Lilacs and irises are longstanding low-maintenance performers in the Kansas landscape that bloom in spring. Sometimes they fail to produce flowers, though, and the gardeners who tend them wonder why. Insect borer infestations are the most likely culprit for bloom failure in established plantings of lilacs and irises, but the insects are difficult to detect without prior knowledge of their existence. To keep lilacs and irises blooming and healthy, gardeners must be diligent in watching for signs of borer presence and removing infested material.
Lilacs and irises are very different plants besides being attractive to insect borers. Common lilacs are shrubs that grow to 5 to 6 feet tall and send up new sprouts from the roots. They bloom in early spring with fragrant purple flowers. Irises are herbaceous perennials with 6- to 18-inch tall, spear-shaped, light green leaves that are emerge from fleshy roots called rhizomes. Large showy flowers in a variety of colors appear atop long stems above foliage in late spring into early summer. Bloom time depends on species and variety.
Lilac borers
Lilac borer is also known as lilac-ash borer because it also feeds within ash trees. Privet is an additional occasional host. Lilac borer is native to North America and widespread. Larval activity stresses plants but is rarely fatal. Look for signs of prior years’ feeding in the form of small exit holes in the lower third of the plant and especially on older, larger stems. Larvae feed within stems to hollow them out and sometimes girdle them.
Larvae of lilac borer are a type of white grub. They overwinter in this form, resume feeding in very early spring, pupate and emerge as moths that looks like wasps. In northeast Kansas, adults appear from April to June. They mate and lay eggs on nearby lilac stems to continue the cycle.
The best control for lilac borer is removal of infested stems at ground level. Removal of older, larger stems is recommended for period maintenance of lilacs anyway, as it encourage plants to produce new growth as well as removing borer infestations. In younger, healthier stems, gardeners sometimes opt to destroy larvae by inserting a wire or cutting into the stem if possible without causing further damage to plants.
Pruning of lilacs to remove lilac borer is most effective after adult moths have completed egg-laying activity in June. Plants with fresh wounds in spring may be more attractive to pests.
Iris borers
Iris borers overwinter as eggs on the previous season’s leaves and hatch in April and May in northeast Kansas. The tiny caterpillars that emerge bore into leaves and work their way down into rhizomes, feeding on interior plant tissues along the way. Feeding activity reduces plants’ stored energy and disrupts uptake of water and nutrients. Larvae feed until July or August, move out into the soil to pupate, and emerge again as a dusty gray or brown moth. Adults then lay eggs on iris leaves to continue the cycle.
The standard recommendation for control of iris borer is to remove and destroy old iris foliage from plantings on or before April 1 of each year. This action reduces new borer infestations but does not help iris rhizomes damaged by previous years’ feeding. Heavily infested plantings may benefit from having damaged rhizomes removed with the rest of the plant debris. Plants should then be monitored through the spring for signs of larvae feeding within leaves. Destroy any larvae that are detected.
If spring cleanup is missed, iris plantings may also benefit from cleanup after bloom is complete in summer. At that time, borer larvae, if present, are easy to find in infested rhizomes. In some cases rhizomes can be salvaged and replanted after borer larvae are removed and destroyed.
Published in Lawrence Journal-World 4-1-2023.
Lilac borers
Lilac borer is also known as lilac-ash borer because it also feeds within ash trees. Privet is an additional occasional host. Lilac borer is native to North America and widespread. Larval activity stresses plants but is rarely fatal. Look for signs of prior years’ feeding in the form of small exit holes in the lower third of the plant and especially on older, larger stems. Larvae feed within stems to hollow them out and sometimes girdle them.
Larvae of lilac borer are a type of white grub. They overwinter in this form, resume feeding in very early spring, pupate and emerge as moths that looks like wasps. In northeast Kansas, adults appear from April to June. They mate and lay eggs on nearby lilac stems to continue the cycle.
The best control for lilac borer is removal of infested stems at ground level. Removal of older, larger stems is recommended for period maintenance of lilacs anyway, as it encourage plants to produce new growth as well as removing borer infestations. In younger, healthier stems, gardeners sometimes opt to destroy larvae by inserting a wire or cutting into the stem if possible without causing further damage to plants.
Pruning of lilacs to remove lilac borer is most effective after adult moths have completed egg-laying activity in June. Plants with fresh wounds in spring may be more attractive to pests.
Iris borers
Iris borers overwinter as eggs on the previous season’s leaves and hatch in April and May in northeast Kansas. The tiny caterpillars that emerge bore into leaves and work their way down into rhizomes, feeding on interior plant tissues along the way. Feeding activity reduces plants’ stored energy and disrupts uptake of water and nutrients. Larvae feed until July or August, move out into the soil to pupate, and emerge again as a dusty gray or brown moth. Adults then lay eggs on iris leaves to continue the cycle.
The standard recommendation for control of iris borer is to remove and destroy old iris foliage from plantings on or before April 1 of each year. This action reduces new borer infestations but does not help iris rhizomes damaged by previous years’ feeding. Heavily infested plantings may benefit from having damaged rhizomes removed with the rest of the plant debris. Plants should then be monitored through the spring for signs of larvae feeding within leaves. Destroy any larvae that are detected.
If spring cleanup is missed, iris plantings may also benefit from cleanup after bloom is complete in summer. At that time, borer larvae, if present, are easy to find in infested rhizomes. In some cases rhizomes can be salvaged and replanted after borer larvae are removed and destroyed.
Published in Lawrence Journal-World 4-1-2023.