Pieter has had a severe aphid infestation on his zucchini plants, and applying a dishsoap/water spray not only killed the aphids, it also killed the plants. He wants some tips on a better way to deal with aphids.
Aphids are fairly easy pests to get rid of without resorting to chemical pesticides. Aphids have a lot of natural predators, but the sprays will also kill these, resulting in more intense pest infestations in future.
Moderate infestations of aphids can be blasted off plants with a jet of water from a hose or a spray bottle set to ‘stream’. For plants with hairy leaves (such as zucchinis) apply water early in the day so that leaves can dry before night. If you blast them with water before serious damage occurs, predators will be able to control any escapees.
Severe infestations can be removed with a soap spray, but not with detergent or bathroom type soap, which has a caustic soda base. The soap used in soap sprays to kill pests has a potassium base, and is available from nurseries, or can be purchased by mail order from Green Harvest. ‘Natrasoap’ does not leave a residue on plants, nor will it burn leaves if used in temperatures below 30° C.
In order to prevent future infestations of aphids, avoid overdoing nitrogen fertilisers because aphids are pests that are attracted to a flush of soft, sappy growth that excess nitrogen, or erratic watering, produce. Controlling weeds in the growing area can also help prevent aphid attack. Many weeds are host plants for aphids, allowing a large colony of pests to establish before you notice them. As aphids can carry viruses for which there is no cure, prompt removal of these tiny pests is advised.
A problem I am frequently asked about is why do immature fruit of the Cucurbit family become soft or discoloured, and fail to mature. The squash or Cucurbit family includes chokoes, cucumbers, grammas, gourds, pumpkins, rockmelons, squash, watermelons, and zucchinis.
If your cucurbit plant is producing small fruit that yellow and fall off before maturity, or turn mushy at the end furthest from the stem, it does not have a disease, or a pollination problem. Your plant is deficient in calcium. Calcium deficiency also causes blossom end rot in tomatoes and capsicums.
Like us, plants need a good balance of calcium and magnesium to form a strong structure. Calcium and magnesium are required for growing tips of plants as well as fruit production and, if there are not enough of these nutrients to go around, growing tips will get priority. Calcium deficiency can occur in several different ways.
Most commonly, it occurs when soil is too acid (soil pH less than 6) and there are insufficient calcium ions in the soil. In soils with a suitable pH of 6 – 7.5, erratic watering can cause it, as plants are unable to absorb nutrients from dry soil, when needed.
To avoid blossom end rot, ensure that your cucurbit (or tomato/capsicum) bed has a suitable soil pH before planting out seedlings. See Changing soil pH. If your soil is quite acidic, and the problem has already occurred, you can raise soil pH slightly by dissolving a generous handful of dolomite (a mixture of calcium and magnesium) in a full watering can, and apply this around the root area (under mulch) of each plant – one full watering can per plant, or two around large vines such as pumpkin and watermelon. If you know that your soil has plenty of magnesium, use agricultural lime instead. This treatment will take several weeks to work, so good bed preparation is worth the effort.
Where erratic watering is the problem, mulch around your plants to reduce fluctuations in soil moisture, and water plants thoroughly once or twice a week, rather than giving them a light watering every day. Pumpkin vines require a lot of water to produce a good crop.
Karen is new to organic gardening and is wants to know what to do about the green vegetable bugs that are attacking her tomatoes.
The green vegetable bug is a sap-sucking pest. They use weeds as hosts and attack vegetable plants that are stressed in some way. Of the chemical sprays required to kill these bugs, one will poison any birds that eat the bugs, and the other is deadly to bees and other beneficial insects. It can also metabolise in humans to a more toxic chemical, and is currently under review.
Chooks show no interest in eating these bugs. The best way to get rid of these organically is to put some methylated spirits into a soup tin or similar container. Put on gloves to protect your hands from any smelly juice they excrete and use a stick to knock them into the tin. Then get rid of weeds and plants that have finished bearing to prevent these pests continuing to breed.
To prevent further attacks (and this is the most important part of the treatment) have a look at why your tomato plants are stressed, and correct the problem. Some gardeners find it difficult to understand that pest and disease attack are only symptoms of unsuitable conditions for a particular species, but it is true. I noticed yesterday, that similar bugs are attacking some silver beet plants we have. We decided after picking the first of this silver beet that we did not like this variety as much as ‘Fordhook’ and decided to just feed the plants to the chooks, a plant or two at a time. Consequently, the silver beet did not receive the same attention as the other vegetables and, before long, these bugs moved in. The bugs are not attacking other vegetables that are receiving normal care.
When pests attack plants, the answer is always in the soil. It may be too dry or poorly drained. If water is in short supply in your area, hill up your tomatoes about 5 cm at a time. They will produce more roots along the stem and allow them to access water more efficiently. Tomato plants need a deep watering (under mulch) several times a week in dry weather, rather than a light daily watering.
Or, the plants may be short of the nutrients they require to produce pest-deterring pheromones. If you have skimped on fertiliser, after a thorough watering, give each plant a light feed of complete poultry-based fertiliser and a drink of seaweed extract tea.
Or, the soil may be too acid or alkaline for the plants and the nutrients they require can be locked out and unavailable to the plants. This can be difficult to determine without testing, except that, with tomatoes symptoms of phosphorus deficiency (purple colouring under leaves and slow growth) are a clue that soil is too acid or alkaline if you have applied adequate complete fertiliser. If soil is too alkaline, put some well-rotted horse or cow manure under the mulch around the plants. If soil is too acid, water in some dolomite or lime around the base of each plant. See Changing soil pH.
These sap-sucking pests are can be found on the twigs and branches of a range of plants, including fruit trees. The pests themselves may not attract much attention unless the plant is heavily infested because they are reddish-brown in colour with black hairs, and only 5 mm in length. The part we notice as a pest is actually the scale’s egg sac – a grooved, white, wax structure, larger than the insect itself. The egg sac will contain up to hundreds of red eggs. (See photos below).
Before treating these pests – check that you haven’t confused them with the larvae of the mealy bug ladybird, which are also white and fluffy. Ladybirds and their larvae consume an enormous amount of scale insects and aphids, but many of the larvae are killed when they are confused with pests. Many pest predators are killed where copper sprays are routinely used, and scale infestations commonly follow spraying for other pests.
When the cottonycushion scale prepares to lay her eggs, she fixes herself to the twig. Give the white structure a poke, if it doesn’t move, it’s scale – ladybird larvae will scuttle off. When temperatures are below 24° C, and pest predators are absent, cottonycushion scale can be suffocated by spraying with white oil at 10 ml per litre of water. This will not kill the eggs, and the spray will have to be repeated in two weeks, to catch the newly hatched scale.
In warmer weather, suffocate spray with enough fine potter’s clay dissolved in water to make it cloudy. Pest infestation is a sign that plants need an improved fertiliser and/or watering program, or that you need to adjust your soil pH.
In autumn, caterpillars of the Oleander Butterfly visit my potted Weeping Fig. The Oleander Butterfly has a wing span of 7.5 cm and is very dark brown or black, with white blotches on both wings and body. The caterpillars arrive in an amusing little caravan formation, travelling head to tail, then spent about a week munching on the fig foliage before each forms a chrysalis. I don’t know where they breed because we don’t have any Oleanders, but they seem to enjoy leaves from plants that have milky sap. I leave these creatures in peace because they do little damage to the tree, and the chrysalises are so pretty.
Slaters and earwigs feed on decomposing organic matter. If they are becoming a pest around your vegetables, you have probably added immature compost or uncomposted manures to your topsoil, or have a lot of semi-decayed organic matter in your soil. We get slaters in our compost heap but we don’t worry about them there as they are contributing to the composting process and dining in the heap keeps them away from the vege patch. Slaters and earwigs are attracted to stressed plants. Give your vege garden and any affected perennials a drink of seaweed extract tea (Acadia, Eco Cweed, or Natrakelp) at weak black tea strength. This will make plants more resistant to pest attack.
Slaters, which are not insects but related to prawns and lobsters, congregate in rotting timber, heaps of rotting vegetation, rock heaps, and shady, dark places. Remove breeding sites from the garden, and turn your compost heap regularly.
Someone told me that they are having great success drowning slaters in beer baits that I recommended in my book for snails and slugs. The slaters are attracted to the yeast smell of beer, as fermenting organic matter has a yeast smell (See post).
Jackie French recommends mixing one part pyrethrum powder to two parts plain flour, and placing baits in, or near, the dark places that slaters shelter in during the day. Try putting the baits in jar lids inside pots laid on their sides with loosely crumpled paper in the top to darken the interior.
Earwigs are slender insects with a set of pincers on the tail end of the body. They can be reddish-brown or black. These little pests will also feed on flower buds and fruit. They like to hide in confined areas in rocks, bark, timber and under debris. Earwigs can be trapped by putting crumpled newspaper into flower pots and leaving the pots on their side in garden beds. In the morning, drop the crumpled newspaper into a half bucket of soapy water to drown the insects, then refill the traps with fresh paper. Remove debris from around your garden to restrict their hiding places. If an earwig refuses to budge when disturbed, it is most likely protecting eggs laid in the soil beneath it. Dig down and dispose of the pale oval eggs.