Sweet pea leaves have a thick waxy coating which gives considerable protection against pathogens. Consequently, sweet peas are less susceptible to pests and diseases than many garden plants. They are most vulnerable as seedlings, before the waxy surface has had time to develop, or when the plants are old or under stress and the natural immune system is breaking down. The most serious problem is greenfly, or aphids, which can transmit serious virus diseases to sweet pea plants.
The waxy leaf coating means that chemical sprays are not readily absorbed so that contact sprays or 'green' methods of control are necessary.
Sweet Pea Pests
Commonly known as greenfly, these are becoming increasingly resistant to chemical sprays, and are important as major vectors of virus. Encourage ladybirds, lacewings, Aphidius and Aphidoletes to build up, and resort to chemical control only if unavoidable.
Tits, finches and sparrows can all cause problems by damaging the flowers of sweet peas, as seen in the photograph. Such behaviour is often learned, and efforts to discourage it should commence immediately it is first seen. Blackbirds will also uproot young plants while searching for food, while hungry pigeons will occasionally decimate a row of young sweet peas.
Minor pests in themselves, they can transmit virus diseases from other plants in the vicinity, especially clover. Try to eradicate any nearby patches of clover, or spray them with a suitable insecticide.
Not a serious problem on sweet peas, and most often seen on late planted crops. The offending creature may be found at the wider end of its tunnel. Common species are easily controlled with pyrethroid insecticides, tropical visitors are resistant to most chemicals, but all are susceptible to "positive biological control" - a quick squeeze between finger and thumb.
Mice can devastate a newly sown crop of sweet peas by digging up and eating the germinating seed. If hungry enough, they will do the same to young overwintered plants, severing the top growth from its roots. Baits and traps are effective means of control, and in vulnerable situations should be in place a week or so in advance of sowing.
A small black beetle which, as its name implies, feeds on pollen. They crawl inside the keel of sweet pea flowers, making them difficut to control, and the flowers useless for cutting. They also transfer pollen from one flower to another, and are a major cause of rogues in UK produced seed. They are most prevalent in areas where oilseed rape is grown, and are attracted to the colour yellow, so yellow sticky traps are useful for protecting your crop. They are also attracted to light, so placing affected flowers in a dark shed with a bright light source may lure them out.
If pollen beetles are a regular problem, consider siting a 'sacrifice plant' near to where you grow your sweet peas. A Hypericum with large, bright yellow flowers might be a good choice. If the sacrifice plant becomes infested with pollen beetles and it is necessary to spray with insecticide to kill them, remember to do this late in the evening when the bees are no longer active.
Also known as Fungus Gnats, these can be a real problem on seedlings grown in soilless composts. The larvae live in the compost and apart from the physical damage caused by them feeding on the sweet pea roots, they transmit plant diseases like phthium and phytophthora. Best controlled by parasitic nematodes similar to those used to control vine weevil.
These familiar garden pests have little liking for the tough waxy leaves of mature sweet peas. They do, however, relish the tender growing point of young plants, and can do great harm to plants before they have started to climb. Methiocarb based baits are more effective that the usual metaldehyde based ones, and pathogenic nematodes (Phasmarhabditis sp.) offer a clean "organic" alternative method of control.
Minute insects, commonly known as thunderflies, which are not generally a problem on sweet peas. One species, the western flower thrips, is a pest of many glasshouse flower crops, and is becoming more of a problem outdoors in the UK as the climate gets warmer. Easier to see with a hand lens than the naked eye.