Horticultural glue is a mixture of rosin, linseed oil or castor oil and other additives as beeswax which allows trapping pests in orchards or in gardens. It is an alternative to pesticides but the preparation is can be tricky and/or messy. This glue is available commercially.
The glue has the property to attract insects, especially if it is placed on a colored surface such as yellow. For vegetable patches, fix yellow plastic rectangles (or any other clear and brilliant color) on posts and drive them regularly into the ground. For trees, coat the glue with a brush or a spatula around the trunks 1m from the ground and at height of at least 10cm. Alternatively, coat a fabric strip and wrap around the trunk. Wear gloves!
Warning: the preparation of a mixture of rosin (derived from pine resin) and linseed oil requires special precautions. If the cooking is too long, the mixture will be brittle after cooling. If the cooking is too strong, linseed oil will release flammable vapors. It is essential to cook the ingredients outside and away from the wind, while keeping within reach a wet cloth and even a powder fire extinguisher. Wear safety glasses. Experiment first with small amounts.
There are other horticultural glue recipes based on cooked sugar syrup, flour or boiled linseed oil, and mistletoe or holly extracts, but it seems they are less effective or (especially if boiling linseed oil) dangerous to make.
Commercial glues may contain 5% of beeswax but the benefits are unclear.
Linseed oil .............................. 100g (paint stores, druggists)
Rosin ...................................... 100-120g (druggists, articles artists)
Terracotta container, or thick glass (Pyrex) or cast iron
Electric hotplate or gas (with a heat spreader)
Heat protection gloves or potholders and safety glasses
Melt rosin (85-95 ° C) over low heat. Add gradually linseed oil with continuous stirring until the mixture becomes homogeneous and acquires the consistency of thick syrup (after cooling). If the cooking is too fast, the mixture will be too thin and will tend to run. If it is too long, the mixture will harden when cold and will be unusable. If it is too strong, linseed oil might boil and release highly flammable vapors.