By Patricia Jordan
WHILE you have been enjoying the summer with trips to the beach or mountains and, I have been researching those dreadful insects that have become the scourge of our gardens in recent years – Scale Insects and Mealy Bugs! Daily investigations on the internet did not bring much hope, but I think I have learned much about these pests as a result, and I will share with you what I have gleaned.
Scale insects and mealy bugs are related to each other. There are thought to be more than 160 species of scale insects and 30 species of mealy bugs, so there will probably be some of each in your garden, as there are over 300 different plants which play host to these parasites!
A major difference between scale and mealy bugs is that mealy bugs remain mobile during their lives, while scale females usually lose their legs, making them immobile. Males of both species mature into small flying midges, but alas they have no mouth parts so they cannot eat. Each female lays between 300 and 600 eggs which can take up to 10 days to hatch, although a few species incubate the eggs within the female and the young are born live. The young move about inserting their mouth parts into plant tissue and excreting honeydew. This honeydew forms the major part of the diet of other insects including ants, some of whom actively transport the scale insects into un-infested areas. These sap-sucking insect pests can weaken the growth of a wide range of plants especially when the honeydew turns into a black sooty mould.
In both species the favourite habitat is underneath leaves and along leaf stems. So before selecting plants in a garden centre, check them out to see if it they playing host to any bugs. Distorted, yellowing or slow growth on plants can often be a sign of scale insects, which look like tiny brown or orange blobs and are often found on the undersides of the leaves near the veins, on the stems or in the leaf joints.
Scale insects are divided into two families – soft scale (Coccidae) and hard scale (Diaspididae). Soft scale insects can breed all year round on shrubs and perennials. Hard scales lay eggs under their scales which hatch into nymphs or ‘crawlers’ after the female dies in early summer. Brown circles are females and the tiny white blobs are males. The nymphs can move only short distances on a plant, but are dispersed over long distances by the wind or animals. Adult female scale insect nymphs become immobile once they find a suitable feeding site. Females are generally headless, legless and wingless, and when mature they produce eggs and then die. The female in some species of soft scale insects can reproduce without the aid of a male.
A winter wash treatment can eradicate scale nymphs, but during the summer months it may be necessary to use a spray of some kind several times over.
Mealy bugs (Pseudococcidae) love hibiscus plants, verbenas, pelargoniums and some daturas and brugmansias. They are about 3 to 7 mm long, and cause distorted leaves, weakened plants covered with shiny honeydew, and sooty mould. Damage can be caused by the direct removal of plant fluids and nutrients and/or by the excretion of toxic salivary compounds into plant tissue. Mealy bugs are covered in a whitish ‘mealy’ wax, which helps retard the loss of water from their soft bodies. Hot humid weather increases their populations as their development is dependent on temperature. After hatching, the juveniles (crawlers) search for suitable feeding sites in sheltered areas. Crawlers can also be dispersed by wind. They have five moulting stages before reaching adulthood. In males, the last juvenile stage pupates into a silk cocoon from which they emerge as a winged adult. The sole purpose of adult males is to mate.
Because mealy bugs are high reproductive and have multiple generations in a year they have the potential to become resistant to pesticides very quickly. Fortunately they can be controlled using ‘soft’ methods including biological agents and low-toxicity pesticides most of which are readily available to the home gardener – parasitic wasps, lady birds and praying mantis, all of which seem to be available through the internet, but alas not here. Use the finger and thumb treatment to squash the bugs first of all, if there is not too much infestation. Using this method ensures that they are killed outright.
We have been using 60cc of Citrole mixed with 10cc Pyrinex in five litres of water but are finding that the bugs seem to have become resistant to this. We are now trialling with some success, Mospilan 20 SP available from garden chemical shops using 12cc in 10 litres of water. Repeat the spraying every three days until you are sure they are dead! Spray under the plant as well and wash any pots thoroughly before replanting.
To prevent infestations of both insects check your plants regularly. First signs could be ants running up and down the stems as they milk the honeydew. Keep the area around the plants clear of weeds and destroy any affected leaves and stems.
WHAT TO DO IN THE GARDEN THIS MONTH
After the heat of July and August, temperatures start to drop a little but it is still hot and there is little chance of any rain even when the first Coptic Winds roll in at the end of the month so leave planting vegetable plugs or bulbs until next month. It’s watering again once the sun has gone off the garden to give plants the most benefit. If you have been away you may find some of your potted plants like pelargoniums are looking jaded. During the summer and indeed in the winter too, the leaf and flower stems are shorter but with a change in temperature they will grow to normal size again. If the weather has been extremely humid there may be botrytis gathering on the leaves and stems in the middle of the plants. Don’t repot yet but cut back any yellowing leaves and old stems from the centre of the plant and give them a good feed, something like Liquid Phostrogen which you can use as you water. They will certainly buck up after that!
One of my friends showed me what she thought was an iron shortage on her grapefruit tree. In fact it was sunburn! Iron shortage shows itself in a fine network of veins on a very pale leaf which can be sorted out by digging in or watering in 2 dessertspoons of iron chelate mixed in with 10 litres of water around the base of the tree, away from the trunk. If your trees have whiteish-yellow blotchy leaves this is caused by a lack of zinc, which usually appears in young growth and can persist as the leaves grow. By spraying straight onto the leaves just under a level dessertspoon of zinc chelate mixed in 5 litres of water, you can hopefully resolve this problem.
Tomatoes can suffer from what is known as ‘blossom end rot’, which is caused mainly by irregular watering. A dose of Epsom salts (Magnesium sulphate), should keep any further damage to a minimum. It also helps seeds to germinate and can deter pests such as slugs and snails.
It’s time to prepare your veggie plots by adding in any compost from your bins or heaps. You might even be able to add in some potted chicken pellets or bagged horse manure which will all help the growth later on.
Plant of the Month Lablab purpureus
Lablab is a most interesting climber, a tender perennial, which originally came from tropical Africa and is a member of the Papilionaceae family. It has many common names such as Egyptian Bean and Chinese Flowering Bean, but is mostly known as the Hyacinth Bean Plant, making it seem like a vegetable, which it is used as in some places. Once established it is drought tolerant and will grow in full sun, but beware as it can grow to over 6 metres. It needs a sturdy support structure on which to grow, as the weight of a mature vine can make it easily topple from any lesser supports. There is no doubt that it would easily cover a wall or fence in just one season.
When the seed germinates either in a pot or in the ground after any danger of frost is past, the emerging plant looks like any kind of bean plant. It has trifoliate leaves and eventually rosy-purple, slightly fragrant flowers will appear in long hanging racemes, which are very attractive to insects. Later these flowers will be followed by bright pink-purple seed pods, which when young can be cooked and eaten. Young leaves can be added to salads and the flowers are also edible. However, dried seeds are toxic, so do not attempt to eat them – just collect them to grow into new plants next season. Lablab is an attractive plant which does not seem to have any disease or insects problems, it just needs space!