Rhagoletis Juglandis
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''Rhagoletis juglandis'', also known as the walnut husk fly, is a species of tephritid or fruit fly in the family
Tephritidae The Tephritidae are one of two fly families referred to as fruit flies, the other family being the Drosophilidae. The family Tephritidae does not include the biological model organisms of the genus '' Drosophila'' (in the family Drosophilidae ...
. It is closely related to the walnut husk maggot ''
Rhagoletis suavis ''Rhagoletis suavis'', also known as the walnut husk maggot, is a species of tephritid or fruit fly in the family Tephritidae The Tephritidae are one of two fly families referred to as fruit flies, the other family being the Drosophilidae. ...
'' (Loew, 1862). This species of fly belongs to the ''R. suavis'' group, which has a natural history consistent with allopatric speciation. The flies belonging to this group are morphologically distinguishable. The adult form of this fly is around 4 mm long. ''R. juglandis'' are distributed in Arizona, California, Texas, Kansas, and Mexico. The species infests the fruits of several species of walnut trees including ''
Juglans regia ''Juglans regia'', known by various common names including the common walnut, English walnut, or Persian walnut amongst other names, is a species of walnut. It is native to Eurasia in at least southwest and central Asia and southeast Europe, ...
'' (the English or Persian walnut), ''Juglans rupestris'' (a species of walnut indigenous to Arizona and Texas), ''
Juglans hindsii ''Juglans hindsii'', commonly called the Northern California black walnut and Hinds's black walnut, is a species of walnut tree native to the western United States (California and Oregon). It is commonly called claro walnut by the lumber industry ...
'' (the California black walnut), and the Arizona walnut ''Juglans major.'' The larvae are small and infest walnut fruits, having hatched from eggs laid by adult females under the surface of the husk of the walnut. The insect overwinters as a pupa in the soil, and adults emerge in mid to late summer. ''R. juglandis'' engages in
superparasitism Superparasitism is a form of parasitism in which the host (typically an insect larva such as a caterpillar) is attacked more than once by a single species of parasitoid. Multiparasitism or coinfection, on the other hand, occurs when the host has b ...
, during which conspecifics infest the same host, even when there are still uninfested hosts available. Although courtship behavior is rare in the ''Rhagoletis'' genus, male flies demonstrate low-frequency wing vibration, accompanied by airborne infrasound; they also turn their wing edges upward. ''R. juglandis'' participate in a resource-defense mating system. Females follow resource-based cues such as ripeness (color), whereas males follow females for more opportunities to mate. The flies also respond to sex ratio to alter the amount of time that copulation takes. When male density is high, copulation times are longer.


Description

''R. juglandis'' was first described by E. T. Cresson, Jr. in 1920 from material from the
exocarp Fruits are the mature ovary or ovaries of one or more flowers. They are found in three main anatomical categories: aggregate fruits, multiple fruits, and simple fruits. Fruitlike structures may develop directly from the seed itself rather th ...
of fruit of ''
Juglans regia ''Juglans regia'', known by various common names including the common walnut, English walnut, or Persian walnut amongst other names, is a species of walnut. It is native to Eurasia in at least southwest and central Asia and southeast Europe, ...
'' (the English or Persian walnut tree) in
Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
. The species was first named the black walnut fly in literature. This served to differentiate it from the walnut husk maggot ''
Rhagoletis suavis ''Rhagoletis suavis'', also known as the walnut husk maggot, is a species of tephritid or fruit fly in the family Tephritidae The Tephritidae are one of two fly families referred to as fruit flies, the other family being the Drosophilidae. ...
''. However, the common name walnut husk fly was later proposed and accepted for this species.


Taxonomy

''R.'' ''juglandis'' belongs to the genus '' Rhagoletis'' and the family
Tephritidae The Tephritidae are one of two fly families referred to as fruit flies, the other family being the Drosophilidae. The family Tephritidae does not include the biological model organisms of the genus '' Drosophila'' (in the family Drosophilidae ...
. It is closely related to ''
Rhagoletis suavis ''Rhagoletis suavis'', also known as the walnut husk maggot, is a species of tephritid or fruit fly in the family Tephritidae The Tephritidae are one of two fly families referred to as fruit flies, the other family being the Drosophilidae. ...
'' (Loew, 1862), '' Rhagoletis completa'' (Cresson, 1929), '' Rhagoletis boycei'' (Cresson, 1929), '' Rhagoletis zoqui'' (Bush, 1966), and '' Rhagoletis ramosae'' (Hernández-Ortiz, 1985). These species do not generally overlap in geographic range in
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
, but there has been some described
sympatry In biology, two closely related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter each other. An initially interbreeding population that splits into two or more distinct spe ...
and parapatry. ''R. suavis'' group flies have history that is consistent with
allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
. Morphological differences implying
sexual selection Sexual selection is a mechanism of evolution in which members of one sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex ...
support specification, including that the different walnut flies are morphologically distinguishable in wing pattern, body coloration and markings.


Morphology

The
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 appear a pearly white color when first laid, but progressively darken as the
embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
develops. The eggs are somewhat curved, with one end slightly tapered and the other end pointed. The eggs measure to be around 0.8 mm by 0.3 mm. Upon developing into
larvae A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect developmental biology, development such as insects, some arachnids, amphibians, or cnidarians typical ...
, these flies take on a transparent look in their early stages and turn creamy white or light yellow as they mature. In addition, they have prominent anterior and posterior orange-yellow spiracles. The larvae measure to be around 9 mm by 2 mm. The
pupae A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
are described as looking like grains of wheat, with a barrel shape and straw color. The pupae measure around 5 mm by 3 mm. As adults, the main body color of the adult fly is tawny with lemon yellow
markings Marking may refer to: Symbols Marking may refer to human-made symbols and annotations in several contexts: On vehicles * Aircraft marking * Emergency vehicle equipment markings ** Battenburg markings, emergency vehicle patterns * Vehicle marki ...
. The wings look translucent and have three parallel transverse dark bands. The distal band continues along the costal margin to the apex (dorsal region). The
abdomen The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal ...
of the fly has dark transverse bands on the dorsum. The female fly is larger than the male, and has a more pointed end of its abdomen due to the telescoping
ovipositor The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typica ...
. Females grow to be around 4 mm long in body length.


Distribution and habitat

There are recorded sightings of ''R.juglandis'' in
Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
,
California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
,
Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
, and
Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...
. Hosts of ''R. juglandis'' infestations include ''
Juglans regia ''Juglans regia'', known by various common names including the common walnut, English walnut, or Persian walnut amongst other names, is a species of walnut. It is native to Eurasia in at least southwest and central Asia and southeast Europe, ...
'' (the English or Persian walnut), '' Juglans rupestris'' (a species of walnut indigenous to Arizona and Texas), and ''
Juglans hindsii ''Juglans hindsii'', commonly called the Northern California black walnut and Hinds's black walnut, is a species of walnut tree native to the western United States (California and Oregon). It is commonly called claro walnut by the lumber industry ...
'' (the California black walnut). Particular
cultivar A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...
s of ''J. regia'' may be more infested than others because of relative softness and thickness of walnut husks. 'Eureka' 'Klondike,' 'Payne,' 'Franquette' and 'Ehrhardt' cultivars of ''J. regia'' are among the most susceptible to infestation. In addition, ''R. juglandis'' has been found to infest the Arizona walnut ''
Juglans major ''Juglans major'' (literally, the larger walnut), also known as Arizona walnut, is a Juglans, walnut tree which grows to with a Diameter at breast height, DBH of up to at elevations of in Texas, Oklahoma, New Mexico, Arizona, and Utah. It als ...
'' in the
southwestern United States The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural list of regions of the United States, region of the United States that includes Arizona and New Mexico, along with adjacen ...
and
Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
. The husk fly also infests ripe
apricot An apricot (, ) is a fruit, or the tree that bears the fruit, of several species in the genus ''Prunus''. Usually an apricot is from the species '' P. armeniaca'', but the fruits of the other species in ''Prunus'' sect. ''Armeniaca'' are also ...
and peach fruits, usually if infested walnuts are located within flying distance.


Life history


Oviposition

''R. juglandis'' is
univoltine Voltinism is a term used in biology to indicate the number of broods or generations of an organism in a year. The term is most often applied to insects, and is particularly in use in sericulture, where silkworm varieties vary in their voltinism. ...
, similar to its close relative ''R. suavis''. Experiments have shown that physical host fruit stimuli such as color and shape enhance
oogenesis Oogenesis () or ovogenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated before birth during embryonic devel ...
in the first egg maturation cycle; this does require sufficient nutritional value of the fruit. Female flies do not completely develop eggs for oviposition until 7 to 14 days after emerging from the soil. Copulation and oviposition occur in the morning or late afternoon and evening. The female searches for a suitable spot for oviposition; when one is found, she forces her ovipositor several millimeters into the husk of the walnut. She then moves her body around in a semicircle to lacerate the tissue of the inner husk and create a cavity for the eggs. The female oviposits eggs below the surface of the husk of walnuts in batches of about 15 eggs. Oviposition takes several minutes. The spot of oviposition darkens after one or two days; then, the egg cavity can be detected with the naked eye.


Larvae and pupae

The larval stage takes 3 to 5 weeks. The larvae hatch from eggs after 4 to 7 days and immediately tunnel into the inner portion of the husk of the walnut. After maturation, as completion of development usually occurs after the infested walnut falls off the tree to the ground, the larvae exit the decaying husk and go onto the soil, in which they burrow and transform into the prepupal stage within 8 to 24 hours. The complete transformation to pupae takes a few days. Larvae spend the winter in the pupal stage in the soil at around a depth of 1 to 4 inches. A study examined the relationship between individual pupal size and the depth of soil larvae were found for ''Rhagoletis juglandis'' and ''
Rhagoletis suavis ''Rhagoletis suavis'', also known as the walnut husk maggot, is a species of tephritid or fruit fly in the family Tephritidae The Tephritidae are one of two fly families referred to as fruit flies, the other family being the Drosophilidae. ...
''''.'' Larger pupae were found in deeper soil, most likely because burrowing deep into soil has a high energy cost. Flies in deeper soil depths had increased mortality; they were less likely than flies who burrowed to a shallow depth to emerge from overwintering. Threats to the larvae in shallow depths include predators like ants and parasites like wasps from the genus ''Coptera''. Deeper soil, however, costs the larvae energy and leads to a higher risk of mortality. In mid to late summer, adult flies emerge from soil prior to ripening of walnut husks and emergence declines in number by September.


Food resources

''R. juglandis'' flies deposit their eggs in the husks of developing walnuts, on which the larvae feed and develop. Adult flies feed on dew and plant sap and can also dissolve solid deposits on foliage for nutrients using saliva. Feeding is most active in the beginning of the day and in the evening.


Behavior


Mating

Courtship behaviors are generally rare in the genus. However, ''R. juglandis'' is unique in its courtship signaling behaviors. Male flies demonstrate low-frequency wing vibration, accompanied by airborne
infrasound Infrasound, sometimes referred to as low frequency sound or incorrectly subsonic (subsonic being a descriptor for "less than the speed of sound"), describes sound waves with a Audio frequency, frequency below the lower limit of human audibility ...
. These bouts of vibration typically last from 5 seconds-15 minutes. Male flies raise their wings during these displays, turning the edges upward. ''R. juglandis'' participate in a resource-defense mating system. Females are inherently aware of resource-based cues and males follow females in order to obtain more opportunities to mate. For instance, females have been shown to have innate responses to ripeness cues. Males then respond based on encounter rates with females at different levels of ripeness. Experimentally, females were shown to land more on green models than yellow or brown ones, and experienced males followed the same patterns. There were more mating interactions on green models. In addition, researchers have found that flies seem to internalize changes in sex ratio in the surrounding environment and respond accordingly. The effect of male density on copulation is stronger than female density. Individual flies copulate for longer in these male-biased environments.


Superparasitism

Superparasitism Superparasitism is a form of parasitism in which the host (typically an insect larva such as a caterpillar) is attacked more than once by a single species of parasitoid. Multiparasitism or coinfection, on the other hand, occurs when the host has b ...
is the use of hosts that already contain a brood from the same species of parasite. ''R. juglandis'' females drag their ovipositors on the husk of the walnut after oviposition, which suggests that they have released a marking pheromone, a behavior typical of the ''Rhagoletis'' genus. However, the flies reinfest the same walnuts, and even the same oviposition sites, created by individuals of the same species. This occurs even when there are still uninfested hosts available. In large, less-infested walnut fruits, adult fly size has been found to be larger and larval survival is higher.


Interactions with humans

''R. juglandis'' infests walnut trees, which has economic importance due to the cultivation of walnuts for human use. A close relative, ''R. completa,'' is known to infest peaches growing near walnuts. Under artificial conditions, females have oviposited in other fruits and vegetables, but larvae failed to develop.


References


External links


''Rhagoletis juglandis'' image
on BugGuide
{{Taxonbar, from=Q7320107 juglandis Diptera of North America Insects described in 1920