''Rhizoglyphus'' is a genus of mites in the family

Acaridae The Acaridae are a family of mites in order Sarcoptiformes. Distribution There are several acarid genera with cosmopolitan distributions, such as ''Acarus', Sancassania'' and ''Tyrophagus''. Ecology Acaridae live in various habitats an ...

. It has a worldwide distribution and is often associated with the

bulb In botany, a bulb is a short underground stem with fleshy leaves or leaf basesBell, A.D. 1997. ''Plant form: an illustrated guide to flowering plant morphology''. Oxford University Press, Oxford, U.K. that function as food storage organs duri ...

corm Corm, bulbo-tuber, or bulbotuber is a short, vertical, swollen, underground plant stem that serves as a storage organ that some plants use to survive winter or other adverse conditions such as summer drought and heat (perennation). The word ''c ...

s or

tuber Tubers are a type of enlarged structure that plants use as storage organs for nutrients, derived from stems or roots. Tubers help plants perennate (survive winter or dry months), provide energy and nutrients, and are a means of asexual reproduc ...

s of plants.

Description and life cycle

''Rhizoglyphus'' begin their lives as whitish, ellipsoidal

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...

s that are 0.12 mm long. These hatch into larvae, which are oval, white and have three pairs of legs. Larvae are 0.15-0.2 mm long on hatching and grow to 0.25 mm long. Larvae then develop into protonymphs, which are 0.4 mm long and (compared to larvae) gain an additional leg pair and two genital suckers. Under certain conditions such as high population density, protonymphs develop into deutonymphs (or hypopi), a quiescent stage that does not feed. This life stage is convex dorsally and flattened ventrally, brown in colour, 0.2-0.3 mm long, heavily

sclerotised Sclerotin is a component of the cuticle of various Arthropoda, most familiarly insects. It is formed by cross-linking members of particular classes of protein molecules, a biochemical process called sclerotization, a form of tanning in which q ...

, lacks mouthparts and has a ventral sucker plate. The purpose of the deutonymph stage is to attach to insects and be carried to other bulbs (

phoresis Phoresis or phoresy is a temporary commensalistic relationship when an organism (a phoront or phoretic) attaches itself to a host organism solely for travel. It has been seen in ticks and mites since the 18th century, and in fossils 320 ...

). Deutonymphs are resistant to starvation and desiccation during adverse conditions. The tritonymph stage follows the protonymph (or deutonymph). Tritonymphs are about 0.5 mm long and have three or four genital suckers. They eventually develop into adults. Adults are 0.5-0.9 mm long, shiny, white and somewhat transparent, and have reddish brown appendages. Unlike tritonymphs, they have distinct genital apertures. Some features which distinguish adult ''Rhizoglyphus'' from other mites are: 4 pairs of prodorsal

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

e and 12 pairs of dorsal hysterosomal setae, external vertical setae small/vestigial and situated about halfway along sides of prodorsal shield, supracoxal setae smooth, and tarsi I and II each with a proximal conical spine near bases of solenidion and famulus. The duration of the life cycle depends on ''Rhizoglyphus'' species, diet, temperature and relative humidity. At a temperature of 25 °C, it can be as short as 12.2 days for ''Rhizoglyphus robini'' and 13.9 days for ''R. echinopus''. Adults live longer (for up to 121 days) at lower temperatures. Males tend to live twice as long as females in this genus.

Host arthropods

As noted previously, ''Rhizoglyphus'' deutonymphs ride on other, larger arthropods for dispersal. They have been collected from scarab beetles,

weevils Weevils are beetles belonging to the Taxonomic rank, superfamily Curculionoidea, known for their elongated snouts. They are usually small – less than in length – and Herbivore, herbivorous. Approximately 97,000 species of weevils are known. ...

, various

flies Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwin ...

and

flea Flea, the common name for the order (biology), order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by hematophagy, ingesting the blood of their hosts. Adult f ...

Host plants

''Rhizoglyphus'' feed on a range of ornamental (e.g. ''

Dahlia ''Dahlia'' ( , ) is a genus of bushy, tuberous, herbaceous perennial plants native to Mexico and Central America. Dahlias are members of the Asteraceae (synonym name: Compositae) family of dicotyledonous plants, its relatives include the sun ...

'', '' Eucharis'', ''

Gladiolus ''Gladiolus'' (from Latin, the diminutive of ''gladius'', a sword) is a genus of perennial cormous flowering plants in the iris family (Iridaceae). It is sometimes called the 'sword lily', but is usually called by its generic name (plural ''g ...

'', '' Hyacinthus'', ''Iris'',

lily ''Lilium'' ( ) is a genus of herbaceous flowering plants growing from bulbs, all with large and often prominent flowers. Lilies are a group of flowering plants which are important in culture and literature in much of the world. Most species are ...

, ''Narcissus'',

orchid Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan plants that are found in almost every habitat on Eart ...

tulip Tulips are spring-blooming perennial herbaceous bulbiferous geophytes in the ''Tulipa'' genus. Their flowers are usually large, showy, and brightly coloured, generally red, orange, pink, yellow, or white. They often have a different colour ...

) and crop plants (e.g.

barley Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...

carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in colour, though heirloom variants including purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild ...

garlic Garlic (''Allium sativum'') is a species of bulbous flowering plants in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chives, Welsh onion, and Chinese onion. Garlic is native to central and south Asia, str ...

onion An onion (''Allium cepa'' , from Latin ), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the onion which was classifie ...

potato The potato () is a starchy tuberous vegetable native to the Americas that is consumed as a staple food in many parts of the world. Potatoes are underground stem tubers of the plant ''Solanum tuberosum'', a perennial in the nightshade famil ...

sweet potato The sweet potato or sweetpotato (''Ipomoea batatas'') is a dicotyledonous plant in the morning glory family, Convolvulaceae. Its sizeable, starchy, sweet-tasting tuberous roots are used as a root vegetable, which is a staple food in parts of ...

taro Taro (; ''Colocasia esculenta'') is a root vegetable. It is the most widely cultivated species of several plants in the family Araceae that are used as vegetables for their corms, leaves, stems and Petiole (botany), petioles. Taro corms are a ...

). While underground plant parts are most commonly attacked, the mites have also been collected from leaves, stems and seeds.

Biochemistry

''Rhizoglyphus'' secrete an alarm

pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...

from opisthonotal glands. This pheromone has neryl-formate as its main component and

citral Citral is an acyclic monoterpene aldehyde. Being a monoterpene, it is made of two isoprene units. Citral is a collective term which covers two geometric isomers that have their own separate names; the ''E''-isomer is named geranial (''trans''- ...

and α-acaridial as minor components. Its secretion triggers escape behaviour in other mites and it may be involved in mite defense against predators. Citral and α-acaridial also have

antifungal An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as ...

activity. These mites also have a range of cuticular chemicals. Hexyl rhizoglyphinate is another antifungal, while the roles of other chemicals remain unknown.

Pest status

As noted previously, ''Rhizoglyphus'' feed on a range of plants. Infested bulbs may rot and fail to produce new growth, or new growth may be discoloured, stunted and distorted. In tulips, ''Rhizoglyphus'' may enter prematurely opened buds and cause bud necrosis. Bulbs that are damaged or infested with ''

Fusarium ''Fusarium'' (; ) is a large genus of filamentous fungi, part of a group often referred to as hyphomycetes, widely distributed in soil and associated with plants. Most species are harmless saprobes, and are relatively abundant members of the s ...

'' fungus are more vulnerable to attack. Due to their pest status, these mites are classified as quarantine species by many countries such as Australia and New Zealand.

Control measures

Rough handling of bulbs should be avoided as this can cause damage that allows ''Rhizoglyphus'' to enter. Keeping bulbs dry also generally prevents them from being attacked, since the mites cannot withstand drought. A range of

pesticide Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all p ...

s have been used, but ''Rhizoglyphus'' are resistant to a number of pesticides and there is concern about pesticides affecting non-target species. There has been research on

biological control Biological control or biocontrol is a method of controlling pests, whether pest animals such as insects and mites, weeds, or pathogens affecting animals or plants by using other organisms. It relies on predation, parasitism, herbivory, or o ...

of this genus of mites. Most of this has involved predatory mites, but

entomopathogenic nematode Entomopathogenic nematodes (EPN) are a group of nematodes (thread worms), that cause death to insects. The term ''entomopathogenic'' has a Greek origin, with ''entomon'', meaning ''insect'', and ''pathogenic'', which means ''causing disease''. T ...

s have also been considered.

Species

* '' Rhizoglyphus actinidia'' Zhang, in Zhang, Jiang & Zeng 1994 * '' Rhizoglyphus algericus'' Fain, 1988 * '' Rhizoglyphus algidus'' Berlese, 1920 * '' Rhizoglyphus alliensis'' Nesbitt, 1988 * '' Rhizoglyphus allii'' Bu & Wang, 1995 * '' Rhizoglyphus balmensis'' Fain, 1988 * '' Rhizoglyphus costarricensis'' Bonilla, Ochoa & Aguilar, 1990 * '' Rhizoglyphus echinopus'' (Fumouze & Robin, 1868) * '' Rhizoglyphus eutarsus'' Berlese, 1920 * '' Rhizoglyphus frickorum'' Nesbitt, 1988 * '' Rhizoglyphus fumouzi'' Nesbitt, 1993 * '' Rhizoglyphus globosus'' Berlese, 1920 * '' Rhizoglyphus longispinosus'' Ho & Chen, 2001 * '' Rhizoglyphus narcissi'' Lin & Ding, 1990 * '' Rhizoglyphus occidentalis'' (Sevastianov & Tamam-Nasem-Marros, 1993) * '' Rhizoglyphus occurrens'' Berlese, 1920 * '' Rhizoglyphus robini'' Claparédè, 1869 * '' Rhizoglyphus robustus'' Nesbitt, 1988 * '' Rhizoglyphus sportilionis'' Lombardini, 1947 * '' Rhizoglyphus termitum'' Womersley, 1941 * '' Rhizoglyphus trouessarti'' Berlese, 1897 * '' Rhizoglyphus tsutienensis'' Ho & Chen, 2000

References

Acaridae {{Sarcoptiformes-stub