Here's a question: how many natural plant scents do you find repellent? The one that comes to the top of my mind is the scent of the newly blossomed corpse flower, Amorphophallus titanum. It smells very much like the name implies. If I could produce a scratch-and-sniff blog post, I would be tempted. The odor is horrible to humans, but sweet to some flying insects which are also attracted to rotting flesh. But perhaps you cannot tolerate the scent of clove, or eucalyptus? Insect pests are like us in this respect.
Pests in the garden and greenhouse
Insects, arachnids, fungi, and viruses. Their numbers are legion. The battle is constant. The most effective weapon is vigilance. In addition to vigilance, there are biological controls made from natural compounds which are generally non-toxic to humans. Included in this category are essential oils. For example cinnamon, rosemary, sage, neem, clove, peppermint and many others. These oils are repellent, and even toxic, to many of the pests in the garden.
It's thrips (singular) and thrips (pleural)
My task in the garden today was to monitor and treat plants for thrips insects. There are more than 6000 different species of thrips, most of them tiny in size measuring about 1 mm. They evolved to feed on fungi, but some feed on plants. The largest thrips feed on other insects. There are thrips species that are beneficial in the sense that some tropical plants (cycads) require specific species of thrips to pollinate their seed cones.
Of those thrips that feed on plants, they are particularly attracted to the blossoms and soft parts of the new growth. Thrips use their mouthparts to slice into the plant and suck out the cell liquid, leaving a scar on the surface. Damage from thrips typically looks like silvery gray spots and curled leaves and petals. They also cause damage by spreading plant viruses between host plants.
Insect repellent of peppermint and clove oil; 6 drops of essential oil per liter of water.
When I found thrips today, I was instructed to give them a good soaking spray with a dilute solution of peppermint and clove oil. There are scientific articles which show this combination is effective to repel and kill some insect pests.
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| Common thrips (photo from the provincial govt. of Ontario, CA). Adult and two larval (nymph) stages. |
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| Cancer bush (Lessertia frutescens), S. Africa. This plant was noted to have thrips feeding on the orange blossoms. They're attracted to red and yellow colors. |
I found a few thrips on this cancer bush, from South Africa. The plant has attractive orange blossoms in the spring. They morph into attractive papery seed-containing sacks. The plant has been used in traditional tribal medicine to treat some forms of cancer, hence the nick-name cancer bush. But, I couldn't find any scientific articles to back up the practice. That doesn't mean it is not useful, just not validated in Western literature.
We also found thrips on a gloxinia, Sinningia macrostachya. Sinningia is a genus of plants from S. America that features a tuberous stem and a preference for growth on stony surfaces. They generally have very attractive blossoms. This particular plant is infested with hundreds of little thrips, although the damage is relatively minor at this point.
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| Gloxinia leaf infested with tiny white larvae |
Bench in 3C, the subtropical room
My work for the day was spent mostly on the collection side of the fence, but also on the display side of the subtropical room. I found most of the infested plants on the collection side, where there are more plants in flower. I used a spray bottle to treat all of the plants on this bench, whether I found thrips or not.
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| Subtropical collection room, lime tree in the foreground (with one lime fruit way at the top) |
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| Solitary remaining lime on this small tree |
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