Walnut husk fly
Rhagoletis juglandis, also known as the walnut husk fly, is a species of tephritid or fruit fly in the family Tephritidae. It is closely related to the walnut husk maggot Rhagoletis suavis (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 (the English or Persian walnut), Juglans rupestris (a species of walnut indigenous to Arizona and Texas), Juglans hindsii (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, 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.
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DiurnalDiurnal animals are active during the daytime, with a period of sleeping or other inactivity at night. The timing of activity by an animal depends ...
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TerrestrialTerrestrial animals are animals that live predominantly or entirely on land (e.g., cats, ants, snails), as compared with aquatic animals, which liv...
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OviparousOviparous animals are female animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive...
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starts withR. juglandis was first described by E. T. Cresson, Jr. in 1920 from material from the exocarp of fruit of Juglans regia (the English or Persian walnut tree) in Arizona. The species was first named the black walnut fly in literature. This served to differentiate it from the walnut husk maggot Rhagoletis suavis. However, the common name walnut husk fly was later proposed and accepted for this species.
The eggs appear a pearly white color when first laid, but progressively darken as the embryo 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, 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 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. 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 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. Females grow to be around 4 mm long in body length.
There are recorded sightings of R.juglandis in Arizona, California, Texas, and Kansas.
Hosts of R. juglandis infestations include Juglans regia (the English or Persian walnut), Juglans rupestris (a species of walnut indigenous to Arizona and Texas), and Juglans hindsii (the California black walnut). Particular cultivars 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 in the southwestern United States and Mexico.
The husk fly also infests ripe apricot and peach fruits, usually if infested walnuts are located within flying distance.
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.
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. 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.
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.
