Pests of Field Crops in Southern Africa

LIRIOMYZA LEAFMINER

(Diptera: Agromyzidae)

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Text Box: A number of species of Liriomyza have been present in Southern Africa for some years, known collectively as “American leafminer” since they are believed to have originated from there, or as serpentine leafminers, because of the long, winding mines they make. In the late 1990’s a new species began to appear, Liriomyza huidobrensis, sometimes known as the “pea leafminer”. This gives a false impression, however, since this pest has an extremely wide host range. Liriomyza spp. are often recognised more by the mines they form in leaves, than by the adult pests themselves, which are rather elusive. These are in fact very small black flies on which there is a varying amount of yellow colouring. In L. huidobrensis the yellow is reduced to a tiny speck on the back of the thorax, only just visible to the naked eye, and the fly tends to be a little larger than L. trifolii, which has more yellow on the head and sides of the thorax. The larvae live within the leaf mines, and are legless, obviously segmented and grow to 2-3 mm in length. As is typical of maggots, they do not have a recognisable head, this being reduced to a pair of black mouth hooks, by which they feed and produce the mine, which widens after each instar is reached. The pupae (strictly puparia) are also 2-3 mm in length and range from light to dark brown in colour. TOP

 

 

In Zimbabwe, Liriomyza spp. (probably mainly L. huidobrensis since the late 1990’s) have been recorded on a huge number of different plant hosts.  Among these are: celery, beans, brinjals, broad beans, cabbage, paprika seedlings, peas, spinach, cucurbits, lettuce, potatoes, tomatoes, in addition to many common flower species (petunias, pansies, Gypsophila, etc), cotton, soyabeans and many weed species.
Text Box: This pest usually appears at the start of the hot, dry season. First symptoms are a few small yellow “stipples” on the youngest leaves, which are the feeding punctures caused by the flies. They do this by inserting the ovipositor into the leaf tissue and making a small hole, from which they can subsequently sip the sap. As the population increases the stippling increases hugely, and may cause some damage in itself. Only a fairly small percentage of these holes actually have eggs inserted into them. Soon, very narrow mines appear in the leaves, and in a relatively short time, these can become extremely numerous. L. huidobrensis mines seem often to be in close proximity to the leaf veins or petiole, and may be rather straighter than the very winding mines of other species such as L. trifolii. The former species also has a greater tendency to mine on the lower leaf surface than the upper. Damage by these leafminers can reduce the plant’s photosynthetic ability and cause leaf drop, particularly in susceptible crops like tomatoes. Sometimes in tomatoes the destruction and loss of the foliage is so severe that developing fruits become sun-scorched. In other crops such as potatoes the foliar damage is not as serious, but can provide entry for leaf diseases that develop rapidly, especially in the wetter months. In this case, disease control becomes critical in the crop. In many green leafy vegetables (celery, lettuce), the plants may not be very adversely affected by a certain amount of foliage damage, but if these vegetables are intended for export, or even for the local market, the mines often render the product unacceptable. One case has been seen where very expensive hybrid tomato seeds were being grown under plastic, and the damage caused not by the mining larvae, but by the feeding adult flies on the first leaves, was so severe that many seedlings were killed soon after emergence. In beans, commercial soyabeans and cotton crops, damage by the mines is often confined to the lower leaves, and the plants soon seem to outgrow it. Occasionally young cotton crops can carry quite a heavy infestation, but the pest has never yet been considered to be of economic importance. TOP Text Box: The life cycle of Liriomyza spp. has been studied in great detail in other countries, and is likely to be very similar in Zimbabwe. On average, the cycle is completed within three weeks in warm weather, with the egg lasting about 3 days, the larval mining stages 3-5 days, and the pupae 8 or 9 days. Eggs are inserted into the leaf tissue, although the positioning may vary from species to species. In L. huidobrensis punctures with eggs in them are only found on the lower leaf surface. In one season, when this species first began to make its presence felt, an average of 250 eggs per leaf inserted into the lower surface of young potato leaves was recorded. The flies themselves live little more than two weeks, but mating can take place after a day and egg-laying peaks after a week. There are three larval instars within the mines, and the larval skins, with mouth hooks quite visible, are left behind in the mine. Pupation takes place when the third-instar maggots cut a semi-circular hole in the widest part at the end of the mine and emerge to fall on the ground, or occasionally to pupate in protected places on the leaves. There is a fourth larval stage just before pupation, lasting just a few hours. The length of the life cycle is obviously temperature dependent, being much longer in the winter months, but may also depend on the host plant (some plants appear to inhibit completion of development). TOP Text Box: Chemical control is a problem, mainly because of the very high degree of inherent resistance to pesticides that exists in Liriomyza species. Recent work in Israel has shown resistance to synthetic pyrethroids, carbamates and organophosphates in L. huidobrensis. However, in situations where no sprays have been applied, very high levels of parasitism have been observed (50-80% of leafminers in one garden that received no pesticides). This is caused by tiny wasps that kill the larvae within the tunnels. The main one is Diglyphus isaea (Eulophidae), which lays its eggs next to the maggot in the mine rather than in it. It is metallic black, with red eyes and pale yellowish stripes on the legs. Characteristically, the wasp pupae that develop in the mines are a bright pale green colour before turning dark. Unfortunately, unlike the pest, these parasites are very susceptible to pesticides, so that unless pesticides are excluded altogether or very “soft”, specific ones are used, the wasps are unlikely to provide sufficient control within field conditions. These parasitoids are commercially available in first world countries, but are mainly used within greenhouse crops. In terms of cultural controls, some measures may help. If possible, infested plant material should be removed and destroyed, particularly at the end of the crop’s life when the pupae may remain to infest a new crop. Non-host crops should be used in the rotation, although these are few and far between. Maize and other grass crops are a possibility. Weed management should be good, as so many weed species are also hosts. In fact, infestations may often originate in weed hosts adjacent to field crops. Although there has been no research on plant barriers, it may be helpful to plant tall barrier crops at field edges. Insecticidal control is unfortunately very costly. The very few products that are effective against Liriomyza belong to new chemical groups, are translaminar and work on the larval stages. Spray coverage should therefore always be thorough. Since the pest has a strong ability to develop resistance, rotation of these insecticides, and restricted usage, is very important. The two main products are abamectin and cyromazine, the latter an insect growth regulator. Neither should be applied more than twice on a crop. A third compound has been of some use in Zimbabwe - a fungicide (pyrazophos) normally used for control of powdery mildews. And a fourth is the naturally-occurring botanical pesticide derived from Neem trees (Azadirachta indica) which has also shown efficacy with this pest, particularly when used as a systemic applied to the root zone. Since the tree is unfortunately not indigenous to Southern Africa, the product can be as costly as the new insecticides mentioned earlier. Sprays should probably be applied early in the mornings, as this seems to be the time when the pest is most active. Although many farmers try to control the flies themselves, there appear to be no effective insecticides against the adult stage. They are, however, strongly attracted to yellow, so that yellow sticky traps or yellow basins filled with mildly soapy water, can trap many of them. This is hardly a very effective control measure though, as many hundreds of flies appear from about August to November when the rains arrive. Once the rains have set in, the pest tends to become less of a problem, presumably because natural elements control the pupae or emerging adults in the soil phase. TOP

Identification

Identification

Host Plants

Damage

Life Cycle

Control

Host Plants

Damage

Life Cycle

Control

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Liriomyza adult on a pea leaf. (2-3mm) and in the drawing below.

Larvae make mines in the leaves of several plants including Tomato.

Liriomyza life cycle. The bottom drawing shows the characteristic winding tunnel occupied by the larva.

Liriomyza mine getting bigger as the larva grows. The larva is visible at the end of the mine.

Liriomyza flies (above) and stippling on pea leaves (above and below).