Answer: Usually a sticky substance on the leaves is a sign that sucking insects are present on the plant. Spider mites leave a tell-tale webbing on the undersides of leaves, especially where the leaf meets the stem. Carefully inspect your plants for the webbing. Most likely the culprits are aphids or whiteflies. Aphids are notoriously messy and leave sticky honeydew wherever they feed. They're tiny enough to fly in through screens and they sometimes hitchhike on people and pets to get indoors. Aphids can be washed off plants with plain water. Whiteflies are a constant problem with greenhouse grown plants. Their populations soar in the warmth and humidty of controlled atmospheres. Whiteflies are plentiful outdoors, as well, and it's not uncommon for them to attack garden plants as well as indoor plants. You can make a trap for whiteflies by finding a piece of bright yellow paper or cardboard and smearing it with petroleum jelly, and then mounting it at the foliage level of your plants. Whiteflies will be attracted to the yellow and when they land, they'll be hopelessly stuck on the surface of the trap. Try to identify the critters and then take appropriate measures to eradicate them.
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