If you can take some close up pics, that would be great. Do you see any bark damage that might look like fungus or some other ailment? How much truly dead wood is there - I think you'll want to remove it?

I don't know how bad your infestation is, but ladybugs do eat aphids.

Good luck with the trees...are they getting watered/fertilized now at least?

Beachy, sounds like you have inherited some neglected trees. For the clementine on the slope I would build a small retaining wall outside of the exposed root system maybe with bricks stacked up or keystones and fill over the roots to cover them. I would also build that wall far enough out so that when the tree does start to recover the roots have room to grow if needed. Then I would start consistently watering it, and when I say consistently, I mean same time of day, same amount of water and same days, odd days or even days or every third day, just be consistent. Same thing goes for the lemon trees watering wise. I would also treat the citrus trees with neem oil weekly and any other plant in your yard that may have aphids. I get aphids on my hibiscus so I spray them at the same time I spray my citrus trees until there is no sign of aphids. Once the trees start to show signs of growth then you can fertilize lightly to make sure it doesn't shock the trees. As for the gnawing on the bottom of the trees I have heard that dryer sheets will deter rodents like gophers away from gardens, if that doesn't work your local Home Depot or lowes has rodent deterrent available that will work well too. As serge said earlier, close up pictures always help with diagnosing problems too. I am no expert but what I posted above is what I would try. There are a lot of experts on citrus on the forum here and I'm sure they will be along soon with more suggestions.
Evan

I would start by getting rid of all those weeds inside the drip line and two feet outside of the drip line. Soak it with a soaker hose. I do not know how much your water cost but get out your check book and invest in water. Soak it deep and often. It will take a while to recover from historical lack of water.

If they were mine, I would start by making firewood of those trees. Citrus wood is great firewood, if you have a masonry fireplace; it is too hot for most wood stoves or metal chimneys. Here's my canned answer for trees in such bad condition.
Best advice, take it out and replace it with something you know and want; John's first rule of trees... you are going to live a lot of years with this tree; you are going to put a lot of energy, money, time, fertilizer, water; so you should begin with the biggest, best, tree that you can find/afford. It is like buttoning your shirt... if you get the first button wrong, it doesn't matter how careful you are after that.
It looks like you are going to have enough work to do on your 1/2 acre without the added struggle of trying to save old neglected citrus.

beachy, the trees can be saved, but it will be some work on your part. We can't really see what's going on as the photo has such poor contrast and is so far away, can't see the specifics. But, here is the list of to-do's:

1. Weed under the trees and mulch.
2. Fix the drip systems so the trees are getting adequate waterings. Nice, deep waterings once or twice a week. At least 2 feet down, depending upon how hot the day is.
3. Prune out all the dead wood.
4. Put out rodent traps. Wrap the trunks with chicken wire to discourage further gnawings on the trunk (probably rabbits, possibly citrus/roof rats).
5. Fertilize VERY WELL at the drip line. Water in well. Repeat in 1 month x 3. Then more to 3-4 fertilizing applications a year. Use a good quality fertilizer with micros.
6. Treat for CLM (Citrus Leaf Miner) which will also take care of any aphids.

Aphids are not the same as psyllids, which is what you are referring to regarding Citrus Greening. Which we do not have here in the state of California - yet. We have the psyllid but not the disease that the psyllids can carry. Aphids do not carry Citrus Greening. They can carry other diseases, but if you treat for Citrus Leaf Miner now, and then a second application in 6 months and continue to do so every 6 months, you'll also control any possible psyllids that might end up reaching your area (you don't say where you are in California, so I can't tell you if psyllids have been found in your area of Calif.) Old trees can be worth saving, especially if they are on sour orange, which produces the absolute sweetest sweet citrus imaginable.

Please let us know how you fare.

Patty S.

What is it with johnmerr and fire wood if a tree cost to much to save he's all cut it down and burn it wow. Kind of offensive to ppl that love plants I understand if it cost $$$$$$ to save a tree but if its $$ there is a different besides to buy a tree that's size would cost $$$$$ anyway. Not attacking u johnmerr but u offered me u always wanting to burn everything down everything its not about money always. I hate ppl that move in to historical neighborhoods and cut down all the 200 year old trees.
Trace

Tcamp, because John is a commercial grower, and it just makes more economic sense to a commercial grower to replace rather than try to revive, in terms of production potential. It is a different venture with dooryard citrus, especially those trees on heirloom rootstocks. If the tree has not succumbed to Tristeza due to being on Sour Orange rootstock, in my opinion, they are worth saving due to the extremely high quality of the fruit. But, it may be a labor-intensive (and pocketbook intensive) venture. All those things need to be taken into consideration before attempting a save. With this tree, it is in good enough shape to save with some effort on the owner's part.

Patty S.

I agree why kill the tree? Saving it might be emotionally rewarding.

It looks like you have room to plant other new trees if so desired. I guess I am a romantic but that tree has been thru a lot and is part of the property you purchased. I would keep it out of respect for the tree and respect for the history of the property.

Trace,
I think you need to lighten up a bit unless you are trying to compete for my title to internationally certified curmudgeon.
With me it is not about money; it is about having the best tree and the tree you want, rather than what someone abandoned and left behind.
Patty is right in the case where a tree could be an heirloom variety, of some sentimental value, or maybe just needs a little water and fertilizer; but a new owner of a seemingly abandoned garden is unlikely to know any of that.
Beyond that, if you don't know the variety, the rootstock, the condition of the roots, etc. it is much harder to treat the tree correctly. Anyway, as always my advice is free; and you can take it for what you paid, or ignore it.

So, I usually take Johnmerr's advice - but in one case I did not. I have a (probably Dancy) citrus tree that came with the house. The remodeling contractors whacked off 2/3 of the tree before we moved in and the tree had already been long neglected. The bark was peeling off north of the graft but had yet to be totally girdled. I posted pictures and asked for advice. I got a lot of advice on how to try and save the tree and a couple of people (including John) said "It's a goner - put your effort into a new tree."

I love saving things - part compassion, part ego, part adventure. I threw all I had at the tree and this has been the result:

1. Better leafing - nice canopy!
2. Fruit set, stayed, and was juicy and delicious.
3. Bark issues got worse and tree has a year or so I would guess before total girdling and imminent death.

So was it worth all that time, money (yes), and anxiety?

No way - I should have taken it out and put in something I really wanted that's going to be healthy for decades (hopefully). Lesson learned! Now for someone else, they might think it's worth it. Everyone has a different opinion based on their own sense of value, which is why it's silly to ask people for their opinion and then bash them because they have an opinion. If we all thought the same, there would be no reason to ask anyone else for their thoughts.

There is also the time issue. A new tree will take years to to get into full production. Secondly, you will need a backhoe to get rid of the roots if you plan to plant a new tree in the same location. Finally, I would really enjoy seeing a picture of that tree in the future looking healthy and producing fruit.

Please note Johnmerr's statement was qualified with a valid comment about saving the tree is also time intensive, and probably would be more rewarding to invest in developing the balance of the 1/2 acre, to which I agree. And even if you can save it, the damage is such that a gardener with even above average skill will be more than challenged to make it a thing of beauty.

But I also know the joy of good fruit on sour orange, having several myself. However, on a good year my Page and Fairchild selections grown on feral trifoliate are too sweet to juice alone. I have to blend it off with lesser fruit or friends just drink a few sips and quit.

Sick trees are also less able to fight off pests and disease, which could imperil the rest of your trees. The Dancy I've been trying to save is thick with scale (the only tree that has it).