Adelges piceae
(balsam woolly adelgid)

A. piceae was introduced from Europe into North America (Maine, first report: 1908) before or around 1900 with imported nursery stock. In the first half of the twentieth century it was spreading all over the continent from the east to the west and to the south-east. It was then reported from New Hampshire in 1916, from California (San Francisco) in 1928, from Oregon in 1930 and from Virginia (Skyland) in 1956. At present, it occurs in the maritime provinces of Canada, in north-eastern USA and in south-western USA (Virginia and North Carolina). The most frequently attacked true fir species are Abies balsamea, Abies fraseri, Abies lasiocarpa, Abies amabilis and Abies grandis (Foottit and Mackauer, 1980, 1983). In Europe, where it is indigenous, the adelgid causes little damage to its principle host plant, Abies alba. In North America it has been reported as an important pest of several economically valuable fir species. Feeding by the adelgid causes damage to the bark and wood, and reduces tree growth, but heavy attacks to the stems and branches, seriously disrupt the trees' metabolism and can eventually lead to tree death.

The impact of A. piceae on true firs in North America has been strong and severe and has caused timber losses, a reduction in tree growth and viable seed production. It has often caused complete mortality of mature stands and sometimes also of the regeneration stands (balsam fir).

Biology

A. piceae is anholocyclic on Abies sp. and is native to Europe where it colonizes Abies alba and other congeneric species. In North America, A. piceae has become a serious pest on many of the indigenous species of Abies since the beginning of the last century.

In central Europe, A. piceae has one to three generations per year while in North America it may have another partial one (Antonelli, 1992). The adelgid usually overwinters as a first or second instar larva (Balch, 1952; Karafiat and Rudinsky, 1956; Harris, 1978; Covassi and Binazzi, 1981). As a rule, the number of generations per year appears to depend mostly on the altitude and on the summer and winter temperatures. It also depends on the local conditions of the fir stands (Mitchell et al., 1961). In Italy, the newly born larvae reach maturity in about 1.5-2 months in the early spring and in the autumn, while they complete their growth in only 1 month in the summer (Binazzi and Francardi, 2001).

In silver fir woods ranging from 900 to 1200 m, four to five generations per year have been counted, which depend directly on the spring/summer weather. In those habitats, A. piceae normally overwinters as a first or second instar larva and reaches the adult stage at the end of February so that the first egg-laying females occur at the end of March. In fir stands at a low altitude (under 300 m a.s.l.), A. piceae has a life cycle of five generations and overwinters as first, second or third instar larvae. They reach maturity from the end of January onwards. A similar behaviour has been observed in other lower mountain fir woods or on single trees of parks, gardens, hills and lowlands. Here pseudohiemosistens larvae start their development at the end of January and reach the adult stage from the middle of February onwards. In warmer climatic conditions, A. piceae does not have a very long quiescence time contrary to what has been observed by some authors in other geographical areas of the northern hemisphere (Mitchell et al., 1961).

The mature females are able to lay an average of 70-80 eggs per cluster, which gradually hatch within 3-4 days. From the beginning of spring onwards, the young larva develops after three moults, into an apterous adult female (pseudoprogrediens) or after four moults into an alate female (exulans). In mid-July, some of them can change into a more resistant, sclerified form (pseudoaestivosistens) without moulting during the whole of the summer. This form can reach maturity in October or spend the following winter as a pseudohiemosistens. However, all of them are able to survive the hardest climatic conditions. In Italy, the adult females have an egg-laying peak in May and June, when they appear to be most numerous from the middle to the highest levels of the host-tree crown, mainly on young tree canopies. The pseudoprogrediens egg-laying females have been observed until mid-November (Francardi et al., 1999). In central Europe, the mature females start their egg-laying in June with a peak during the summer (August to September). Egg production decreases then ceases during the early autumn (Eichhorn, 1968).

Ecology

The newly born larvae are mobile. They spread along the leading axis and lateral twigs of white fir trees. They settle on the bark or under the ring of perules at the bud bases, along the trunk or branches, and even on the protruding roots from the ground (Binazzi and Covassi, 1991). Each young larva may cover more than 30 m in the crawler stage (Harris, 1978) and this stage is more susceptible to the hardest environmental conditions and to predators (Eichhorn, 1968). After the moult, individuals of A. piceae can change their feeding sites by moving along the stem. Dispersal of the woolly aphid on the host plant can occur in a relatively short time. All of the different instar larvae of A. piceae tend to gather in small groups of at least 10-15 individuals or settle in scattered positions along the lower surface of the twigs. Here they can find damper and shadier sites, more favourable to their requirements (Balch, 1952).

When the woolly aphids increase in number, all the epigean parts of the plants (i.e. trunk, branches and twigs) can be heavily colonized even when exposed to the sun and the colonies can also be found on the exposed roots. In addition, sooty mould fungi grow plentifully on the adelgid honeydew and this fact, together with the adelgid sap sucking, often leads to the decline or death of the attacked trees. This aggressive behaviour is more common on isolated trees in parks or gardens, in critical physiological conditions. When the attacks persist, they can heavily damage the trees but when the infestation breaks down, the trees can recover. The woolly aphids overwinter inside the bark crevices (trunk and branches) or under the bud perules. In addition, the importance of other vegetative structures as feeding and overwintering sites for A. piceae is known in Europe. These include buds left by larvae of the tortricoid moth, Epinotia nigricana (Francardi and Covassi, 1992, 1994). Individuals of A. piceae have also been found in the bottom of stems mined by larvae of Argyresthia illuminatella, in regeneration areas of Abies alba (Covassi and Francardi, 1994).

Studies on the predator complex of A. piceae have been carried out since the 1950s by Delucchi (1956) and other authors, i.e. Pschorn-Walcher and Zwölfer (1956); Karafiat and Franz (1956); Pschorn-Walcher (1964); Thomas (1968); Eichhorn (1968, 1969); Smirnoff (1970); Harris and Dawson (1979); Coulson and Witter (1984); Fowler et al. (1986); Kevan and Klimaszewski (1987); Binazzi and Covassi (1991); Humble (1994); Carter and Winter (1998); Francardi et al. (1999); and Binazzi and Francardi (2001).

The orders and families that include important natural enemies of A. piceae are:

- Coleoptera - Coccinellidae (Aphidecta, Pullus, Anatis); Derodontidae (Laricobius).

- Diptera - Syrphidae (Neocnemodon); Chamaemyiidae (Leucopis); Cremifaniidae (Cremifania); Cecidomyiidae (Aphidoletes, Lestodiplosis).

- Neuroptera - Chrysopidae (Chrysopa); Hemerobiidae (Hemerobius).

- Moniliales - Tuberculariaceae (Fusarium).

The control of A. piceae brought about by the antagonists may regulate and sometimes minimize insect attack in small, isolated plantations of fir but it has never appeared effective in forest situations. In Europe, where A. piceae is native, damage by it is found only occasionally. Most of the antagonist species listed in the natural enemy table were introduced into the USA and Canada. Despite the acclimation of some of those enemies during the second half of the last century, many fir forests were injured by A. piceae (Smirnoff, 1971; Harris and Dawson, 1979; Coulson and Witter, 1984; Fowler et al., 1986). Over 20 species of insect antagonists of A. piceae have been introduced into North America from Europe, Asia and Australia during the last 50 years but only six of them have become established. However, neither the introduced nor the native ones have caused any significant reduction in the adelgid populations.

The native insect antagonists or the exotic species that were established in areas of North America infested with A. piceae, did not have any satisfactory effect in reducing the adelgid populations during the second half of the last century (Thomas, 1968; Smirnoff, 1970, 1971; Edimann and Ehnstrom, 1975; Harris and Dawson, 1979; Coulson and Witter, 1984; Fowler et al., 1986; Kevan and Klimaszewsky, 1987; Humble, 1994; Zilahi-Balogh et al., 2002).

A. piceae can be detected on tree trunks by the specks of white wax wool of the mature females, on the shoots by the gout-like growths caused by the adelgid sap sucking, and on the stems of young plants, by their shortening and tapering (progrediens form). The sistens generation individuals are partially concealed under bud scales or in crevices, scars or lenticels of the bark.

A. piceae is morphologically very similar to other congeneric species from Europe and Caucasus. The identification to specific level of those species is rather difficult and based mostly on morphological features of the first instar larvae of the sistens generations, mainly of the hibernating forms (pseudohiemosistens). The main character used for identification is the number of pores within the ten 'middle fields' of the spinal plates of the meso- and meta-thorax and of the first three abdominal segments. In addition, the shape and the relief of those pores plus the prothoracic dorsal sclerotization are useful. Other suitable differentiating characters are the body length and width of the neosistens forms, the ultimate rostral segment size, and the lengths of the third antennal segment and apical seta of the same forms. See Binazzi and Covassi (1991) for a key to the species.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

There are few control techniques for A. piceae available to the land manager.
Biological Control

The native insect antagonists or those species that were established in areas of North America infested with A. piceae, did not have any satisfactory effect in reducing the adelgid populations during the last century (Thomas, 1968; Smirnoff, 1970, 1971; Edimann and Ehnstrom, 1975; Harris and Dawson, 1979; Coulson and Witter, 1984; Fowler et al., 1986; Kevan and Klimaszewsky, 1987; Humble, 1994; Zilahi-Balogh et al., 2002).

Chemical Control

Insecticides sprayed from the air over forest stands were ineffective in some experiments as A. piceae females are effectively protected by the copious wax wool covering their bodies (Coulson and Witter, 1984; Hastings et al., 1986; Carter and Winter, 1998). This method is often impractical when large areas need to be sprayed because of the harmful impact this may have on the balance of the biocoenosis. Otherwise, insecticides, used in ground applications to protect Christmas trees, seed orchards and highly valued trees in parks and gardens, can be effective in reducing A. piceae population levels but they are extremely costly.

In land management, restrictions on moving true fir logs out of the infested areas or importing true fir nursery stock or ornamentals have been useful methods of control. Other effective attempts to control A. piceae have included harvesting true firs from an infested area to protect the uninfested nearby stands, shortening of the rotation age of true fir stands, or conversion to non susceptible or less susceptible fir species or to other tree species.