The larvae of all Agromyzidae species (Diptera) are internal feeders of living plant tissue. They can be leaf miners (leaf miner.pct), stem miners (stem miner.pct, Oph phaseoli larva.pct), Melanagromyza larva.pct), seed parasites and gall inducers among others. Since the larvae normally are not able to change their host plants or even the leaves, ovipositing females are very important in the determination of the larval feeding sites. Apart from larval food selection the adults' function as in many other insect groups is mainly reproduction and dispersion.
Here I try to summarize some general patterns in agromyzid biology with special emphasis on life history and phenology. Further information can be found in the Species and Higher Taxa Modules. Especially the extraordinary life history of the Phytobia species in the cambium of trees is reviewed there (see Genus Phytobia). The evolutionary significance of the different feeding habits are discussed in the section on Evolutionary History. Morphological adaptations concerning different food types are presented in Immature Stages.
The female deposits single eggs with the ovipositor (Female ovipositor) into that plant tissue the hatching larva will feed on. Oviposition always requires copulation; there is no case of parthenogenesis known in the literature.
The larvae emerge only a few days after oviposition. As in most other cyclorrhaphan flies there are three larval instars. Of these, the third one possesses the most diagnostic features and is often depicted in the literature. Apart from size, the second instar normally is very similar to the third.
The leaf-mining larvae lie on their side and feed by wide sweeping movements of the first body segments. They use the mouth hooks to tear off the plant tissue that will happen to be ingested. The pupariation occurs in most species in the soil. Prior to pupariation, the last instar larva cuts a characteristic semicircular exit slit in the epidermis of the mine. Then the larva leaves the mine and drops to the soil. From some Liriomyza species of economic importance, it is known that the fully grown larva normally leaves its mine early in the morning and shows negative phototactic behaviour (Leibee, 1986). This behavior enables the larva to find a secure pupation site and to avoid exposition to drought and heat. One can assume that other agromyzid species, which leave the mine for pupation, behave similarly. Some subgroups exclusively pupate in the mine (e.g. Ophiomyiini, Chromatomyia, Cerodontha). They pupate immediately below the upper surface of the plant part they lived in. Prior to pupation the last larval instar may perforate the epidermis. The emerging adult tears the plant surface by means of the ptilinum (a temporary bladder-like inflatable structure on the frons, see External adult morphology). Of course the puparia originating in the host plant are also frequently present in the soil or plant litter around it. The puparia can be displaced in the course of the decaying process of parts of the host plant.
Leaf-mining Agromyzidae normally complete their larval development within a few days but during dry or cold periods, considerable delay may occur. Most stem miners seem to grow slower and make larger mines.
The mouthparts of the agromyzid adults are only suitable for ingesting fluids or pre-orally dissolved food. However, females and to a lesser extent males normally do not only feed on honeydew or nectar as other adult Diptera, but also from potential host-plants. To obtain cell sap, a female punctures leaves of the species' host plant with her ovipositor. While doing so she may or may not lay eggs. After puncturing the female turns around in order to consume cell sap emerging from the puncture. It was also observed that males feed on punctures previously created by females (Minkenberg and van Lenteren, 1986). Sometimes there are dozens of the so-called feeding punctures occurring on only a single leaf (more on the females' behaviour: Bethke and Parrella, 1985). This behaviour may play an important role in finding suitable host leaves for their offspring and in maturing of the eggs (Spencer, 1973). Feeding punctures are often found on the upper-side of the leaf whereas the same species often oviposit on the under-side only. Sometimes the feeding punctures alone can become the cause of plant damage (Parrella et al., 1985, Hendrickson and Barth, 1978).
Most agromyzid flies are restricted to a group of closely related plants or even to a single host plant. Hence, the host-plant is often a useful hint for identification. On this CD-ROM a database is included containing the main cultivated host plants and their agromyzid consumers. Most species feeding on dicotyledon plants have a narrow host-plant relation, but in contrast grass miners often have a considerably wider host range (e.g. Chromatomyia fuscula, Cerodontha denticornis, Pseudonapomyza spp.). That may be due to the lower content of phytochemicals normally found in grasses. Identification of species feeding on grasses is often more complicated and unusual species should be expected.
Quite a small number of species among the Agromyzidae (Chromatomyia horticola, Liriomyza trifolii, L. sativae, L. strigata, L. bryoniae, L. huidobrensis and Tropicomyia spp.) are extremely polyphagous and cause damage on several crops (overview in Spencer, 1990). Most of these belong to the generally, world wide most devastating pest species. Although they can develop on many plants, the polyphagous species may have some host preferences that can also show regional variations (Morgan et al., 2000).
Agromyzids normally depend on certain developmental stages of their host plants. This often results in a limited number of generations or distinct distribution patterns over a vegetation period. Yet particularly among species of economic importance high flexibility and many generations are observed. The emergence time and the synchronization at the beginning of a vegetation period are often crucial for the severity of the following damage. Some research was done to predict the agromyzid population by interpreting temperature and other environmental data (Nechols et al., 1983).
Although often the younger, vulnerable plants are most seriously threatened by heavy agromyzid infestation (e.g. Talekar, 1990), young plant parts are not always preferred (for example Phytomyza ilicicola and Phytomyza ilicis).
Agromyzidae distribution is almost worldwide (except some isolated and unsuitable regions). The same is true for the larger genera. The highest diversity appears to be in the temperate region of the Palaearctis (von Tschirnhaus, 1991). Due to incidental human spread, especially many species of economic importance nowadays have a cosmopolitan distribution (e.g. Liriomyza trifolii, Liriomyza huidobrensis, Chromatomyia horticola).
The natural populations of most agromyzid species are normally controlled and maintained on a rather low level by numerous hymenopteran parasitoids (mainly Braconidae and Eulophidae) (parasitoid.pct). Many of them apparently parasitize rather unspecifically a wide range of agromyzid flies (Minkenberg and van Lenteren, 1986). Since parasitoid species also quickly adopt alien agromyzids established in their area, for some Liriomyza species remarkable high numbers of parasitoids are known (Waterhouse and Norris, 1987, Murphy and LaSalle, 1999).
In cultures, the influence of parasitoids is often reduced by the indiscriminate use of insecticides that often damage the parasitoids even more than their hosts (Spencer, 1973, Spencer, 1990, Saito et al., 1993). Hence, in biological and integrated pest management the establishment and maintenance of parasitoids as control agents plays an important role (e.g. Kumar, 1985, Schelt and Altena, 1997, Letendre et al., 1991).
Certainly both larval and adult agromyzids are also pursued by many rather unspecific predators. So far, not much is known about the predators' impact on agromyzid populations. Larvae can be hunted even within their mines by predatory Heteroptera and, especially in tropical areas by ants (von Tschirnhaus, personal communication). Adults are a potential prey for any predatory animals which feed on small flying insects. One example are some predatory Muscidae (house flies) in the genus Coenosia which are recently introduced as biological control agents in greenhouses (KÜhne, 2000, Coenosia.pct).
Although they apparently are not easy to maintain in greenhouses, they may contribute to the control of noxious agromyzids.
Generally agromyzids are also susceptible to bacterial and fungal pathogens and also to nematodes. There have been some attempts to use the latter as biological control agents (LeBeck et al., 1993, Williams and Macdonald, 1995). However, leaf miners normally are better protected against pathogens than external feeders (Connor and Taverner, 1997).
The knowledge about the bionomics of certain species differs widely. About some species not more than the host plant is known, others were subject of vast research projects. A fair part of our biological knowledge about agromyzids come from only a few well-studied species. The following (hyper-linked) taxa are a selection of well known or otherwise interesting taxa.