We all want to have healthy butterflies visiting our garden and know that to produce healthy butterflies we need healthy host plants. Take for example the monarch butterfly: it’s important that you grow healthy milkweed, and you don’t want to lose the milkweed to diseases, parasites or other predators (in fact, the monarch butterfly caterpillar is considered a ‘predator’).
How do we grow healthier swan plants, the most common milkweed in New Zealand? A recent article in the Winter magazine of the MBNZT was helpful, but we need to know more about the telltale signs that our plants are not thriving. At this time of the year (midwinter) little can be done to help the swan plants that are still alive - they will either survive or not survive.
Tropical milkweeds look sad, with rusty leaves and sometimes deformed leaves.
Swan plants, however, that have survived the summer may be carrying a virus and it is best to dispose of them, carefully, as you don’t want to spread the virus further.
Viruses are among the smallest and simplest entities that can cause disease. They can only be seen when magnified thousands of times using an electron microscope, so very hard to diagnose. There are over 2,000 known viruses, ¼ of them attack and cause diseases in plants.
The symptoms can be caused by environmental factors, insect damage or improper nutrition – so they are hard to diagnose. Most are dependent on a vector such as aphids, thrips, leafhoppers or whitefly. It is most important to prevent the infection and eradicate the source.
Never buy sickly plants. Growing from seeds is a reliable way of starting healthy plants.
Always disinfect tools used for propagation or pruning. Dip them in a 10% solution, making sure it touches all parts. This will kill any viruses within seconds. Rinse and dry your tools well.
Also remove weeds that may harbour viruses of their insect vectors from around your desired plants. And the biggest challenge of all: maintain insect control.
Aphids are probably the most problematic of the pests you’re likely to find on your swan plants and other milkweed. Aphids are small sap-sucking insects and typically flightless females can give live birth to female nymphs that may already be pregnant! These aphids will breed profusely so that the numbers can increase very quickly. And later in the season they may produce wings which will allow them to colonise new plants.
Not only that, but ants are soon attracted to the aphids. The aphids produce a byproduct called ‘honeydew’ and ants are known to milk the aphids. Honeydew is rich in sugar an as the weather gets warmer ants become more active, seeking more honeydew. (Note: ants will also eat insect eggs!)
The honeydew will also fall onto lower leaves of your plant, attracting other insects… and mould, sooty mould. Another vector to bring diseases to your plant!
So in summary, as spring approaches, remove any diseased plants and start afresh when the weather gets warmer. Put plants that you suspect have viruses straight into a plastic bag and into the rubbish – do not compost them or the sick leaves.
When it’s warmer, plant your new seeds, or look out for healthy plants in the shops. And each day, check your plants for the likelihood of vectors such as aphids. If you want to raise caterpillars your strategy will need to be pesticide-free.