Instructions for preventing rice spider mites

Stem spiders damage leaves, flag leaf sheaths and rice ears, causing purple sheaths, half-formed or completely flattened flowers. Instructions on how to recognize, outbreak conditions in the hot, dry season and how to handle when the leaf sheaths have symptoms of 5-10%.

Summary: Spider mites are very small, almost invisible to the naked eye, but the damage is great because they cause damage in the flag leaf sheaths, on growing cotton and seeds. Severely affected fields have purple sheaths and flower heads that are partially or completely flattened. This article guides you on how to recognize symptoms, treat them at the right stage, and avoid habits that cause spider outbreaks.

Applies to: Summer-autumn, fall-winter rice and fields experiencing hot, dry weather, lack of water, thick sowing, and excess nitrogen fertilization.
Duration: Monitor from planting to flowering, especially the leaf sheath stage.
Difficulty level: Medium. The hardest part is early detection because spiders are very small.
Estimated additional costs: 250-600 thousand VND per acre if spider insecticide must be sprayed.

How harmful are spider mites

Semite spiders appear all year round, but are heaviest in the summer-autumn crop when the weather is hot and dry. Spiders live and cause harm in:

Leaves.
Leaf sheath.
Inside the flag sheath.
Rice ears and grains are growing.

When spiders suck, they create wounds. These wounds open the way for fungal diseases to attack, causing poor seed development and increased grain sagging.

Identification signs

Because spiders are very small, they are mainly recognized by symptoms:

The damaged leaf sheaths have a characteristic purple color.
The flower blooms unevenly and can cause choking.
Seeds are flat or completely flat.
The leaves and sheaths have dark spots, the tree looks damaged.
The field has an abnormally increased rate of grain smearing.

The most visible symptom is that the flag leaf sheath changes color to purple-brown. Once you see a lot of flat cotton, it's too late to deal with it.

Conditions for spiders to grow

Spiders often explode when:

High temperature and humidity.
The weather is hot and dry.
The fields are sown thickly.
Excess nitrogen fertilization.
The field lacks water.
Spray crop-protection products early and many times before 40 days after sowing, causing loss of natural enemies.

So, preventing spider mites is not just about spraying spider insecticide. The field floor must be moderately sparse, with enough water and balanced fertilization.

Prevention by farming

After harvest

Plowing and turning over stubble.
Bury or treat rice residue.
Cut crops or rotate to other crops if the area is often severely affected.
Don't let rice rot exist for too long.

Remnants and rice residue are places where many pests stay during the crop.

In case

Sow thinly as recommended.
Do not fertilize with excess nitrogen.
Increase potassium and silicon as needed to harden plants.
Keep water properly, avoid letting fields dry for a long time.
Do not spray crop-protection products early if the threshold has not been exceeded.

Natural enemies such as predatory spiders, parasitic wasps and beneficial insects help control the population of spiders. Spraying widely too early will cause this natural protective layer to be lost.

When to treat

Usually consider spraying when:

Symptoms detected on the leaf sheaths range from 5-10 percent.
The field is in the planting and flowering stage.
Long hot and dry weather.
The field has a history of heavy spider mites.

Don't wait until the cotton is very flat. The best stage is when you first see symptoms in the leaf sheaths.

How to spray effectively

Before spraying:

Add water to cover the field at the appropriate level.
The goal is to make the spider move up, making it easier to come into contact with the drug.
Check the field for no rain in the next few hours.

When spraying:

Use spider repellant according to the label and local recommendations.
Spray enough water, focusing on the leaf sheaths and the middle layer.
Spray in the cool afternoon.
If you have to spray again, change the active ingredient group.

Don't use regular crop-protection products and expect to kill spiders well. Spider mites need treatment that is effective against spiders and sprayed in the right location.

Combine grain prevention

The spider mite increases the risk of grain smearing by creating wounds for fungal pathogens. There are spiders in the field:

Monitor flowers before and after flowering.
Prevent sprinkling at the right time if the weather is humid.
Keep plants healthy with balanced fertilization.
Avoid late protein.

Management of spider mites and grain smearing should go together at the flowering stage.

Monitor periodically

[ ] Percentage of rice leaf sheaths with purple stains.
[ ] Rice stage: paddy field, flowering, milking.
[ ] Is the field lacking water?
[ ] Amount of nitrogen fertilized.
[ ] Number of crop-protection product spraying times before 40 days after sowing.
[ ] Ratio of flat/half-open flowers after flowering.

If the field is often infested with spider mites, it is necessary to increase inspection before flowering, not waiting to see flat seeds.

Common mistakes

Just look for the spider with your eyes: spiders are very small, so you have to read the symptoms on the sheath.

Wait until the cotton is flat before spraying: the damage has already been done, it's difficult to save.

Thick sowing, high nitrogen fertilization: creates dense canopy and soft plants, favorable spider growth.

Spray crop-protection products early many times: kills natural enemies, spiders easily break out.

Do not add water before spraying: spiders are still hidden deep in the sheath, the drug has poor contact.

Take notes

Date of first discovery of purple sheaths.
Proportion of symptomatic sheaths.
Rice stage.
Medications/active ingredients used and date of injection.
Results after 5-7 days.
Rate of grain smearing at the end of the season.

Taking notes helps identify which crops and which fields are susceptible to spider mites so that you can prevent them earlier.

References

*Instructions for preventing rice spider mites* — Hainong technical document.
*Integrated pest management for rice* — Plant Protection Department.
*Rice IPM Handbook* — FAO Vietnam.