Liriomyza trifolii (Burgess, 1880)
The taxonomic diagnosis by Spencer, 1973, which is based on external characters only, cannot be used uncritically since the distribution of trifolii and some other pest species has increased considerably. More species than previously should be expected to be sympatric with trifolii on cultivated plants.
Some external characters might be, however of some value to exclude Liriomyza trifolii as possible candidate.
Wing length: about 1.3 - 1.8 mm. As in Liriomyza huidobrensis, third antennal segment is darkened (not yellow but rather light brown). Frons including frontorbits yellow or orange. Inner vertical bristles on yellow cuticle, outer one on either light or dark cuticle.
Mesonotum mat grey; acrostichals in 3-4 rows normally restricted to the central part of mesonotum. Dorsocentral bristles gradually increasing in size.
Color of legs can be variable, but femora normally lighter than tibiae and tarsi.
Tooth at the distal part of the distiphallus longer than figured by Spencer.
Surstyli, broad with one apical spine each.
Posterior spiracle each with three bulbs, the posterior bulb of each spiracle is elongated.
The leaf mine varies in form with the host-plant but when adequate space is available it is normally long, linear, narrow and not greatly widening towards the end, with frass in well defined strips at alternate sides of the channel (Spencer, 1973).
The larvae are leaf miners and feed mainly on the palisade mesophyll (Parrella et al., 1985).
Under laboratory conditions Schuster and Patel, 1985 investigated the larval development at different temperatures on tomato and found a strong decrease of the developmental time at higher temperatures: 10.1 days at 15.6°C, 7.1 days at 21.1°C, 4.4 days at 26.6°C and 3.5 days at 32.2°C. Similar results were obtained by Minkenberg, 1988.
The pupal stage in the laboratory covered about 28 with a variation of 6 days depending on the temperature (Leibee, 1984, Minkenberg, 1988).
Most adults live for 14 days on caged chrysanthemums, a few specimens survive up to 30 days (most individuals live much shorter, about 14 days). During their life the female lays up to 250 eggs (Parrella et al., 1981). As in other agromyzids the longevity increases with decreasing temperature (Parrella, 1984) and increases through availability of moisture and food. There are many successive generations during a vegetational period depending on the availability of host plants and temperature.
However, the various other studies dealing with the life cycle of Liriomyza trifolii (e.g. Smith et al., 1962 (as Liriomyza sp. n.); Harris and Tate, 1933 (as Agromyza pusilla), Harvir et al., 1996, Park, 1996, Chandler and Chandler, 1988, Prieto and Chaco de Ulloa, 1982, Parrella, 1984, Reitz and Trumble, 2002) indicate that all these ecological data are variable depending on the host plants and other environmental circumstances. Among specimens on different host plants local lineages or even cryptic species differing in the bionomics or host specificity have to be expected (Parrella et al., 1983, Reitz and Trumble, 2002).
Highly polyphagous species, Spencer, 1990 reported 25 families as potential hosts. Altogether about 400 host species are known (Parrella, 1987). The main crop species known to the author are listed here but additional cultivated plants should be expected:
Asteraceae (Compositae): Helianthus annuus (sunflower), Lactuca sativa L., Chrysanthemum spp.
Brassicaceae (Cruciferae): Brassica chinensis, Brassica juncea (mustard), Brassica campestris (rapeseed).
Chenopodiaceae: Beta vulgaris L., Spinacia oleracea L.
Cucurbitaceae: Cucumis melo L. (muskmelon), C. sativus L. (cucumber), C. anguria (gherkins, cucumbers), Cucurbita pepo L. (squash), Citrullus vulgaris (watermelon).
Luffa acutangula (L.) Roxb. (Chinese okra, sponge gourd, ridge gourd).
Euphorbiaceae: Ricinus communis (castor).
Fabaceae (Leguminosae): Phaseolus lunatus L. (lima bean), Phaseolus vulgaris (beans), Pisum sativum L. (pea), Trifolium repens L., Vigna unguiculata ssp. sinensis, Lablab purpureus (Indian bean), Arachis hypogaea (Groundnut, Peanut).
Liliaceae: Allium cepa L. (onion), also A. schoenoprasum L. (chives).
Malvaceae: Hibiscus esculentus L. (okra), Gossypium spp. (cotton).
Solanaceae: Capsicum annuum) (pepper, bell pepper), Lycopersicon esculentum Mill. (tomato), Solanum melongena L. (egg-plant), S. tuberosum L. (potato).
Apiaceae (Umbelliferae): Apium graveolens L. (celery), Daucus carota L. (carrot).
Ornamentals are also very important host plants since trifolii was spread around the world mainly with cut flowers. The main ornamental host plants are Chrysanthemum, Gerbera and Tagetes (Marigold).
The economically most important host plants appear to be Chrysanthemum, tomato, potato, celery and beans.
There are some studies which compared the suitability of different host plants, Parrella et al., 1983 found, that number of viable eggs on tomato is significantly smaller than on Chrysanthemum and Celery. Fagoonee and Toory, 1983 found in laboratory experiments, that adult females prefer bean to potato. However, there may be also some differences between some populations of the species, which may be accustomed to plants grown in their environments (see Parrella et al., 1983 and Reitz and Trumble, 2002). Among the large number of host plants the suitability can differ significantly but moreover, the hosts' tolerance of agromyzid damage may differ as well. For example Martens and Trumble, 1987 found very different reduction of photosynthetic activity on damaged leaves of different host plants. Lima beans were found to compensate leaf-mining activity better than celery and tomatoes. That can be partly explained by the development of callus-tissue in lima beans.
Spencer, 1973 listed some localities in North and South America only. Some years after his publication, Liriomyza trifolii became a serious pest of increasing importance both in the Americas and many other areas, where the species was introduced. Around 1977 it emerged in some European and African countries and subsequently spread further. It is widely accepted that the main reason of dispersal was trade with cutting Chrysanthemum. Nowadays trifolii has also a high pest status in the Near East, in the East palaearctis, the Oriental region and many Islands. Therefore, the species can be expected wherever a suitable climate is present.
In North and Central Europe and other temperate regions the species is usually restricted to greenhouse cultures but, however, there are reports of occasional overwintering and outdoor occurrence in those Countries (e.g. van der Linden, 1993).
Detailed information on the state of distribution until 1986 is given by Minkenberg and van Lenteren, 1986.
Because its wide host range and its ability to develop resistance against various insecticides (see Parrella et al., 1981, Parrella and Lindquist, 1983) Liriomyza trifolii is one of the most dangerous agromyzid flies worldwide. If large populations occur, trifolii can damage the leaves heavily and damage especially young plants (but see Keularts and Lindquist, 1987). Furthermore Kapadia, 1995 found evidence for Liriomyza trifolii to be a disease vector for gourd. Due to its wide host range the species can survive in the field feeding on weeds when no suitable crop is available (cf. Smith and Hardman, 1986, Zoebisch and Schuster, 1987).
Apparently, the infestation of trifolii and other Liriomyza species is often the result of indiscriminative use of insecticides, applied either against the agromyzids themselves or other pest insects (e.g. Saito et al., 1993 and other References Parrella). This is because many naturally occurring parasitoids are more vulnerable to many insecticides than their hosts. Thus, the support of parasitoids and the application of specific chemical treatments (e.g. the application of insect growth regulators) have shown to be a promising attempt to control trifolii (Parrella and Lindquist, 1983, Heinz et al., 1988). Especially in greenhouses through inoculative release of parasitoids a good control can be achieved (references). In Europe several parasitoid species are commercially available. However, in many cases the careful application of modern insecticides might be inevitable.