What is red lily beetle?
Red lily beetle and its larvae are leaf-eating pests of lilies and fritillaries. The adult beetles occasionally eat other plants but lilies and fritillaries are the only plants on which they will lay eggs and the grubs can develop. Apart from spoiling the plants' appearance, damaging attacks in early summer will result in undersized bulbs developing, which may not flower next year.
Non chemical control
Where only a few lilies and fritillaries are being grown, the plants should be regularly inspected from late March onwards so that adult beetles, larvae and eggs can be removed by hand.
For more extensive infestations, it may be necessary to use an insecticide, such as thiacloprid (Bayer Provado Ultimate Bug Killer) or acetamiprid (Scotts Bug Clear Ultra).
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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Red lily beetle overwinters as adult beetles that go down into the soil and leaf litter in the autumn. This could be anywhere, not necessarily in the vicinity of lilies and fritillaries (consequently, there is no advantage in attempting to treat the soil below lily plants). The beetles begin emerging on sunny days in late March and April when they seek out the foliage of host plants. Eggs are laid in small batches on the underside of leaves during April to mid-summer. The eggs hatch and the larvae feed on the foliage. When fully fed, the larvae go into the soil to pupate. The next generation of adult beetles emerges from mid-summer onwards. These beetles add to the feeding damage but there is only one generation a year and these late summer adults will not mate and lay eggs until the following year.