Liriomyza  spp.

Leafminers

General  

• Rather polyphagous insects widely distributed in the world which are classified in the order Diptera and the family Agromyzidae.
• Many of these flies are classified as quarantine pests.
• Several species found in tropical areas:
  
  • Liriomyza bryoniae Kaltenbach, often wrongly referred to as Liriomyza strigata [Meigen],
  • Liriomyza trifolii Burgess the California leafminer,
  • Liriomyza huidobrensis Blanchard South American leaf miner,
  • Liriomyza sativae , the market garden leafminer,
  • Liriomyza strigata Meigen etc.
  • Nemorimyza maculosa Malloch, a leaf miner that damages lettuce
    
• Controlling the populations of these pests is often problematic because of their possible resistance to several insecticides, the latter also eliminating useful fauna (parasitoid Hymenoptera).
• Observed in the open field and under shelter.

• Sensitive botanical family (s) * : 

* These polyphagous leaf miners multiply and survive on many alternative cultivated hosts (tomato, cucumber, lettuce, melon, pepper, celery, bean, potato, chrysanthemum, gerbera), as well as on weed plants present in or near outside of culture.

• Affected production areas :

• Organs attacked

Symptoms, damage

• Symptoms :
  • Numerous chlorotic nutritional punctures observed on the lamina and produced by the females with their ovipositor. Galleries, mines (figures) more or less sinuous appear thereafter on the leaves.
  • Yellowing of the most affected leaves which may wilt and dry out. The photosynthetic activity of plants, their growth and yields are thus greatly reduced.
  • Burns on fruits ( Sunburns - sunscald ) linked to the degradation of many leaves no longer protecting them from solar radiation.

• Signs : Presence of flies and larvae on and in the affected organs, and in the culture (figures ).
• Possible confusion :

Biology

• Development cycle (figure ): 6 stages of development are observed (egg, 3 larval stages, pupa and adult).
  • Eggs, cream-coloured and oval in shape, laid in the tissues during egg-laying stings using the ovipositor of females. A female can produce several hundred eggs which then hatch and give birth to transparent larvae.
  • Larvae digging in the leaflets of the galleries that the presence of black excrements makes quite visible. The white third-stage larvae pierce the leaf blade, leave the leaflets, drop into the folds of the plastic or onto the ground and bury themselves shallowly.
  • Subsequently, they turn into barrel-shaped pupae and which vary in color as they age from yellow to dark brown, the blackish pupae are often the ones that are parasitized.
  • Adults, small flies 2 to 3 mm long, yellow and black ( Liriomyza spp.) or blackish gray ( Chromatomyia horticola ). The adult females, present on the upper face of the lamina, perforate the epidermis thanks to the veritable auger that is their ovipositor, suck up the plant juice (nutritional bites) and deposit their eggs (laying bites). It should be noted that the males, without augers, also take advantage of feeding bites to feed themselves.
• The duration of their cycles varies according to temperature and species.
• Adults fly easily and disperse in crops and their environments. Newly infected plants (carrying eggs or very young mines) can also contribute to the spread of these insects.
• The evolution of the population levels of these insects is rather influenced by high light intensities, certain rather vigorous host plants, high hygrometry (80-90%) in particular.

Protection 

• Disinfect reused substrate or soil.
• Weed the greenhouse and its surroundings.
• If possible, produce the plants in an shelter insect-proof . The use of nets insect can protect the nursery, but also the crop.
• Control the sanitary quality of the plants.
• Arrange yellow sticky panels under shelter to capture the adults, preferably position them above the crop to increase the probability of capture.
• Collect and destroy heavily mined leaves, and remove and destroy crop residues.
• Favor natural enemies in field cultivation or under open shelters**.
• Consider  chemical protection ( e-phy )***, in particular if you use auxiliaries or biopesticides ****. This is not always effective and should preferably be combined with cultural, physical and biological methods. Repeated use of insecticides can lead to resistance phenomena in leaf miners, rendering the active ingredients ineffective in the short term.

***: Broad-spectrum insecticides also impact the natural enemies (helpers) of leafminers and reduce or eliminate their natural control.

****: Bio-control methods have been tested abroad, they include the use of Bacillus thuringiensis var kurstaki (entomopathogenic bacteria), Beauveria bassiana (entomopathogenic fungus) and Steinernema carpocapsae (entomopathogenic nematode). Only the latter is authorized for this use in Guyana but its effectiveness has not been tested.