My Leyland Cypress is browning and has Cypress tip moth signs. I’m worried about the brown spots, and wonder if it can survive this attack? How can I control the pests, if it might survive?
From what I can determine, your Leyland cypress (x Cupressocyparis leylandii) trees are probably going to
survive this attack unless they are weakened in some other way.
x Cupressocyparis leylandii in California survive the Cypress tip moth, though they can be unsightly. Since California is a bit too dry for this tree, the conditions are
not identical, but Natural Resources Canada does not
indicate that infestations are fatal. Because you said you found
evidence of tip moth (Cypress tip moth = Argyresthia cupressella), I will
assume that is what the problem is, but a bit of browning, even in
conifers, is not unusual right after trees are planted. Be sure that you
are not overwatering, as one effect of that is the same as underwatering
(i.e., tip die-back or yellowing) because too much water prevents the
plant from taking water and oxygen into the roots.
The Organic Gardener’s Handbook of Natural Insect and Disease Control
(ed. by Ellis and Bradley, 1996, p.183) says about pine tip moth
(Rhyacionia frustrana):
“Handpicking works if only a few caterpillars are present. Pruning off
and destroying infested tips in winter is a very effective control.”
I would recommend a prune-and-wait-and-see approach. April is a bit late to
prune (and puts root establishment in competition with shoot regrowth),
but you may be able to slow the infestation down, so go ahead and do
it. Watch the trees this season and then prune again in the winter next
year. Be sure to destroy (burn or bag and put in the garbage) the
debris so you don’t reinfect your tree.
A good gardening resource is the UBC Botanical Garden Forum. Personal
testimony/experience is valuable, especially if it’s regional. (You
might find it useful in the future.) Several people commenting about
x Cupressocyparis leylandii note that it is not a very desirable tree; one of its
parents, the Cupressus nootkatensis, also called Callitropsis
nootkatensis or Chamaecyparis nootkatensis, or, for that matter, plain Nootka cypress) is better. One person
recommended planting small trees in the beginning, since they grow very
fast. This might save you some money, should you have to replace your
trees. The site does not need a password; just click on “Search” in the upper right
corner.
Below is some additional information from Oregon State University about cypress tip moth. This site recommends pesticides, but from everything I read, they are not
effective without multiple treatments. Since this pest is generally not
fatal to the trees, it is probably not worth it to use chemicals which would be dangerous and time-consuming to apply. If you know something about the
life cycle of the pest, your observations will yield more information and any
manual control attempts are more likely to be effective.