Genus Phytobia

Phytobia Lioy, 1864

Taxonomic history
In earlier literature the genus was interpreted in a wider sense than today. Calycomyza, Amauromyza, Nemorimyza, Dizygomyza, Poemyza and Icteromyza were sometimes treated as subgenera. The former two are now separate genera whereas the latter three groups are nowadays subgenera of Cerodontha.
At present, Phytobia consists exclusively of the cambium miners. The monophyly can be confirmed by larval characters and the peculiar lifestyle, whereas the adults show rather plesiomorphic characters. So far, about 50 Phytobia species are described worldwide but many undescribed species should be expected especially in the tropical regions (von Tschirnhaus personal communication).

Adults, external
For identification of imagines, several characters have to be used in combination with each other: Orbital setulae reclinate or erect; pre-sutural dorsocentral bristles strongly developed; pre scutellar bristles present, usually more than two; frons normally dark, lunule often silvery (Spencer, 1974, 1986).
Subcosta often partly fused with R1 Phb cambii wing id.pct and therefore the wing sometimes appears similar to Agromyza wings Ag albipennis wing id.pct. In most cases the similarity is quite superficially and pose no risk of misidentification. Anyway, to avoid confusion, the stridulation mechanism missing in Phytobia and only present in Agromyza should be checked (Tschirnhaus, 1972, see External adult morphology) (Ag intermittens strid file.pct).

Male terminalia
Gonites rod-like as in other Phytomyzinae. Surstyli are distinctly articulated (Phb cambii epandrium.pct).
Female oviscape and male aedeagal apodeme usually very long. The aedeagus has often strong, conspicuous appendages but altogether there is a wide range of shapes in this genus.

Immature stages (mainly based on the descriptions of Phytobia cambii and Phytobia cerasiferae)
The larvae cannot be confused with any other dipteran larva because of their unusual slender and thin body shape. They can reach up to 3.0 cm (but often shorter) in length and a width of only about 0.5 mm. The posterior spiracles are directed posteriorly, each with three bulbs of equal size Phb cambii Larva SEM3.pct. The locomotion welts consist of flat scales rather than denticles Phb cambii Larva SEM2.pct.

The cephalopharyngeal skeleton phb cambii larva.pct shows some typical characters of most agromyzid larvae: thin, inconspicuous dorsal bridge; elongated intermediate sclerite; mouth hooks alternating; right mandible higher than left one Phb cambii Larva SEM1.pct. The following characters are distinctive for Phytobia larvae: lateral sclerites of mandibles mainly present but less heavily sclerotized than e.g. in Napomyza lateralis; hind margin of mandibles with a pronounced dorsal projection; dorsal margin of middle and basal part of cephalopharyngeal skeleton almost straight with a lower part of the dorsal arm missing.
Larval diagnostic characters to distinguish between different Phytobia spp. are not yet known.
The puparia are large but rather broad and short as found in other agromyzid taxa. They are never as heavily elongated as the larvae.

This is a general summary of the available information on Phytobia feeding habit and life history. The data are based on studies of several species. Present knowledge indicates a rather uniform lifestyle all over the genus. However, since differences between species are poorly known, I include the name of the species under study behind the authors cited.
The larval feeding habit of these flies is unique among Diptera in that they mine in the stems of living trees Phb cambii mines.pct. ash tree.pct Previously, the cambium-cells themselves were suggested as food source but recent investigations indicate that "they mine in the zone of differentiating xylem between the cambium and the lignified xylem elements" (Ylioja et al., 1998, on Phytobia cambii). Some years earlier, Wallner and Gregory, 1979 (on Phytobia setosa) stated "the initial [cambium] cells were unaffected, hence although called a cambium miner P. setosa does not mine in the cambium". Although limited information from a small number of species are available, it is generally assumed that all Phytobia larvae feed on living trees. The females oviposit in younger shoots of the tree. The hatching larvae mine downwards to the root collar or even proceed further into the root. They can be surprisingly long (up to 15 m) (Ylioja et al., 1999). Pupariation occurs normally in the soil near the host plant after the fully grown larva has left the stem. However, it is frequently observed that a fair part of the mines terminate at the base of the tree crown. This can be due to mortality (Ylioja et al., 1998, on Phytobia cambii) or early maturing of the larvae before entering the stem (Wallner and Gregory, 1980, on Phytobia setosa). This can be true especially for older trees, since Rexrode and Baumgras, 1980 (on Phytobia pruni) observed that most piths flecks occurred during the first 10-15 years of cambial growth. Hence, the flies possibly prefer the younger branches of old and huge trees.
All species with known biology appear to have a single generation per year. Normally, the species hibernate as puparia and the adults emerge in spring between April and June of the subsequent year. Conversely, Phytobia cerasiferae overwinters as larva in the tree and pupariate in the same year, in which the adults emerge (Pitcher, 1956).

Host plants
Known from several angiosperm tree families.

Economic importance
Although the tunnels produced by larval feeding are refilled with callus cells, the feeding galleries remain visible in the wood. These so-called pith flecks Phb betulivora wood.pct can reduce stability and commercial value of the wood. Sometimes it is reported that the vigor of infested trees can be reduced especially in nurseries (e.g. Lahey and Agrios, 1977, on Phytobia pruni; Martinez et al., 1985, on Phytobia cambii). Sometimes even insecticides are applied against Phytobia (see Moraal and Grijpma, 1987).