How can I get rid of caterpillars?
Q What can I do about the caterpillars eating my lovely willow leaves and leaving them warty? Sandra Wallbu on, by email
A Willows are the food source for the caterpillars of some 90 species of moths but the caterpillar-like grubs often encountered, as with your tree, are the larvae of the lesser willow sawfly ( Nematus pavidus).
These are voracious feeders, like their relatives the berberis, gooseberry and Solomon’s seal sawflies, and can strip a small tree of foliage very quickly. The bright green and yellow larvae have black heads with distinctive black stripes along their sides. They’ll rear up dramatically in an effort to ward off predators. The insignificant dark copper-coloured adult sawflies are wasp-like and barely 6mm (¼in) in length. Sawflies derive their common name from a saw-like ovipositor, which is used to pierce leaves where the eggs are laid. The larvae are distinguished from caterpillars by having at least six pairs of fleshy prolegs (abdominal limbs) compared with the four or five of caterpillars. When mature, the lesser willow sawfly larvae drop to the ground and pupate in the soil. There may be two or three generations a year, with the last overwintering in the soil.
Hand-picking off the larvae can give good control. Heavy infestations can be dealt with with a range of insecticides approved for caterpillars. Digging around willows in winter exposes the pupae to the cold or foraging birds.
Interestingly, the wart-like galls on the leaves of your tree are caused by other species of sawfly, where the grub lives inside the leaf. The most common is the willow redgall sawfly ( Pontania proxima).
The adult female lays a single egg in each leaf in late spring, which hatches into a tiny, pale green grub with a dark head which eats the soft leaf tissue. This stimulates the leaf to produce a bean-shaped gall, which can be green, red or yellow. The grub eats out cavities within the gall and then pupates in the soil with a second generation in late summer.
Although unsightly, the galls cause no harm to the tree. Control is simply a case of picking off and destroying infected leaves.
Q Can I prune my Japanese maple hard back? Gordon McGhie, Blyth, Northumberland
A These trees will usually tolerate cutting back hard, but you should avoid such brutal treatment as it will spoil the tree’s natural elegance. WhenW the trees are dormant and out of leaf over the winter, you can reduce the height or width by thinning out some of the longer branches. Use a pruning saw to take branches back to a well-placed, shorter side branch, at least one-third the diameter of the branch you’re cutting out. Very low branches can be taken back to the main trunk. Look to reduce the canopy by no more than a third overall. Q Can you identify the white-bo omed bumblebees nesting in my bird box? Linda Wilkerson, by email A There are a surprising number of bumblebees with white bottoms, but Q Why won’t my containergrown fig fruit properly? Mandy Rye, Lincolnshire
A Figs need a container at least 45cm (18in) in diameter and a potting compost of John Innes No 3 with 25 per cent added grit. Ideally, repot every three years in winter.
When repotting, remove about 20 per cent of the potting compost and cut away a similar proportion of root. Figs are very drought-tolerant planted in the ground, but in a container you need to water well during the most nest in holes in banks. An exception is the tree bumblebee ( Bombus hypnorum), which prefers to nest above ground and is very partial to bird boxes. It differs from the white-tailed bumblebee ( B. lucorum) in having a distinctive, tawnycoloured thorax. The tree bumblebee is widespread from Asia and across Europe but is a summer, otherwise the tree will naturally jettison fruit if it dries out.
Incorporate a controlledrelease fertiliser each spring and, during the growing season, feed every couple of weeks with a high potassium fertiliser, such as tomato food.
Once a shoot makes five leaves, pinch out the growing tip to encourage bushy growth and the formation of embryo figs for the following year. Trees are frost tolerant but the fruit isn’t, so protect them over winter with three or four layers of fleece. new species to the UK, first found in Wiltshire in 2001, but now common throughout England and Wales. It’s active from March but colonies decline by late-July, with just the new queens hibernating over winter.
Generally, as with most bumblebees, they’re pretty docile if undisturbed and very rarely sting humans.
Why don’t my growing bag sweet peas flower? Dennis Haynes, Oxfordshire
I wouldn’t grow sweet peas in growing bags. Firstly, the volume of compost is very small and sweet peas hate drying out. The other reason is that in front of a south-facing wall, I suspect the roots are getting poached in the summer sun.
Sweet peas do best in deep, well-cultivated soil, where their roots are cool and moist. If growing in containers, choose ones about 45cm (18in) deep and painted white to reflect the sun. Alternatively, if the containers are rectangular, line them with a
Why are my plum leaves curling? Donald Parkes, West Midlands
The damage is due to the plum leaf-curling aphid ( Brachycaudus helichrysi), a sapsucking insect that feeds on the foliage of many prunus species.
It’s active from April through to late May and will have left trees by now. You can simply prune back badly distorted shoots. To avoid it next year, use a winter wash or approved insecticide for fruit as the leaf buds are opening.
It’s worth examining the underside of the other leaves carefully, however, as you may also see whitishgreen insects which are mealy polystyrene sheet, which will keep the roots cool.
Even with tomatoes that tolerate warm roots, I prefer to sink some bottomless 10 litre pots into the growing bag and plant into these so there’s a larger volume of compost to hold moisture and which is also less likely to overheat. plum aphid ( Hyalopterus pruni).
These are active right into late summer, when dense colonies may develop, so a timely spray may help. Never spray any fruit tree when in flower due to the danger to beneficial pollinating insects.