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Insects—scale [photos from web visitor]
(From an actual email) "I brought my plants inside to keep them warm and they have developed some form of scale growth, see attached photo. What is this and what should I do?"
Oh my lord! That looks like scale all right—an insect infestation.
http://www.ext.vt.edu/departments/entomology/ornamentals/scaleinsects.html
You don't really need an insecticide—this is an unhealthy plant because of unhealthy growing conditions. Light? Temperature? Water?

TOO MUCH NITROGEN!
Excessive plant food makes the plant produce carbohydrates that sucking insects exploit. This is a common gardening problem. Aphids and similar ones that suck out the juices appear where you fertilize other plants.

In this case start by washing the scale off. Wipe it with neem oil, or lightly spray it on. Go to a local houseplant store to discuss this and buy a small container of neem oil - it organic.

I think they use dormant oil spray in orchards around here for that. (Oil smothers the insects) But I'd first recommend you talk locally with a plant nurseryman (or nursery person to be non-sexist). They might best know your local insect varieties and recommend a specific insecticide. You don't have to use an evil poison—there are always organic type ones.

One way is to start with mechanical removal—stiff spray of water, a brush like you wash your skin with, soap, etc. When I farmed vegetables for the market the plants with insect problems always were the ones with a problem like damaged roots, too much water or too little water, too little light, etc. The healthy plants didn't get insect problems. If a plant had a lot of aphids you just give it a stiff spray of water every day to knock them off. Worked.

*** Too much nitrogen fertilizer, given to vegetables, can result in too much plant sugar that attracts sucking insects. Maybe its the same with San pedro.

I'd want to check the roots and would lift the plant out onto a table to see if the roots were rotting or all healthy. If the plant is "loose" and rocked around in the pot that would be a bad sign. It should have a tight grip on the soil—healthy roots.

If the roots are rotting you can cut them back and let the plant rest for a week, then repot in dry soil. Leave it alone for months and it will regenerate.

A healthy plant should not have an insect problem. The alkaloids in the skin are supposed to kill them, but maybe these insects get high on the plant chemicals—sort of like kids who sniff gasoline.

This time of year (Winter) your plant doesn't need water. Remember cacti breath at night through their stoma and thus obtain water vapor with the CO2 they need. So take it out of the warm house and into a cooler place until Spring.

FEEDBACK:
• What I don't like from the photo is that the plant is in a house at room temperature when it should be cooler and dormant. Is there a spare room that you don't heat?

• Don't like those pots—not enough drainage.

• Water once a month, lightly, this time of year. Don't try to force a plant to grow during its dormant time. Farmers don't water & fertilize fruit trees off season for this reason.

• Make sure the soil dries back before re-watering. Don't keep it always wet.

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