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Description & Statistics
The predaceous habit of locust egg capsules was first found by Riley (1880) in studies on the Rocky Mountain locust. Many species are external parasitoids of the larvae, prepupae and pupae of Hymenoptera (Apoidea, Sphecoidea, Vespoidea and at times Tenthredinoidea and Ichneumonoidea) in their cells or cocoons, and other species are parasitic externally on coleopterous larvae or pupae in the soil (Meloidae, Cicindelidae and Scarabaeidae). Some species attacking locust eggs also develop on the pseudo nymphs of meloid species that are predaceous on the same host stage. Villa alternata Say was reared both from noctuid and tenebrionid pupae. Anastoechus mylabricida Zack. was found to attack prepupae and pupae of Zonabris and at times the larvae of Carabidae and other insects in the soil (Zackvatkine 1934). Anthrax oophagus Par. attacks locust eggs and Zonabris larvae and pupae as well as occasionally developing as a secondary parasitoid through other Bombyliidae. Systropus conopoides Kunckel and other Systropus spp. are solitary parasitoids of Eucleidae larvae in their egg-like cocoons (Clausen 1940/62). Other species develop internally in larvae or pupae of other Lepidoptera (Pyralidae, Noctuidae, Tortricidae and Tineidae), in larvae and pupae of Coleoptera (Tenebrionidae), and in pupae of Diptera (Muscidae, Tachinidae and Asilidae).
Species that parasitized pupae of Lepidoptera enter the active caterpillar while it is feeding. Anthrax lucifer F. was reared from pupae of Laphygma frugiperda S. & A. that had developed from larvae collected and isolated when about half grown (Allen 1921). First instar larvae had thus entered the caterpillars prior to that time, though extensive feeding and development did not occur until after pupation. Baer (1920) believed that the young larvae enter the mature maggots at the time when they have left their hosts and are entering the soil to pupate. Clausen (1940) commented that it is not yet established that these species are true internal parasitoids, for they may feed externally on the pupa within the puparium.
Hemipenthes sp. was found to be hyperparasitic in Lepidoptera through some Ichneumonidae, from the cocoons of which several species of that genus were reared. Clausen (1940) noted that there was doubt as to the true host relationships. First instar larvae may enter the body of the caterpillar hosts and thus gain access to ichneumonid larvae, or they may attack the latter directly and only after the cocoon has been formed. It seems that the family as a whole can probably be regarded as ore beneficial than injurious because of the extensive attacks of many species on egg masses of Acrididae. Apart from the species attacking Tenebrionidae, Scarabaeidae, Lepidoptera and Tenthredinidae and those attacking Glossina, the long list of hosts of the parasitic species consists of insects that are themselves entomophagous in habit (Clausen 1940/62).
Bombyliidae, are cosmopolitan but are most common in the Mediterranean area. There have been more than 2,508 species described as of 2000. They are usually robust and densely covered with fine hair, and usually have wings that are clouded.
Parasitic Bombyliidae are solitary and most are ectoparasitic, although endoparasitic species are known. Also, there are both primary and hyperparasitic species known. Host preferences are exceedingly varied, though the species themselves are confined within relatively narrow limits (Clausen 1940). A number of species are predators in egg pods of Orthoptera. Bombyliids also attack various Hymenoptera, including beneficial species, or they are larval-pupal parasitoids of Lepidoptera. Other species at times will attack hosts in other insect orders. Bombyliids have not been deployed extensively in biological control. There have been a few species used against grasshoppers, with little success. Early information regarding host preferences were given by Bezzi (1924) and Painter (1932).
Adult bombyliid flies are most often observed during periods of bright sunshine, although some species prefer shady places. Almost all species are flower feeders, subsisting on nectar and pollen, although several genera lack functional mouth parts and probably do not feed.
Oviposition.-- The manner of oviposition varies among the species. Callistoma desertorum, which develops in acridid egg pods, lays its eggs (80-100 at a time) in holes and fissures in the soil (Zackvatkine 1931). This species is capable of laying 1,600-2,000 eggs, however. Eggs of Cytherea setosa Par. are laid in groups of 1-5 on the soil surface in shaded places, or in crevices. Anthrax oophagus Par. and A. jazykovi Par. oviposit similarly. Female Glossista infuscata Meig. probably inserts her eggs directly into the freshly formed egg pod (De Lepiney & Mimeur 1930). Meilis (1934) recorded that female Bombylius variabilis Lw. apparently oviposits while in flight, merely touching the abdomen to the ground near an ovipositing locust or newly formed egg pod.
Species attacking hosts which are contained in open burrows or cells seem to have developed a method of oviposition which is considerably different from that of those species that develop as egg predators. Female Bombylius major L. insert the egg into the entrance of a Andrena sp. nest during the absence of the female bee (Dufour 1858). Female B. fugax Wied. projects the egg into the nest opening of Panurgus sp. while the latter is in flight, the same behavior being shown in Hyperolonia morio F., when parasitizing Monedula sp. (Seguy & Bandot 1922). The eggs of Villa sp., developing in cells of solitary bees, are readily projected into glass vials buried in the soil (Painter 1932).
Larvae.-- The young larva in searching for a host has not far to seek as the egg is usually deposited in the host's vicinity. Such larvae are well equipped for movement in the soil and have little difficulty reaching hosts. Species developing as predators in locust egg capsules are usually solitary, although some are gregarious. In B. variabilis Lw., after consuming one egg mass the larva searches in the soil for a second mass. Such species as A. trifasciata Meig. (Fabre 1886), developing on larvae of the mason wasps, have to penetrate an exceedingly hard cell well to reach the host. Those attacking parasitic Hymenoptera seem to have to make their way into the cocoon (Clausen 1940/62). Larvae mature quickly after feeding begins on inactive host stages. The larva of A. anale first attaches itself to the thoracic venter of a 3rd instar Cicindela larva (Shelford 1913). A thickened chitinous ring is formed around the feeding incision, and growth is slight until the host forms its pupation cell, which may be 8 months after the parasitoid larva has attached. From then onward, development is rapid. Larvae of Exoprosopa fasciata Macq. (Richter & Fluke 1935), parasitizing Phyllophaga pupae, attach themselves to the pupa's venter. Sparnopolius fulvus Wied. is occasionally found parasitic on grubs of the same genus (Clausen 1940/62).
Young larvae of Spogostylum delita Lw. (Niniger 1916), developing in cells of Xylocopa, are frequently found in the cells even before host eggs have hatched. They may wander about over the food for a month or more, feeding voraciously, before quieting down, during which period very little growth occurs. A definite feeding position is ultimately taken on the 3rd or 4th abdominal segment of the bee larva, and the body contents are then consumed in ca. 5 days. First instar larvae of B. pumilis also feeds on food material stored in the cell of the host, Colletes daviesana Smith (Clausen 1940/62).
Pupation and Adult Eclosion.-- Pupation sites differ considerably among species, being dependent on the kind and stage of host attacked. Larvae that develop in acridid egg capsules consistently leave the capsule and form a pupal cell in the soil at some distance away. Larvae of Systoechus albidus Lw. burrow downward 8-20 cm. in compact soil and form a distinct cell (Potgeiter 1929). A. anale on Cicindela larvae and E. fasciata on Phyllophaga pupae pupate in the host pupation cell. Species that are externally parasitic on larvae of solitary bees, sawflies and wasps, pupate within the host cell or cocoon, and those on or in Diptera do so within the puparium. Internal parasitoids of pupae of Coleoptera and Lepidoptera transform within the host's pupal shell (Clausen 1940/62).
Prior to adult eclosion, there is a period of pronounced activity of the pupa, the purpose of which is to free it from any covering or enclosing wall and to permit the adult fly to emerge directly into the air. Species found in soil come to the surface after traversing 1 m. or more of soil, and at least the anterior portion of the body protrudes from the burrow before the adult fly emerges. Pupae contained in cells or cocoons must cut an opening equal to their body width, which involves repeated body rotations to rasp away a hole large enough to permit complete or partial extrusion. Those pupating within the pupal remains of the host rupture the body wall ventrally in the thoracic region before escaping. Those contained in puparia either force off the operculum or cut away a portion of the puparial wall. In every case, repeated bending and twisting of the abdomen causes the head crown of the pupa to penetrate the soil or rasp away the cocoon or cell wall in its path (Clausen 1940/62). Systropus conopoides emerging from eucleid cocoons, has pupae that can fill their digestive tubes with air, thus inflating the body in an aid to emergence (Kunckel d'Herculais 1905). Such inflation is not associated with a dilation of any part of the tracheal system, and it gives the body greater leverage within the confined space of the cocoon, and pressure is essential to the efficient use of the specialized cutting structure on the head (Clausen 1940/62). The adult fly emerges from the pupal skin through a longitudinal split along the dorsum, and this is done rapidly. The time elapsing between the cessation of movement of the pupa of Thyridanthrax lloydi Austen and the flight of the fly is only 2-3 minutes (Clausen 1940/62).
Life cycles in Bombyliidae of temperate regions generally take a full year. In tropical species this may be only two months. Adverse environmental conditions, such as a lack of moisture, profoundly influence development of many species, causing them to enter diapause for long periods. Mature larvae of Systoechus albidus were kept in dry sand for 4 years, after which completion of development and emergence quickly followed when sufficient moisture was provided (Potgieter 1929).
The incubation period has been determined in Bombylius fugas as 8-12 days. However, in Anastoechus mylabricida Zack. (Zackvatkine 1934) the egg persist through winter. The larval feeding period represents only a very small portion of the entire cycle. In Hyperalonia the consumption of the mature Pseudagenia larva is finished in 3-4 days, followed by a resting period of 5-6 days. Hyperalonia oenomanus takes 5-8 days for feeding. However, in most other species a much longer feeding period exists, ca. on month in S. albidus and ca. 7 seeks in Spogostylum delila Lw. Copello (1933) found that the 1st instar larva of Hyperalonia morio reaches its Monedula host in late autumn, feeds only slightly until spring, and then rapidly consumes the prepupa the following spring. The winter is passed as 1st instar larvae in B. variabilis, B. pumilis and Hyperalonia sp., but most species hibernate as mature larvae. However, the 2nd instar larva of Anthrax anale is found in winter (Shelford 1913).
The pupal stage lasts from a minimum of 7-9 days in Systoechus albidus to just lest of on month in H. oenomaus, with 12-16 days as normal for most species. Occasionally individuals of a few species hibernate as pupae (Clausen 1940/62).
Parasitization Rates.-- Field parasitization rates can be high, indicating a considerable degree of natural control effectiveness. In those species which attack hosts in soil, the condition of the soil seems to be the main factor governing effectiveness. High mortality may prevail in one locality, while in another there may be hardly any attack at all. Usually the range of the parasitic or predaceous species is more restricted than that of the host. Among species attacking egg pods of Acrididae, Glossista infuscata Meig. destroys up to 85% of eggs of Dociostaurus macroccanus Thbg. in Morocco (Lepiney & Mimeur 1930), while Zackvatkine (1931), as a result of observations on several species in Turkestan, estimated that ca. 20% are destroyed each year by Callistoma desertorum and up to 40% by Cytherea setosa Par.Potgieter (1929) noted finding 1,143 larvae of Systoechus albidus in 1-sq-yd, which contained more than 100 egg pods of Locustana. A large number of larvae may develop in each egg mass. Wilson (1936) found 62.4% of Camnula pellucida Scudd. egg pods destroyed by Aphoebantus hirsutus Coq. This species and most others attacking similar hosts are solitary. All eggs in the cluster may not be consumed, but those remaining do not hatch, because of desiccation and disease. In areas with heavy attack by Aphoebantus, the leafhopper emergence the following spring was very low. This predator is very scarce in arid sections with sparse vegetation (Clausen 1940/62).
When attacking other groups of hosts, the parasitoid population also may be quite high, as revealed in the 55-65% parasitization of the cocoons of Tiphia sp. by H. oenomaus in India (Clausen 1928b) and 18-25% of the pupae of Laphygma frugiperda by Anthrax lucifer in the southern United States. Parasitization of the larvae of Monedula surinamensis is high among those which mature during December to February (Copello 1933).
For detailed descriptions of immature stages, please see Clausen (1940).
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