Rats seek out extra vitamin C in lemon rinds

Q: We have a critter attacking our lemons. It does not eat the fruit, only the peel. It eats the peel so cleanly that the bare fruit is left hanging on the tree. The white pith is gone, too. It is a strange thing to see a naked lemon hanging on the tree. Do you have any idea what this critter could be?

A: This is most likely rats. I have not seen it, nor has it been reported to me directly, but several reports in the U.S. and Australia attribute the eating of lemon peels but not the pulp to rats. This can happen to fruit on the tree or on the ground. Rats will also gnaw on the bark and branches of citrus trees.

Contrary to this, it has been reported that rats will eat the pulp of oranges and pomegranates but not the rind or outer covering. It is not understood why, but lemon peels and pomegranate juice is high in vitamin C and calcium.

Rats produce their own vitamin C inside their bodies, so it is not clear what they are going after by selectively eating rind and not the fruit. It is also reported to me that rats will eat guava fruit and papaya in the tropics, another source of high vitamin C.

Research on rats and vitamin C in the early 1900s report that rats may benefit from extra vitamin C in their diets in growth and reproduction. Better get the rat traps out.

Q: What should we be doing to our fruit trees this time of the year?

A: Focus on three things in January: finish pruning, fertilize fruit trees if they need fertilizer and put on a preventive pest control application.

When pruning, control the size of the tree first and establish its general architecture or structure before pruning it to improve production. Reduce the size of the tree so that it is easy to harvest the fruit, prune and spray for pests. Fruit should not be on branches so low that fruit touches the ground.

Download a copy of my general pruning recommendations for fruit trees located on my blog, Xtremehorticulture of the Desert.

Fertilize fruit trees in our climate any time in January before the first week in February. If you are late by a couple of weeks, you can still do it. Use conventional fertilizers or 100 percent compost. Apply it on top of mulch, or pull the mulch back and put it on top of the soil, then put the mulch back.

Water fertilizer or compost into the soil. Fertilizer and compost cannot move to the roots if it is not in contact with wet soil.

Be careful how much phosphorus, the middle number, you apply. Phosphorus can hang around in soils a long time, and it can build up concentrations if you aren't careful. Apply it only once during a growing season.

January is the best time to apply iron to the soil for all peaches, nectarines, plums, apricots, apples and pears. Use EDDHA iron chelate for best results in all soils. One or two tablespoons per tree is usually enough.

Apply a dormant oil to trees at least once, if not twice, during the winter. It is a very important preventive pest control application and aimed at controlling future aphids, scale insects, spider mites and a few other general pests that might appear this growing season.

Dormant oils "suffocate" insects, so apply it on a warm day with no wind. It is best applied with high pressure application equipment such as compressed air or backpack sprayers. Applying it with high-pressure equipment will break the oil into a finer spray, which helps the oil cover as much surface as possible without leaving gaps in the application.

Q: My apple tree is still full of leaves and hasn't lost one yet. Should we wait to prune once it starts losing leaves, or is it OK to prune in the next couple of weeks? The leaves have turned a purply brownish color but are still very much alive.

A: Go ahead and prune. We have not had a freeze hard enough to knock off the leaves. The tree is fully asleep for the winter.

Actually, you can start pruning fruit trees as soon as they stop growing in late fall. We normally wait until leaf drop because we can't see where to make the pruning cuts easily. You can also wait to prune until near February, and that will be fine as well.

Leaves can be removed by running your hands down the branches if they are in reach. Try shaking the limbs or rap them lightly with a broom, and they should drop as well.

If you want the leaves to drop on their own, turn off the water to the tree until leaves begin dropping, and then resume your irrigations as needed.

As for me, I would just wait for normal leaf drop if it is difficult to see where to prune.

Q: Our walnut tree is at least 10 years old and producing wonderfully for the past three years. This year, toward the end of the season, the husks started getting black. I opened one and saw little white worms. I opened the nut shell and saw no damage to the nut itself, so we did nothing. By the end of the season, most of the husks were on the ground, all black. Of the husks with nuts, the nuts were OK, but there were a lot of husks with dried up black nuts. We also noticed this larger branch on the tree that seems to have split its bark.

A: This sounds like walnut husk fly damage to the husk. If walnut husk fly damage starts early enough to the husk, they can cause a nut failure just like you describe. If damage to the husk is later in the summer, the nuts inside will fully develop.

The walnut husk fly adult is around the size of a housefly and lays eggs just below the surface of the husk. This blackens the outside of the green, immature husk and causes it to get soft. Eggs from walnut husk fly hatch into maggots, or worms as you call them, feeding on the inside of the husk.

After a few weeks, older maggots fall from the husk to the ground and burrow into the soil where they spend the rest of their life before emerging as adults the following summer. They emerge as very colorful, mature flies ready to repeat the cycle. This lifecycle is repeated once a year with egg laying on husks starting in mid- to late summer.

The University of California has an excellent fact sheet on the Internet concerning the walnut husk fly. Control measures are mentioned but a bit difficult. If there are other walnut trees in the area, this could cause an increase in the population of this pest in coming years if they are left uncontrolled.

The pictures of the tree limb you sent look like borer damage to me. The same borers that attack fruit trees such as peach also attack walnut. Pull off the loose bark from the limb, and look for damage from borers on the surface of the exposed wood.

If borers damage is present, remove all of the loose bark with a sharp knife. If the damage to the limb is more than halfway around its circumference, remove the limb. If it is less than half, the tree has a good chance of recovery if all the loose bark is removed down to healthy wood.

Q: I have a Eureka lemon tree and a Rio red grapefruit tree. Is it best to stop watering them through the winter weather, or should I continue with their regular watering schedule?

A: They should have a winter watering schedule, and water to them should not be turned off. Get a general idea when to water by monitoring the soil moisture with an inexpensive soil moisture meter. Purchase these at most nurseries or garden centers where they cost less than $10.

A winter watering schedule should be somewhere between 10 days and two or three weeks. Plants in containers must be watered much more often.

Push the tip of the meter as deep as you can in the soil somewhere beneath the canopy of the tree and midway between drip emitters if they are present. Do it in two or three locations. These meters are inaccurate, but they give you a general idea if the soil is dry or wet. Water when they meter is midway between wet and dry.

Trees that have leaves present use more water than trees without leaves. A layer of wood chips as a surface mulch will cause you to water less often. A lack of water or very cold weather may cause leaf drop.

Q: I transplanted a Joshua tree into our new backyard in March. It is doing great. Do I need to cover it as temperatures dip down below freezing?

A: No, you do not need to protect it with any kind of freeze protection through the winter. They are good down to about 10 degrees Fahrenheit. Make sure you are watering it infrequently, or it can develop root rot and die. Fertilize it once lightly in the early spring with a fruit tree or rose fertilizer.

— Bob Morris is a horticulture expert living in Las Vegas and professor emeritus for the University of Nevada. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.