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Tuesday, October 21, 2014

What is Causing the Webbing in My Desert Willow

Q. My desert willows in Kingman, Arizona, are being eaten up by worms. The web-like pods are all over the branches, but I have not found anything inside. I certainly do not want to spray as the trees are over one of my gardens and close to some windows.

A. I did some quick checking but could not find this critter mentioned anywhere. How do you k now they were being eaten up? It is possible they have done their damage and now have moved on leaving their webbing behind.
Sphinx moth found dead on sidewalk in May in Las Vegas.
Desert willow is used in butterfly gardens but it is usually for the adult butterfly rather than the immature larva (worms) or caterpillar. If they are causing a great deal of damage then they should be controlled.
Tent caterpillar webbing
Worms or caterpillars that do cause damage to desert willow are the Sphinx moth larva or “worm” and relatives of tent caterpillars. The larva of the Sphinx moth is gigantic with the horn coming off of its rear end. You would've recognized that one if you had seen it.
Sphinx moth to give you perspective on size of the moth.
Bt formulation from Monterey
The other caterpillar is much smaller, along the sizes you're talking about, but they usually form a webbing one or 2 feet across and they feed inside this webbing or tent. That's why they call these “tent caterpillars”. Tent caterpillars reproduce quickly and do a lot of feeding over a short period of time.
My guess is that these critters will not last very long and be on their way for the season. The desert willow will respond and survive. But in the meantime that tree will have some damage.

You could use a spray of BT, called usually either Dipel or Thuricide which is an organic control. It will only target “worms” that become moths or butterflies. Also Spinosad will work as well. This way you could avoid more poisonous conventional sprays.


  1. A few years ago I sent you some pics of worms that were chomping on my desert willow trees. I have finally been able to identify them as fall cankerworms. They did not come back in 2015, but reappeared last fall in 2016. I guess the severity of each winter determines whether or not I will see them each year. Up in the high desert areas we do see a lot of tiny moths...almost like snow sometimes. My trees do recover so I am not as upset as I used to get. Thought you would like to know. Sue in Kingman Arizona

  2. Thanks for this update, Sue. I seldom get them from readers so it is appreciated and informative. If it is fall cankerworm then I would guess that Bt and Spinosad would work on them. I tracked it down and I see it is called both fall and spring cankerworm and reported mostly in the east. I did find University references to it from Colorado and Texas. I liked the Colorado one best.