Phytomyza orobanchia Kaltenbach, 1864
Ph. orobanchia is one of the best known examples of an agromyzid fly beneficial for humans. This is because the larvae feed on the seeds of a parasitic weed that is a real danger for many cultivated crops.
Especially when found on the host plants, Orobanche, the species cannot be confused with other agromyzids.
IMPORTANT CHARACTERS
Adults
Wing length: 3.5 - 4 mm. Body dark except the yellow or light brown head. The large third antennal segment is elongated, rounded at tip and reddish-brown colored. Legs dark with yellow knees.
Immature stages
Third instar larva with strong cephalopharyngeal skeleton and strong mandibles. The asymmetry of the mouth hooks is less pronounced than in leaf miners. Anterior spiracles with rather long stalk and wide plate bearing numerous bulbs. Posterior spiracles with numerous spiracular bulbs arranged in a circle (after Tawfik et al., 1976).
BIONOMICS
The larvae mainly feed in the fruit of their host plants. Occasionally also the stems can be affected. Oviposition mainly takes place into the petals, but also in other parts of the developing fruit. Immature fruits are infested at different developmental stages, the earlier the oviposition the greater the damage for the plant. Although the larva never consumes all the seeds of a fruit, the whole fruit can completely decay due to the mining of the larva. Frequently more than one larva feeds in a fruit. Pupation normally occurs within the seed capsule, but several authors (see Kaltenbach, 1864, Spencer, 1973, Tawfik et al., 1976) observed that at high larval densities, the larvae might evade into the stem and pupate there. The complete development from egg to adult can last 14 - 19 days under laboratory conditions (25.7°C and 48.9% relative humidity) (Tawfik et al., 1976). Under natural condition the development is normally slower, in their literature survey Klein and Kroschel, 2002 reported up to 40 days per generation. There are several generations per season (2-6 reported by different authors, cited by Klein and Kroschel, 2002.
HOST PLANT
Reported from 21 species of Orobanche (Klein and Kroschel, 2002).
DISTRIBUTION
Widespread in the palaearctic Region, also known from Ethiopia.
Palaearctic records are Germany, Great Britain, Italy, Spain, the former Yugoslavia, Malta, Turkey, Israel, Morocco, Kirghiz Republic, Moldova, Ukraine, Uzbekistan, Egypt, Bulgaria, Hungary, Ethiopia, Yemen. Ph. orobanchia is particularly abundant in the Mediterranean region but occurs in smaller number in Central Europe (Spencer, 1973).
Recently the species was established in Chile in order to control accidentally introduced Orobanche species (Norambuena et al., 1986).
ECONOMIC IMPORTANCE
All the 140 species of Orobanche are parasites of other plants. Some of them can be a serious threat for cultivated plants and might reduce the yield by more than 50% (several examples were reported in Spencer, 1973). Since several other control methods for the weed gave limited results, Ph. orobanchia received some attention as control agent (Amsellem et al., 2001, Klein and Kroschel, 2002). The seed reduction caused by the agromyzid fly can be high and range between 11 and 79% (Klein and Kroschel, 2002). However, that can still be insufficient to control the host plants effectively. Under field conditions, the efficiency of the flies can be reduced by parasitoids, culture methods being detrimental for agromyzids, too low winter temperatures and applications of insecticides against other targets. Therefore to achieve a tolerable level of Orobanche, an innundative release of adult insects seems to be necessary. This approach was pursued in the former Soviet Union and later also in Morocco. The extensive literature, often in Russian language was recently summarized by (Klein and Kroschel, 2002). In those areas where Orobanche was introduced, the so called inoculative release of Ph. orobanchia could be enough if no naturally occurring parasitoids diminish the agromyzid population. This had been tried in Chile (Norambuena et al., 1986).