Nalepella
Nalepella danica
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Arachnida
Order:	Trombidiformes
Family:	Phytoptidae
Genus:	Nalepella Keifer, 1944

Nalepella, the rust mites, is a genus of very small Trombidiform mites in the family Phytoptidae. ^{[2] [3]} They are commonly found on a variety of conifers, including hemlock, spruce, balsam fir, and pine. ^{[3] [4] [5] [6]} They sometimes infest Christmas trees in nurseries. ^{[3] [5]} Nalepella mites are vagrants, meaning they circulate around the tree; females overwinter in bark cracks. ^{[5] [7]} Infested spruce emit a characteristic odour. ^[8]

The genus is holarctic, and species are found in North America, Europe, and China. ^{[2] [9]}

The mites feed on the cell sap of the tree's needles, sometimes causing severe damage. ^[5] Typical effects from a Nalepella infestation include needle discolouration and premature needle drop. ^{[3] [7]} The colour of discolouration varies by species; for example, Nalepella tsugifoliae causes yellowed or grey discolouration, while Nalepella halourga's discolouration is more bronze in colour. ^{[7] [10]} Some species are considered serious pests of ornamental coniforous trees. ^[11] They are commonly found on Christmas trees in North America and Europe, and they may seriously damage the tree. ^[3]

Spruce infested by Nalepella were found to increase emissions of certain compounds that may cause the characteristic smell of infested plants. ^[8] Another study in 2009 found that some compounds emitted by infected spruce attracted or repelled Hylobius abietis, another pest of conifers. ^[12]

Nalepella mite eggs overwinter on needles, then hatch early in the spring. ^{[3] [13]} As cold-season mites, they are most active in the early spring and the fall. ^{[3] [13]} The mites deposit eggs during the fall, but may continue to be active into the winter. ^[3] They have multiple generations per year. ^[13]

N. brewrieanae, first discovered in 2003 on Picea breweriana. ^[15] It was first described from Germany, but is also known from Poland. ^{[15] [23]} Besides P. breweriana, it is also known from P. abies and P. glauca. ^{[23] [24]}

Nalepella danica infests members of the Abies (fir) genus. Specifically, it has been recorded from A. alba, A. concolor, A. lasiocarpa, and A. nordmanniana. ^{[23] [25] [26]} It causes small rusty brown to bronze spots on the needles of its host plant, but a severe infestation can result in defoliation. ^{[23] [26]} Nymphs typically grow between 90 and 108 μm, while female adults 145 and 240 μm. ^[26] They are known exclusively from Denmark. ^{[25] [26]}

Nalepella ednae is distributed across the central and Northwestern United States, as well as in British Columbia. ^[27] They are of concern in Mexico, where they may be introduced via cut Christmas trees. ^{[27] [28] [29]} Although it is only known from a few fir species, all may be hosts. ^[27] The damage they cause is unknown. ^{[23] [30]}

Nalepella haarlovi is known from Denmark and Finland. ^[9] It has been recorded infesting Picea sitchensis. ^[31] They are one of the most economically important members of the genus. ^[9] This species has four to eight generations per year. ^[9]

Nalepella halourga, commonly known as the spruce rust mite, is restricted to Picea (spruce). ^[3] Their colour varies throughout the year; during the growing season, they are colourless to pale yellow, but in the fall they turn reddish-purple. ^[10] They are found in Eastern North America. ^[9]

Nalepella longoctonema was first described in 1991 from two fir species in Oregon. ^{[32] [33]} They grow to 206 μm in length, and have been collected in large numbers on fir plantations. ^[9] They are one of the most economically important members of the genus. ^[9]

Nalepella shevtchenkoi lives around the bases of the host plant's needles, as well as on its stems. ^{[23] [34]} It is known from Abies (fir) and Picea (spruce) species. ^[23] The species is considered one of the most damaging of the eriophyoid mites. ^[23] It is found in parts of central and eastern Europe. ^{[35] [36]}

The hemlock rust mite is reddish-orange in colour, and has relatively large eggs. ^[37] They infest fir, hemlock, larch, and yew to high densities- there may be as many as 100 mites on one needle. ^{[3] [7] [38]} Infested trees turn bluish, then yellow, before beginning to drop needles. ^[39] They feed on both sides of the tree's needles. ^[40]