Descriptions of L. trifolii refer to fresh materials. Dry specimens may be distorted due to the manner in which they have been preserved. Also, the age of the specimen, when killed, will have some effect on its preservation characteristics.
For accurate identification, examination of the leaf mine and all stages of development are crucial.
Egg
L. trifolii eggs are 0.2-0.3 mm x 0.1-0.15 mm, off white and slightly translucent.
Larva
This is a legless maggot with no separate head capsule, transparent when newly hatched but colouring up to a yellow-orange in later instars and is up to 3 mm long. L. trifolii larvae and puparia have a pair of posterior spiracles terminating in three cone-like appendages. Spencer (1973) describes distinguishing features of the larvae. Petitt (1990) describes a method of identifying the different instars of the larvae of L. sativae, which can be adapted for use with the other Liriomyza species, including L. trifolii.
Puparium
This is oval and slightly flattened ventrally, 1.3-2.3 x 0.5-0.75 mm with variable colour, pale yellow-orange, darkening to golden-brown. The puparium has posterior spiracles on a pronounced conical projection, each with three distinct bulbs, two of which are elongate. Pupariation occurs outside the leaf, in the soil beneath the plant.
Menken and Ulenberg (1986) describe a method of distinguishing L. trifolii from L. bryoniae, L. huidobrensis, and L. sativae using allozyme variation patterns as revealed by gel electrophoresis.
Adult
L. trifolii is very small: 1-1.3 mm body length, up to 1.7 mm in female with wings 1.3-1.7 mm. The mesonotum is grey-black with a yellow blotch at the hind-corners. The scutellum is bright yellow;the face, frons and third antennal segment are bright yellow. Male and female L. trifolii are generally similar in appearance.
L. trifolii are not very active fliers, and in crops showing active mining, the flies may be seen walking rapidly over the leaves with only short jerky flights to adjacent leaves.
Head
The frons, which projects very slightly above the eye, is just less than 1.5 times the width of the eye (viewed from above). There are two equal ors and two ori (the lower one weaker). Orbital setulae are sparse and reclinate. The jowls are deep (almost 0.33 times the height of the eye at the rear);the cheeks form a distinct ring below the eye. The third antennal segment is small, round and noticeably pubescent, but not excessively so (vte and vti are both on a yellow ground).
Mesonotum
Acrostical bristles occur irregularly in 3-4 rows at the front, reducing to two rows behind. There is a conspicuous yellow patch at each hind-corner. The pleura are yellow;the meso- and sterno-pleura have variable black markings.
Wing
Length 1.3 -1.7 mm, discal cell small. The last section is M(sub)3+4 from 3-4 times the length of the penultimate one.
Genitalia
The shape of the distiphallus is fairly distinctive but could be mis-identified for L. sativae. Identification using the male genitalia should only be undertaken by specialists.
Colour
The head (including the antenna and face) is bright yellow. The hind margin of the eye is largely yellow, vte and vti always on yellow ground.
The mesopleura is predominantly yellow, with a variable dark area, from a slim grey bar along the base to extensive darkening reaching higher up the front margin than the back margin. The sternopleura is largely filled by a black triangle, but always with bright yellow above.
The femora and coxa are bright yellow, with the tibia and tarsi darker;brownish-yellow on the fore-legs, brownish-black on the hind legs. The abdomen is largely black but the tergites are variably yellow, particularly at the sides. The squamae are yellowish, with a dark margin and fringe.
Although individual specimens may vary considerably in colour, the basic pattern is consistent.
L. trifolii are small black and yellow flies which may be detected flying closely around host plants or moving erratically and rapidly upon the leaf surfaces. Inspection of the leaf surface will reveal punctures of the epidermis and the obvious greenish-white mines with linear grains of frass along their length. For accurate identification, examination of the leaf mine and all stages of development are crucial.
L. trifolii larvae will be found feeding at the end of the mine, or the mine will end with a small convex slit in the epidermis where the larva has left the mine to pupariate on the ground. Sometimes the puparium may be found adhering to the leaf surface, although in most cases the fully-fed larva will have found its way to the ground beneath the plant to pupariate. This is especially true in hot, dry conditions where the larva/puparia would quickly desiccate if exposed on the leaf surface. Empty puparial cases are split at the anterior end, but the head capsule is not usually separated from the rest of the case.
Mined leaves should be collected into polythene bags and transferred to a press as soon as possible. Leaves containing larvae intended for breeding should be collected into individual polythene bags, which on return to the laboratory should be slightly over-pressurized by blowing into them before sealing the end. Blowing up the bag by mouth and sealing it adds valuable carbon dioxide to the moist air mix. Constant attention is required to ensure that puparia are transferred to individual tubes until the fly emerges. If the plant material begins rotting, good material with feeding larvae must be removed to more sanitary conditions.
When puparia are observed they can be very carefully removed to tubes containing a layer of fine sand, or a small strip of blotting paper or filter paper. This should be kept damp (never wet) until the adult emerges.
On emergence, the fly should be kept for at least 24 hours to harden up. Do not allow condensation to come into contact with the fly, or it will stick to the water film and be damaged.
Field collection of the adult L. trifolii is done by netting. The use of sticky traps, especially yellow ones, placed near host plants is a very effective method of collection and estimation of infestation.
If the puparial stage is collected from the soil, care must be taken not to damage the puparial skin or death will almost certainly follow. The pupae should be stored in glass tubes on a layer of clean sand or, better still, thick filter paper. The tube must have high humidity, but be free of condensation.
When the fly emerges, it must be allowed to harden for 24 hours before killing for identification purposes. Ensure that the tube has no condensation present.
Newly emerged adult L. trifolii are generally softer than specimens aged for several days and may crinkle as drying proceeds, especially the head. The ptilinal sac may still protrude from the suture between the frons and face obliterating some important characteristics. Adults should be dried slowly in the dark in a sealed receptacle over blotting paper. If preserving wet is preferred, the live specimen should be dropped into 20-40% alcohol, and transferred to 70-90% alcohol after 2 days.
Related invasive species
- Liriomyza trifolii
Related Farm Practice
- Activity
- Soil
- Feeding
- Breeding
- Materials
- Bulbs
- Host plants
- Development
- Identification
L. trifolii feeding punctures appear as white speckles between 0.13 and 0.15 mm in diameter. Oviposition punctures are usually smaller (0.05 mm) and are more uniformly round.
L. trifolii leaf mines can vary in form with the host plant, but when adequate leaf area is available they are usually long, linear, narrow and not greatly widening towards the end. They are usually greenish white.
In very small leaves the limited area for feeding results in the formation of a secondary blotch at the end of the mine, before pupariation. In Kenya, Spencer (1985) notes the growth of many L. trifolii from mines which began with a conspicuous spiral. This is not a characteristic associated with L. trifolii on other continents.
The frass is distinctive in being deposited in black strips alternately at either side of the mine (like L. sativae), but becomes more granular towards the end of the mine (unlike L. sativae) (Spencer, 1973).
Fungal destruction of the leaf may also occur as a result of infection introduced by L. trifolii from other sources during breeding activity. Wilt may occur, especially in seedlings.
The host range of L. trifolii includes over 400 species of plants in 28 families including both ornamental crops (Bogran, 2006) and vegetables (Cheri, 2012). The main host families and species include: Apiaceae (A. graveolens);Asteraceae (Aster spp., Chrysanthemum spp., Gerbera spp., Dahlia spp., Ixeris stolonifera, Lactuca sativa, Lactuca spp., Zinnia spp.);Brassicaceae (Brassica spp.);Caryophyllaceae (Gypsophila spp.);Chenopodiaceae (Spinacia oleracea, Beta vulgaris);Cucurbitaceae (Cucumis spp., Cucurbita spp.);Fabaceae (Glycine max, Medicago sativa, Phaseolus vulgaris, Pisum sativum, Pisum spp., Trifolium spp., Vicia faba);Liliaceae (A. cepa, Allium sativum) and Solanaceae (Capsicum annuum, Capsicum frutescens, Petunia spp., Solanum lycopersicum, Solanum spp.) (EFSA, 2012).
It is now considered to be the most important pest of cowpea (Vigna uniguilata), towel gourd (Luffa cylindrica), cucumber (Cucumis sativus) and many other vegetable crops in southern China (Gao, 2014). In Europe, L. trifolii is a major pest of lettuce, beans, cucumber and celery, Capsicum sp., carnations, clover, Gerbera sp., Gypsophila sp., lucerne, Senecio hybridus, potatoes and tomatoes (EFSA, 2012). It is now a major pest of the Compositae worldwide, particularly chrysanthemums (including Dendranthenum, the commercial 'Mum') in North America, Colombia, and elsewhere. It also causes severe damage to different open field crops, such as chili peppers in Mexico.