Here are a few photos of the recent bark damage to a lemon
tree in Central Davis, taken March 20, 2024. It was noticed
a few days earlier and increased since then. A few limbs
are girdled completely, most have a narrow band of bark on
the upper surface.

The steel fencepost in the trunk_view image is forty inches tall.

Prime suspects are squirrels and/or rats, but neither has
bothered the lemon tree in the past sixteen years. Squirrels
do feed on mulberry buds this time of year but haven't 
stripped bark in this way ever. The only fece (just one)
found underneath was rat-sized, not squirrel sized.

Rats are abundant, but have never attacked trees in 
any visible way until now.  

There's a WCS tube trap wired to the top of one well-stripped
limb, baited with a little peanut butter above the trigger.
After 24 hours, no action.

A nocturnal inspection with a light revaled a small creature
moving around the tree crown, trying to stay out of the light.
Never got a good look, but by all odds it was a small to medium
sized rat... mystery solved.

At this point some Liquid Fence repellent spray was offered to me.
I applied it to the upper trunk, hoping to forestall further damage.
That night I again heard movement in the upper crown. Damage down
low seemed to slow, so maybe it helped a little, but not much.

After a couple more days I added cheese to the trap, just
to see if it would get any attention. It didn't. The next
evening I searched again with a light, but didn't see or
hear anything in the tree.  

On March 30 a nocturnal light chase revealed two smallish
rats, moving together with mutually hostile intent. During
their flight and fisticuffs I got a good look and observed 
their path out of the lemon and over to an oak. I've now 
pruned the oak, leaving about a one-foot gap between frail 
branches of the two crowns, to see if they can jump it.
A larger gap was made to a nearby crepe myrtle.

On April 1 a flashlight search found one small rat, which
did not try to exit the tree, even under close scrutiny.
How it entered the tree is unclear.

20240330

More photos, taken May 18, 2024 show considerable additonal
damage and paint applied to the trunk to minimize sunburn.
Applying the paint seems to have slowed damage considerably,
although there are still signs of rat activity elsewhere in
the yard.

The canopy has thinned, not least from one major limb removed
as it was completely girdled at the point where it was cut off.

Many other limbs appear to be nearly as bad, but a surprising
amount of foliage remains and the tree is still opening flowers,
setting fruit and pushing out new leaves. For now I'll not try
to remove anything, to see what survives.

20240518

After some discussion on rec.gardens Leon Fisk found a discussion of
bark damage and how it affects the tree:
https://treenet.org/resource/ring-barking-and-girdling-how-much-vascular-conne
ction-do-you-need-between-roots-and-crown/

The takeaway is that the rats removed the phloem but left the xylem. Water
still moved from roots to crown, but nutrients couldn't get from crown to
roots. Thus, the roots would die first, invisibly. Time to act....

I decided to take the opportunity to trim the tree quite harshly, 
down to a size more appropriate to the space available. 

Deepest thanks to Leon for finding the article. 

20240529

After watching the various signs of rat activity in the back yard
I moved the WCS tube trap from a branch (now removed) in the tree 
to a spot where the rat(s) seemed to be dining. It (they?) collected
sprigs of alfalfa planted as a groundcover, carried them to the more
secluded dining area and stripped the edible leaves off the stems.

I didn't replenish the bait (peanut butter and cheese), having little
faith it would do any good. True to expectation, it didn't. No takers.

After a couple of weeks I put out a large bucket of juiced-out lemon
rinds for disposal and was surprised to find the rat(s) were picking
the rinds out of the bucket, carrying them to less-exposed places and
eating the remainder of the lemon pulp and some seeds. The seeds were
a surprise, as they don't seem much of a meal. 

So, I sliced a lemon and reached for the safety catch on the trap,
being quite surprised to find the trap tripped with a medium sized
male rat.

It being unclear how many rats were in the offing I baited the trap
with the lemon half and put it back in the same spot.  

Whether the rat went for the peanut butter and cheese or it simply
entered the trap by mistake is unclear. That alfalfa leaves, lemon
rinds and seeds were actively foraged suggests some degree of hunger.
The trap is designed as a "passage trap", open at both ends to invite
the critter to run through on its way elsewhere. 

20240602

On August 11, 2024 a much smaller than usual rat was found caught
by the tail in the WCS tube trap and dispatched by the "bucket
method" using soapy water. The rat was caught by the tail on its
way _into_ a concealed area chosen by the rats to feed on clipped
stems of alfalfa. That the rat very nearly escaped is impressive. 

The feeding area had become apparent a month or two earlier, and
a plywood barrier was set up to restrict access, with the trap
placed inside, facing one opening, unbaited. After two months of 
failure I'd largely given up hope of catching any more rats, even 
though feeding behavior remained evident. Indeed, the rats had
taken to chewing the stems off tomato plants, apparently checking
edibility. No further damage to the lemon tree has occurred. 

20240811 

Summer is essentially over, the the tree looks like it's recovering.
Adventitious growth is vigorous, some of the bark is re-growing 
over exposed wood. Perhaps 10% of leaves appear to be deformed, but
they're green and appear productive. No further rat activity seen.

20241009