauka, Siberian Publishing Firm of the Russian Academy of Sciences, 147 pp. (in Russian).
Kondakov, Y. P. 1979. Phyllophagous insects - forest pests of Baïkal lake region. In: "Fauna of Forests of Baïkal Lake region". Novossibirsk:Nauka, Siberian Department, pp. 5 - 44 (in Russian).
Mattson, W.J.; Addy, N.D. 1975. Phytophagous insects as regulators of forest primary production. Science 190: 515-522.
Pavlovskii E. N.; Shtakelberg, A.A. 1955. Forest pests guide. Moscow- Leningrad: Edition of the Academy of Sciences of the USSR 1, 421 pp. (in Russian).
Pleshanov A. S.; Vassilieva, T.G. 1974. Outbreaks of larch defoliators at Baïkal lake. In: Transactions of VIII Congress of the All-Union Entomological Society, Part II. Leningrad, pp. 233 - 234 (in Russian).
Pleshanov, A. S. 1982. Insect defoliators of larch forests of Eastern Siberia. Novossibirsk: Nauka, Siberian Publishing Firm of the Russian Academy of Sciences, 209 pp. (in Russian).
Pleshanov, A. S.; Issaev, A.S. 1981. Ecological and physiological aspects of the resistance of larch to insect defoliators. In: Role of Plant-Insect Interactions in the Dynamic of Forest Pest Populations. Abstracts of the Symposium, Irkutsk, p. 40 - 41 and 80 - 81 (in Russian).
Andrei Dorian Orlinski
Name and Address of the First Author:
Andrei Dorian Orlinski
1, rue Le Nôtre
France 75016
CREATION DATE:        05/13/01