1. Background

Unlike most leafminer fly species, Liriomyza huidobrensis, is highly polyphagous, tolerant to many insecticides, and readily develops resistance to insecticides used to control it.  Originated in the neotropics, this specie was restricted to America until the 1980s. But since then, L. huidobrensis is rapidly spreading to other areas is reported in several countries of Africa, Europe, and Asia. Liriomyza huidobrensis, as a serious pest of potato, has been reported in areas of Peru, Chile, Argentina, Brazil, Central American countries, Mexico, Northern Africa, Kenya, Indonesia, Malaysia, and Israel. This pest also attacks many other vegetables (Figure 1) and ornamental plants. The leafminer fly has become a key pest in all countries where it has been introduced, capable of completely destroying potato fields, in absence of control measures. In many places, leafminer flies have shown significant levels of resistance to most carbamate, organophosphate, and pyrethroid insecticides, commonly used to kill larvae and adult flies.



Figure 1. Cultivated host of leafminer fly, potato (A), tomato (B) and bean (C)
2. Characteristics of leafminer fly (L. huidobrensis)
2.1. Life cycle

Leafminer fly has four life stages: egg, larvae, pupa and adult (Figure 2), lasting 3-5 weeks. Up to 13 generations per year. The eggs are layed by the female individually inside the leaf’s tissue, they are rounded and translucid (3-6 days). The larvae period has three stages (5-8 days) that could be visualized by the length and thickness of the mine the last stage on the larva development is the bigger. The larvae can be found by opening the main vein in the leaves. The pupae develop on the top of the leaves (10-17 days). After that time, the pupae fall to the ground and stays there until the emergence of a full-developed adult. Adults are small (1.7-2.3 mm), black flies with a bright yellow spots in the thorax. Mating and oviposition occurs after 6-24 hours and 1-3 days respectively after the emergency.

Figure 2. Life cycle
2.2. Recognizing damage
Adult damage

Female adults of L. huidobrensis uses its ovipositor to make holes in the top and/or bottom of the leaves, promoting the production of exudates that will feed males and females. This type of wound is known as the “feeding puncture”(Figure 3). When the wounds are made to lay the eggs, they are called “oviposition punctures”.

Figure 3. Feeding puncture produce by female adult
Larvae damage

The larvae due to its mining habits, which reduce the photosynthetic ability of the plant, cause the worst damage. The larvae get to the plant from the bottom and after some time, the rest of the plant becomes infected (Figure 4).

Figure 4. Feeding puncture produce by larvae damage
2.3. Damage pattern

Larval damage in a growing plant follows a fairly well established pattern. The first leaves to show damage are those of the lower part. Middle and upper leaves are progressively damaged as the plant grows and infestation continues (Figure 5). Damage in the upper leaves usually occurs when the canopy stops growing. Necrosis of the infested leaves follows the same pattern, until the whole plant dries out.

Figure 5. Pattern of LMF in potato plant
2.4. Seasonal variation of LMF population

In the Cañete valley, LMF host plants are present the year round. Climatic conditions are favorable for the development of the fly. In normal years, monthly average temperatures lie between 23.6 ºC (18.4º-28.7ºC) in summer and 15.6 ºC  (13.2º-18.0ºC) in winter. Despite these conditions, there are clear seasonal trends in LMF population levels (Figure 6):

i) Moderate adult infestation. - Occur in the warmer months from December to April.

ii) High adult infestation.- occur in the winter and spring season from June to October.

Figure 6. Moderate and high adult infestation in Cañete valley
3. Standard evaluation for LMF

The procedure for evaluation of advanced clones to LMF resistance involves trials over 2 seasons under moderate and high adult infestation according to the scheme show in Table1.

3.1. Moderate infestation
First season (1-1)

This is the first resistance evaluation for any clones. Clones are separated by groups of 15 to 25 clones. Experimental design is a RCB with 3 replications. The experimental plot consists of one row with 5 or 10 plants. Each group must include a susceptible local variety as check.

Second season (1-2)

Selected clones in first season are re-evaluated to confirm the characteristics of resistance. Experimental design is a RCB with 3 replications.  Experimental plot consists of 3 rows with 5 to 10 plants/row. Susceptible local variety is included as check.

3.2. High infestation
First season (2-1)

This screening begins with the selected clones in the first season of moderate adult infestation (1-1). Experimental design is a RCB with 3 replications. Experimental plot consists of one row with 10 plants. Susceptible local variety and resistant variety are included as check.

Second season (2-2)

Selected clones in first season are re-evaluated to confirm the characteristics of resistance. Experimental design is a RCB with 3 replications.  Experimental plot consists of 5 rows with 5 to 10 plants/row. Susceptible local variety and resistant variety are included as check.

3.3. Field management

Cultural management of the experimental area will be similar to a commercial potato field and/or as required in each experimental site. N-P-K will be applied according to local recommendations. Half of the nitrogen at planting and the other half at first hilling. Water will be supplied by furrow irrigation according to requirement of plants. Pesticide applications will be carried out against late blight and mites as needed. Field experiments will include local varieties as test crops.   

3.4. Field evaluations
a. Monitoring adult population

Monitoring pest populations has the purposes to study the population dynamics of the pest. Adult populations are determined by using yellow sticky traps. These traps are placed in fields of the clones evaluated. Samples are taken at week intervals.

b. Monitoring foliage damage

Three reading of foliage damage percentage will be made on individual plants within plots. At 60, 75 and 90 days after planting in early clones and at 75, 90, and 105 days after planting in later clones. A damage score on a scale of 1-5 will be used as described in Table 2. Examples of damage levels are show in Figure 7.

Table 2. Evaluation scale of Leafminer Fly on potato clones



Figure 7. Damage level on Revolucion variety at 60 (A), 75 (B) and 90 (C) days after planting
3.5. Qualification

Trials field are considered acceptable if susceptible local variety has a score damage over 3, under moderate LMF adult infestation; and a score damage over 4 under high LMF adult infestation.

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