Mole Gopher Slug Soft Rot Leaf Spot Borer Other

The Iris Borers, Macronocture onusta, can cause serious problems for the iris grower.  The attack all kinds of irises.  The "Iris Borers" are actually the larval form of the moth.  The iris borers were first discovered in the 1890's in Chicago on some daylilies. 

The Lifecycle:

moth 01 To the left is the borer moth.

To the right, is the borer pupae.

I do not have a picture of the borer eggs. 

The borer larval form is seen below in several pictures.
borer molt

Borers begin their life cycle as eggs, laid on garden debris in the late summer and fall.  Each spring, the borers larvae begin to hatch after the first two days of temperature greater than 70 degrees.  The 1/4 inch larvae crawl up the iris leaves.  Near the top they chew into the leaves.  Then they eat their way down inside the leaves to the rhizomes, where they gorge themselves until they reach a length of about 1 to 1 1/2 inches. 

borer 04 Note the size of the larvae compared to the penny next to it.

Also note that is is on the fan of an iris, not in the rhizome yet.

Some time in the summer, the borer larvae change into a pupae with a chestnut brown cocoon.  These pupae reside in the soil for about a month and then a moth emerges and lays eggs.  Borers in the pupae, moth and egg stages do not feed.  Only as a larvae do they eat and do damage.  At this stage they are most vulnerable to the efforts to control them.

Signs of Borer Infestation:

In an article in the September, 2000, issue of "Tall Talk", Barbara Nicodemus gives a great description of how to recognize borers in your iris. 

The presence of the iris borer may first be recognized by the wet stains along the leaf edges, notched out leaves, small pinholes, fine silk threads left from their spinning, and 'sawdust' looking remains at the base of the plant from their waste. 

borer 07 To the left, the small and young larvae eatting holes in the fans -- beginning at the top and working towards the rhizome.

To the right, the larvae is much larger and eatting at the base of the plant, soon to be within the rhizome.
borer 06

Later, the newly developing central leaves will have larger, more ragged, "saw-toothed" edges, due to the growing borer feeding inside the lower base of the leaf sheaths.  The outside base of the plant will become slimy and look water-soaked due to the "bleeding" of the leaves.

In the advanced stage, the central leaf may yellow and will be easily pulled out, sometimes even the whole fan.  Bloom stalks will topple over and, upon inspection, the base will show the slimy, riddled work of the borer. 

borer 11 borer 10 borer 09
Early signs of damage. Bloom stalk toppled over Borer seen inside the stalk

The preceding two signs might be all that is noticed on smaller rhizomes, like MDB, etc.  This is especially true on some beardless irises, like Siberians.

borer 08 borer 05 borer rhizome
Larvae ready to eat
into the rhizome
Larvae eating the rhizome And empty Rhizome eatten
out by a borer larvae

Basic Borer Control:

Keeping a clean garden is the first step in minimizing borer problems.  In the summer and fall, the borer moth lay their eggs on the iris foliage.  In the fall, cut back the iris leaves and remove them from your property if possible.  This will reduce the number of borers hatching next spring.

In the spring, watch for signs of borer infestation.  When seen, simply pinch them in the leaves.  If the borer has chewed its way further down the leaf, it maybe easier to remove the portion of the leaf than search for and destroy the borer. 


Imidacloprid (Merit) is a systemic, persistent pesticide manufactured by Bayer Corporation with a low toxicity to mammals.  Merit is the brand marketed for home use while Marathon is in the nursery trade.

To control borers, Merit only has to be applied once in the spring.  Read the directions carefully and follow them.  Do not apply near vegatables or plants that will be eaten.  Wear rubber gloves and long pants during the application. 

Read the rest of this article on your own.  Click the very first hyperlink on this page.  It is a good source.  It sites Barbara Nicodemus and Don and Ginny Spoon.