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Louise Thomas
Editor
A mild winter, wet spring and a very changeable summer have all gone some way to boosting slug and snail activity this year.
The RHS has received three times more inquiries about slugs and snails up to June this year than it did last year in the same period, while calls about box tree moths have also increased – two species which have been affected by the weather.
“When weather is warmer, insects go through their life cycles more quickly, but they slow down as it cools,” says RHS entomologist Stephanie Bird. “Generally, if it is warmer, you’ll get more life cycles fitted in of any given insect, unless it’s a really long life – butterflies tend to have longer lives,”
Horticulturist Monique Gudgeon, garden director at Sculpture by the Lakes sculpture park in Dorset, says that the special mulches they use to protect plants have not deterred the molluscs this year.
“Delphiniums, dahlias and lupins have all become prey, and vegetable seedlings and young bedding plants have also fallen victim.”
The insects and molluscs that are more likely to withstand extremes of temperature better are those that aren’t already struggling, Bird adds.
So, how will this year’s weather – the earlier wet period followed by heatwave – have affected slug and insect populations?
Slugs and snails
“We don’t know whether they are just more active because of the weather, so there might not necessarily be more slugs, but they are more active,” says Bird.
“They like damp, mild conditions, like we had in spring. And in June we received more inquiries about them than the previous June.”
Are populations likely to increase in warm, damp weather?
“If they are active, feeding well and the conditions are good, they are more likely to reproduce,” she says. Slugs and snails breed all year round, but there are peaks related to conditions being good for them, when it’s mild and wet.
Brief heatwaves, as we’ve experienced this summer, are unlikely to affect numbers.
“Snails aestivate, which means they go into a summer sleep and seal up the opening to their shell, which prevents them from desiccation,” Bird explains.
“Slugs will go down within the soil and find more sheltered places. It would have to be very hot for a very long period of time for it to have an effect on them.”
Solutions
Trap them rather than use slug pellets, says Gudgeon. Use upside down hollowed out oranges or melons, or marrow skins where they will hide during the day. Upside-down bin lids on top of bare soil can also work. Remove them to somewhere well away from your garden, such as a hedgerow.
Beer traps are also effective, as are commercially available nematodes, which can be watered around plants that will infect slugs and snails with parasites, which will eventually kill them.
Keep things neat and tidy, and clear away leaf litter so there are less places for them to hide.
Choose your plants carefully, adds Bird. If you have a lot of slugs in your garden, avoid plants they love, such as hostas, dahlias and delphiniums.
Aphids
“There are around 500 species of aphid in the UK and their development is related to the weather and the temperature,” says Bird.
“They have a range of different life strategies. Some will spend their whole time on one plant, others will start off on one plant early in the year and then move to a second plant to continue their life cycle.”
Colonies tend to multiply in warm weather. “They are not prone to drying out like slugs, and as the weather warms you’ll get more aphids, but then that will be offset as you’ll get more predators like the parasitoids, which will eat the aphids,” she adds. Typical predators include parasitoid wasps, ladybird and hover-fly larvae, and ladybirds.
Box tree moth
The increase in inquiries about box tree moths might be down to the mild winter and spring, which would encourage the caterpillars to be active earlier and feed earlier, Bird explains.
However, if the moths have produced an extra generation, they may not be good at overwintering.
“We can’t say for sure how how it’s going to be affected, even in normal conditions,” she says, given that the box tree moth is a fairly new arrival from south-east Asia, first recorded in Kent in 2007.
Eggs and larvae
Bird does not think these will be affected by torrential downpours or mini-heatwaves.
“For instance, with the box tree moth, a lot of the eggs are laid on the underside of leaves in dense foliage, where they will be protected. Rain won’t affect eggs unless they’ve been laid in a poor position. Temperature would have more of an effect than rainfall.
“A late frost, for example, might harm populations because they are more vulnerable. As the climate changes we are seeing different insects being able to expand further north.”
Survival methods
“In general, insects can cope with a few wet days, a few cold days and a few hot days via different strategies. Insects can go into a state where if it gets cold, they use very little energy, essentially becoming paused.
“Very small caterpillars can enter this diapause state over winter, and then require a set number of days over a threshold temperature to become active again,” says Bird.
“If the temperature then drops, they can re-enter diapause.”
What you can do
Gudgeon says: “In most cases, there are biological controls which can be used to deter pests and reduce numbers of repeat offenders like white fly, asparagus beetle, cabbage white caterpillars, and vine weevil.
“I use many different predators especially for vine weevil grubs during the summer in our nursery and the pots around the gardens, but also in the greenhouse over winter, which can be a cosy place for pests to hide.”
