Training on ocular identification of Leafminer flies and its natural enemies in farmers' fields

Module I

Identification Of Leafminers And Their Natural Enemies

Objectives:
After this unit, the participants are expected to be able to recognize leafminers from their natural enemies

A. How to Differentiate Leafminers from Other flies?

Flies are very abundant, you could even see them in your own garden. Flies are source of food for spiders and other animals. Many species are parasitic or predaceous on other insects, while others are scavengers. However, large numbers are nuisances because they bite and irritate humans and some transmit human and animal diseases. Many attack and damage cultivated plants, but few are capable of transmitting plant diseases. 

Flies are normally found near the habitat of their larvae. Adults often are seen feeding on nectar of the flowers. Many are bloodsuckers and are found near or on the animals that they feed on. 

Their young ones may live in water, soil, decaying materials or animal or plant tissues. The plant-feeding species live in various parts of the plant. 

This group is sometimes confused with bees, wasps, and beetles especially at the larval stage.

Among the dipterous flies, the family Agromyzidae is one of the economically important groups because of its leaf mining activities. Leaf miners as compared to common houseflies are smaller, less hairy and have a yellowish triangle on the thorax or between the wings (Figure 1). They feed on many cultivated and wild plants and hence knowing their characteristics is very important to facilitate decision-making regarding control for future incidence.

Figure 1.  Adult male Leafminer

Compared to the other adult insects with 2 pairs of wings, the adult flies are distinguished by having only one pair of wings. Under the microscope, the hind wing is reduced to a knob-like structure called the halteres. (Figure 2) The halteres are used for balancing while at flight. They also have large compound eyes and the type of mouthparts is sponging type (Figure 3). They suck up juices and sap from plants by the use of a sponge-like structure at the tip of their mouth.

Figure 2. Haltere

Figure 3. Dipteran mouthpart

The young ones or larvae of leaf miners are legless, yellowish, worm like and lack developed heads (Figure 4). They also have strong chewing mouthparts that make tunnel-like structures on the leaves. The first and second larval stages of the leafminer burrow into the green tissue of the leaves. The third larval stage, on the hand, concentrates it's feeding towards the upper leaf surface. The mine produced by leafminers compared to other insects has distinctive feces which are being deposited in black strips (Figure 5) alternately at either side of the mine. When the larva matures, it cuts a longitudinal slit in the leaf to pupate either on the leaf surface or on the ground. The adults live for about 10-20 days depending on the environmental conditions. Adults feed and lay eggs at sunrise, peaking at mid-morning. Mating may occur at any time but frequently during the daylight hours. The adults are not fast and long distance fliers. They move about the plant in rapid jerky movements. The female adult punctures the leaves during egg laying or while feeding. On the other hand the larva damage leaves by creating serpentine-like or snake-like tunnels.

Figure 4 Leafminer larva

Figure 5 Mines with frass

B. How to determine the differences between Four Leafminer Genera?

In the Philippines, there are 4 genera in the family Agrornyzidae, which are plant feeders and are economically important. These are Aqromyza, Chromatomyia, Ophiomyia and Liriomyza.

Ophiomyia are common bean flies that attack legumes at a younger stage by tunnelling through the stem. This eventually result to wilting and death of seedlings.

Agromyza flies are found everywhere and their larvae tunnel or mine leaves. Many species are tree feeders but some attack Poaceae (grass family- rice, wheat, barley). They may occasionally attain pest status on a local scale/ outbreak level. They have restricted host range. The Agromyza flies are normally distinguished from Chromatomyia as they are black and wing length is shorter than Chromatomyia. The body size is about 3-3.5 mm.

Chromatomyia flies on the other hand, consist of polyphagous species like C. horticola (Goureau) and C. syngenesiae (Hardy). is an Old World species, which means that it occurs in Asia and the Pacific and is able to adapt to tropical climates. On the other hand, C.Syngenesiae is a cosmopolitan species meaning it adapts everywhere. The species damages a wide variety of plants but what is alarming is the effect of C. Horticola on peas and C. syngenesiae on lettuce and chrysanthemum. Adult f lies are usually black flies with yellow frons. The body size ranges between 3.5-4 mm including wingspan. Wings are generally longer than Agromyza and Liriomyza. Mines are uniformly whitish in color with irregular linear mines.

 

Pupae and exuviae may be present at the end of the mines. The frass is on short intervals along the mine in irregular well-separated grains (Figure 6). Rot and decay of seedlings is secondary effect. They produce necrotic effects on the leaves, aside from internal feeding. The mining causes the stems to wilt in seedlings.
	Figure 6 Chromatomyiz mines with frass

Liriomyza flies on the other hand, are generally leaf miners with a capacity to damage wide array of host plants. Man has either transported or introduced by accident many of them into countries to which they were not native. Among the serious pests are L.huidobrensis, L.trifolii, L. chinensis, and L. sativae. International and domestic trades accidentally introduced consignment of commercial chrysanthemum cuttings infested with leafminers. The yellowish head, yellow triangular structure at the thorax or between wing area and the yellow-banded abdomen, could easily recognize this genus. The body size may range from 2.5-3 mm. The mines formed are tightly coiled as compared to the irregular linear shaped mines of the Chromatomyia species. The frass is distinctively deposited in black strips alternately at either side of the mines like in L. sativae and produce granular structures towards the end of the mine as in L. trifolii. Necrotic areas are produced on the leaves. Internal feeding is done. An abnormal coloration and form on the leaves results. The production of honeydew or sooty molds appears on the leaves. An abnormal leaf fall occurs as well as wilting.

Liriomyza species have their distinct mines that can aid in the preliminary identification of the species.

• L. huidobrensis has its tunnel tightly coiled and irregularly shaped. The tunnel starts with the midribs and lateral leaf veins. The tunnel gets wider as the larva matures.

• L. trifolii has its frass alternately on either sides and a granular material is formed towards the end of the mine (Figure 5).

• L. sativae, on the other hand, has its frass alternately on the other sides but there is no granular material formed towards the end of the mine.

• L. brassicae has a greenish white mine and produces a continuous ribbon of black frass.

C. How to Differentiate Leafminers from its Natural Enemies?

Knowing the characteristics of leafminers from previous discussion. Natural enemies are parasitoids, parasites or predators. They feed on other arthropods by either internal or external feeding. These are farmer friendly organisms, which naturally reduce pest population to a level that economic damage is not attained. They naturally occur in the pesticide-free environment and therefore, conservation is the best way to preserve and enhance their potentials.

A parasite is an organism that lives in or on the body of another organism (the host) during some portion of its life cycle.

A parasitoid is an arthropod that parasitizes and kills an arthropod host; it is parasitic in its immature stages but is free-living in its adult stage. They have a habit of dancing on their prey before attack or oviposition.

A predator is an organism that attacks and feeds on other animals (usually smaller or less powerful than itself and consumes more than one animal in its lifetime).

The beauty about the use of natural enemies is that resurgence of pests rarely occurs unlike with the misuse of pesticides. The resurgence of pests in the field is relatively high because pests develop resistance to chemicals rapidly. Moreover, chemical control increases the pest population as it kills and reduces the population of natural enemies.

D. How to Detect the Presence of Natural Enemies of Leafminers?

There are numerous parasitic (friendly) wasps that occur naturally in the pesticide free- fields. They help to regulate leaf miners to reach damaging levels. Though their presence is not easily recognized or identified because of their minute size, but there are ways to determine the presence of parasitoids or natural enemies of leafminers. Arthropods that come near the snake-like mine of leafminer larvae are probable natural enemies. This is if the characteristics do not fit the leaf miner characters. The behavioral pattern of parasitic wasps is to perform first a waggle dance or following an 8-like flight pattern near the host. When the larvae has been located. The ovipositor is then pierced in the tunnel where the larva feeds on. Then a darkening of the larvae will be observed. The parasitized larvae eventually become immobile in their mines. The larva may become bloated or blackened as the parasitoid develops internally. Only one parasitoid larva develops per leafminer larvae/pupae. They pupate in the remains of their host and the color is glossy black.

E. How to differentiate various Species of Natural Enemies of Leaf miners?

Among the parasitoids or parasitic wasps observed to be present in the field in controlling leaf miners are Opius spp., Asecodes delucchii (Boucek), Diglyphus isaea (Walker), Hemiptarsenus varicornis (Giroult), Pnigalio katonis (Ishii), Neochrysochalis formosa (Westwood) and N. okazakii Kamijo. As parasitic wasps, their action is to oviposite on leafminer larvae/pupae and feed on their host internally until the host organism dies.

NOTE: For more details on the identification of leafminer parasitoids please refer the brochure on [FARMER' S GUIDE TO IDENTIFY NATURAL ENEMIES OF LEAFMINERS IN THE CORDILLERA ADMINISTRATIVE REGION]

Activities:
1. After collecting specimens in the field, classify whether they are leafminers, natural enemies or associated insects.
2. Identify the leaf miners and natural enemies to genus level using hand lens.

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