Prevention of yellow fruit flies in mango

Yellow flies can damage 30-60% of mango output if not prevented. Instructions on proper fruit bagging, pheromone traps, attractant traps and orchard hygiene management for mango growing areas in the Mekong Delta.

Summary: Yellow flies (*Bactrocera dorsalis* and *Bactrocera correcta*) are the biggest enemies of mangoes in the Mekong Delta. Cloudy fruit not only loses its commercial value but is also completely rejected by export markets in the US, Japan, Korea, and Australia. This article focuses on the three most effective measures — fruit bagging, pheromone traps and orchard sanitation — that have been successfully applied by growers in Tien Giang and Dong Thap.

Applies to: Hoa Loc mango, Cat Chu mango, Taiwanese mango, Keo mango in the Mekong Delta, Southeast and part of Khanh Hoa.
Duration: From the time the mango sets young fruit to harvest.
Difficulty level: Medium. The important thing is to do it at the right time.
Estimated additional costs: 3-7 million VND per hectare for fruit bags and pheromone traps.

Characteristics and reasons why yellow flies are dangerous

Yellow flies lay eggs in mango fruit when the skin is still green but has begun to soften. The eggs hatch in 2-4 days, the larvae burrow into the fruit and eat the flesh. Fruits are often cloudy:

The outer shell has small brown spots — egg-laying marks.
There is a nest of larvae inside, the fruit's flesh is dark.
The fruit softens early and falls before ripening.
Harvested fruit has larvae — cannot be sold.

Yellow fly life cycle is fast — 18-25 days from egg to adult. A crop can have 5-8 generations overlapping each other. This is the reason why spraying is not effective: spraying once kills adult flies, but the eggs and larvae in the fruit continue to develop.

Unlike fruit borers that destroy from the inside, yellow flies attack the fruit after it has successfully landed. The closer the fruit is to ripeness, the more attractive it is to flies.

Covering the left — the most effective measure

Bassing mangoes is the best prevention method, 85-95 percent effective if done properly.

Bag type and selection

Specialized paper bag (paraffin paper or 2-layer paper): durable, breathable, waterproof. Price 500-1,500 VND per bag depending on size.
Thin mesh fabric cover: allows light in, resulting in beautiful colors. Suitable for mango export to the US and Australia.
Perforated plastic bags: cheapest but less effective — susceptible to moisture, susceptible to fungus in the bag.

Don't use completely sealed plastic bags — if moisture doesn't escape, the fruit will get fungus or rot.

What time

When will the fruit reach its size:

Hoa Loc mango: 60-80 days old fruit, 4-5 centimeters in diameter.
Taiwanese mango: 50-70 days old fruit, 3-4 centimeters in diameter.
Acacia Mango: similar to Taiwan.

Bagged too early — the fruit is too small and can easily fall due to the weight of the bag and friction.

Bao is too late — the yellow flies have laid eggs, Bao can't save them.

Covering technique

Use paper to surround the fruit and stem. Tie the stem side tightly with the bag's built-in string.
Do not touch the fruit directly — hand oils can stain the skin.
Before packaging, check that the fruit shell has no major scratches.
Fruits showing signs of pests and diseases — unwrapped, picked and discarded.

Time to harvest

Bag the fruit until harvest — usually 45-70 days depending on variety. During this time:

No need to spray the fruit — the bag is protected.
Still spray the leaves if there are other pests — does not affect the fruit.

Pheromone traps and attractant traps

Pheromone trap

Pheromone is a sexual attractant that attracts male yellow flies. When trapped male flies die, the mating rate decreases — the number of eggs laid decreases.

Use pheromones *Methyl Eugenol* for *Bactrocera dorsalis*, *Cuelure* for *Bactrocera cucurbitae*.
Hang 5-10 traps per hectare at a height of 1.5-2 meters.
Each trap contains pheromones that last 6-12 weeks. Replace when the deadline expires.
Place traps around the perimeter of the orchard — not in the middle of the orchard.

Self-mixed attractant trap

Simple preparation from available materials:

100 milliliters of water.
30 grams of red sugar or molasses.
10 milliliters of vinegar.
5 grams of *Trichlorfon* (insecticide *Organophosphate*) — to kill flies entering the trap.

Place it in a wide mouth plastic bottle, punch a few holes. Hang 5-10 traps per hectare. Change the solution every 7-10 days.

This trap attracts both male and female flies, effectively complementing the pheromone trap.

Cleaning the orchard — removing natural fly nests

Cleaning the orchard is the cheapest method but many people ignore it:

Collect fallen fruit under the tree every 2-3 days during the fruit season. Fallen fruit still has larvae — if not treated, it will become a source of flies in the next crop.
The fallen fruit is buried 50 centimeters deep or burned. Do not throw them in piles in the orchard.
Prune dense branches to let light into the canopy — yellow flies like shade.
Clean thoroughly after harvesting — do not leave fruit left on the tree or on the ground.

A well-cleaned orchard reduces yellow fly pressure by 40-60 percent for the next crop.

When to use treatment

Spraying on mangoes is not as effective as expected because:

Eggs and larvae in the fruit are not destroyed.
The fruit is ripening — cannot be sprayed because it leaves residue.
Destroy natural enemies.

Only spray in the following cases:

The orchard had not yet had time to cover the fruit and flies appeared in high density. Spray *Cypermethrin* or *Deltamethrin* (synthetic pyrethroid) once, then quickly cover the fruit.
After harvest, spray the entire orchard with broad spectrum to kill residual flies.

Absolutely comply with the pre-harvest quarantine period stated on the label — usually 14-21 days.

Monitor every week during fruit season

[ ] Check the pheromone trap — count the number of flies trapped. High density is a sign of an outbreak.
[ ] Look at the fruit — are there small egg-laying marks on the shell?
[ ] Falling fruit — check for larvae.
[ ] Condition of the bag — is it torn, wet, or damaged?

When fly density increases rapidly — bag fruit more carefully and increase the number of traps.

Common mistakes

Ignore fruit bagging because it's "wasting effort": the cost of fruit bagging is many times smaller than the damage to perforated fruit. Uncovered orchards often lose 30-60 percent of their yield.

Spraying to replace fruit sacks: not effective against eggs in fruit. Leaves drug residue on harvested fruit.

Packing fruit too late: flies have already laid eggs, bagging has no effect. Keep a close eye on the size of the fruit.

Let the fruit fall under the tree: becomes a fly nest for the next crop. Each fallen fruit can contain dozens of larvae.

Hang the trap in the middle of the orchard: flies concentrate in the middle of the orchard — more harm. Hang around the orchard perimeter to catch flies from outside.

Take notes

Time to start bagging — date, quantity of fruit.
Percentage of successfully coated fruits after harvest — proportion of fruits that are not opaque.
Number of flies caught in the trap per week — graph.
Number of fallen fruits containing larvae in the season.
Total cost of bags + traps compared to the increased selling price due to the fruit not being opaque.

References

*Prevention of fruit flies on fruit trees* — Plant Protection Department, 2022.
*Technical handbook for growing mango for export* — Southern Fruit Institute, 2023.
*Packaging mangoes for export* — Department of Crop Production, 2021.