This ugly little creature is the larva of the leaf-eating ladybird. Stressed plants in prolonged hot, dry conditions attract these pests. The larvae become almost black as they reach pupa stage. Both adults and larvae of leaf-eating ladybirds are particularly fond of the Solanum family (tomato, potato, eggplant) and the melon or squash family where they do a lot of damage to leaves.
The adult leaf-eating ladybird has 26 or 28 spots in rows across its wing covers. They are slow moving and drop to the ground when disturbed. In summer, if you see the adults on leaves in your garden, be sure to look under the leaves for their eggs. Remove small leaves containing eggs and, on large leaves, use a knife to scrape the eggs into a container. As I dislike spraying my garden, I just squash the adults and larva with a gloved hand.
Unfortunately, the damage done by these ladybirds and their offspring have resulted in many gardeners spraying other species of ladybirds that are voracious pest predators. Both adults and larvae consume a considerable quantity of pests such as aphids, scale and mites, and one type of ladybird feeds on fungus. Peter Chew and his family have an excellent website, Brisbane Insects and Spiders, where gardeners can easily identify which creatures are beneficial to their gardens and which are pests, and includes a Ladybird Field Guide.
The photo below shows both larva and pupa stages of the 28-spotted ladybird.
A reader has asked about powdery mildew on zucchini plants and fungus-eating ladybirds:
Hi. Wonder if you can sort this.
1. Most fungi need moisture and organic material. This seems to be supported by my zucchinis which seem to get worse powdery mildew when I get water on the leaves. I have read that they like dry weather. Is there evidence for either opinion?
2. Some people say that the ladybirds that feed on this mildew spread it by carrying spores, others reckon they are a controller, eating the fungus down. What is the evidence please for either of these? Many thanks, Barb
Powdery mildew is likely to occur on stressed plants in humid weather when temperatures are between 11-28° C. and, once established will continue to affect the plants even if weather becomes dry. Avoid wetting leaves whenever possible Barb. However, because they like low-humidity weather, it doesn’t mean that they are drought tolerant. Zucchini and some other members of the cucurbit family (melons and squash) produce a lot of foliage and need plenty of water and fertiliser. An efficient way to water this group of plants without wetting the leaves is to put a large drink container (with the base and cap removed) neck downwards near the roots so that all the water goes directly to the root area where it is needed, (see photo). Keep topping up the container until it empties slowly.
The yellowish ladybirds with 26 or 28 spots are the only pests of the ladybird family. They eat the leaves of stressed plants of cucurbits. The beneficial fungus-eating ladybird and larvae can be clearly distinguished from the pest in the photos below. From the far left is the ‘Fungus-eating ladybird’, then the leaf-eating ’26 spot Ladybird’ that damages plants. Next is the larva of the ‘Fungus-eating Ladybird’, which also eats fungus and, last of all is the prickly larva of the ’26 spot Ladybird.
Rather than blame the fungus-eating ladybird for spreading the disease, gardeners should check that their plants have sufficient water and nutrients to avoid stress, and the soil pH is suitable for them to absorb what they need for healthy, disease-resistant growth. Also see Powdery Mildew for treatment of this disease.
In hot, dry weather the 26 or 28-spotted ladybirds can do a lot of damage to the vegetable patch. Plants can only absorb nutrients from the soil as water-soluble ions. As soil dries out, vegetable plants are unable to absorb the nutrients they need to produce the compounds that deter pests, and these troublesome ladybirds can move in, feeding on leaves until only a network of veins remain. The larvae (pictured below) tend to feed on the underside of leaves while the adults feed on the upper sides of leaves. Badly damaged eaves can become papery, and brown. Their favourite foods are the cucurbit or squash family, which includes the melons. They can also attack, bean, potato and tomato plants.
The problem with using sprays to get rid of these pests is that whatever will kill them will also kill the beneficial ladybirds that help keep many garden pests under control, including the bright black and yellow ladybird that eats powdery mildew and other fungi. Fungus-eating ladybirds are often seen wandering over leaves of the squash family.
The Brisbane Insects website now has a field guide to ladybirds to make it very easy for you to identify whether the tiny creatures crawling on your plants are pests or ladybird larvae, which come in a range of colours and shapes.
Click here: Ladybird Field Guide
The best way to solve the problem of 26 (or 28)-spotted ladybirds is to knock the adults and their larvae into a soup tin with some methylated spirits in the bottom of it. Also scrape off the eggs, which can be found in a cluster on the underside of leaves (see photo below).
Then give the foliage and soil around the plants a generous drink of seaweed extract tea, to supply potassium and trace elements plants need to build resistance to pests, because only stressed plants are attractive to pests. Ensure the plants have sufficient fertiliser for healthy growth, and that the soil is not too acidic or too alkaline. Water the plants thoroughly, under mulch, when the top cm. of soil is dry, rather than giving them a lighter, daily watering. If drought conditions are making it difficult to spare enough water, you can help deter them by spraying both sides of the leaves with chilli spray, but you will need a lot of chillis. The solution is – 2 cups chillis to 2 cups of water. Chop chillis finely while wearing gloves. Steep them in water for an hour, strain mixture, and spray liquid over leaves. (Garlic spray can be used as a deterrent but breaks down quickly in hot weather.) Finally, get rid of any blackberry nightshade plants. These act as a host for this little pest. See:Blackberry Nightshade
Eggs and larvae of the 26 (or 28)-spotted ladybird.