iGrow Gardening: Wormy apples

David Graper
SDSU Extension Horticulture Specialist

Question from a reader

Q Help! I have two honeycrisp apple trees which I have had for about ten years and have been bearing very well for the last two years. I picked at least five five-gallon buckets of apples and only got about six apples that didn’t have worms. I sprayed with Bonide fruit tree spray as directed on the label, and it says to only spray twice. I sprayed at blossom fall and two weeks later. I practice good sanitation, always clean up fallen apples and leaves when they fall. Other people who have apple trees do not spray them at all and have no problems with the apple maggots or any other pests. I am very frustrated by this. I live in Redfield and do not travel very well (too old and don’t like to drive), so I would need to order something so please advise what I can use as the apples taste wonderful. The sprays I can order usually don’t list the ingredients in the catalogs. If I should spray more or use some other spray, please advise. I also tried the gallon jugs with various lures and the sticky red balls. I am very frustrated as the apples were large (3 to 4 inches) and looked beautiful until I cut them open. Any help you can give me will be much appreciated or pass the inquiry to someone who might help. Thank you in advance.

A I can certainly understand your frustration with having what could be a beautiful apple crop but soon learn that apple maggots have ruined it. The most important thing to remember about apple maggots is that they do not emerge from the soil until about the first part of July. That is when you need to have protective sprays on your trees, if they have been a problem in the past. The Bonide product you used is a good product but if you only spray twice, as you mentioned, you would likely only get protection from some fruit and leaf diseases and possibly codling moth, that attacks the young fruit, shortly after they form in the spring. The gallon jugs with a variety of stuff fermenting inside is generally not effective for controlling any apple pests, except some types of fruit flies perhaps that would usually only attack rotting apples.

Here is a portion of John Ball’s Pest Update newsletter that he publishes each week during the summer. If you want to see that week’s full text (June 27, 2018) with pictures, got to:

Apple maggot (Rhagoletis pomonella) is the most serious apple pest and treatments should begin by the end of June.

Symptoms of a maggot infestation are dimpled, lumpy appearance to the surface of the apple and the flesh often turning mushy and containing the brown trails of the larvae, hence the other common name “railroad worm.” A sure sign of the pest – an unpleasant one if you happen to find one, or half of one, while eating the apple – is a small (1/4”), creamy white and legless larva in the fruit. The adults, resembling houseflies with banded wings, should be flying and placing eggs on the developing apples in another week or two and will continue egg-laying for another month. Once the eggs hatch the larvae burrow into the apple. The apple maggot pupates in the soil and probably will be emerging as an adult beginning next week. However, egg laying does not really begin until a week or so later so there is still time plenty of time to begin treatments (even if any eggs are laid now, the egg is either crushed by the expanding fruit or the larvae cannot survive in the high-acid of the newly developing apple). Treatment is either Carbaryl (Sevin) or Malathion applied starting in another week or two with subsequent applications every 7 to 10 days for three or four applications. Apple maggots tend to emerge from the soil after a 1/2-inch rains so some producers time applications with rainfall, but this is not necessary for the home production.

Another means of management is to place 3-inch diameter bright red balls in the tree, about 2 in semi dwarf trees (about 10-15 feet tall) and 5 in standard size trees (about 20-30 feet tall) that are covered with a sticky material called Tanglefoot. The female apple maggot always flies to the biggest, brightest apple to lay her eggs and these will be the biggest, brightest “apples” in the tree. You cannot eliminate the pest by using this control, but the population can be significantly reduced. The “apples” can be made from material found in almost any garden store – even can find Tanglefoot at most hardware stores or you can buy the completed “apples” from the Internet, try

Still another possible control measure is to spray Kaolin clay on the fruit. The clay is not a true pesticide, but it irritates the adult apple maggot and they tend to fly to other fruit. The clay must be reapplied if we have some heavy rains so expect to make several applications during a season. It often takes at least three applications to work. The clay is sold as “Surround At Home” and can also be obtained from

Did the cold temperatures kill emerald ash borers?

Emerald ash borer must survive our winters like any other insect that wants to make South Dakota home. The trick for many insects is not hibernating like bears or head south like many birds and the monarch butterflies, instead they supercool in late autumn (which sounds kind of cool!). This is the ability to cool liquids, like water, below their normal freezing point without forming ice. Ice crystals forming in a cell is a killer for any life.

Your car radiator also has liquid that better not freeze as the ice will damage the radiator much like ice formation damages a living cell. We use an antifreeze in our car radiators to keep the liquid from freezing. Insects make their own antifreeze out of alcohol, proteins, and sugars. Much like the antifreeze in your car, this allows the insect to survive a lower temperature than 32°F, but there is still a limit to supercooling. Once the temperature drops below the specific threshold for an insect, the liquid will quickly freeze, and the insect die.

The supercooling ability for emerald ash borer differs throughout its natural range in Asia. Those from the southern part of the range, central China, may not be able to supercool as deeply as those from northeastern China. It appears that the introduction of emerald ash borer to this country came from more of the middle of its range – we got lucky – they might not be as cold tolerant as ones from the north.

Studies in Minnesota and Ontario have found that emerald ash borer as a larva can supercool to about -25°F in midwinter. This does not mean every larva dies once the temperature dips that low. Some will be protected by thicker bark (and bark can provide about 5°F of insulation) while others survive in lower trunks covered by the snow.

But all the larvae up in high branches, which have thinner bark and are exposed to the cold, may not be so lucky. If the temperature drops overnight to -25°F for several hours or longer, we might kill more a third of the population. If we have the temperatures drop to -35°F we might kill almost all of them, as well as most of our car batteries.

This past spring while we were stripping bark from infested trees, about one-third of the larvae found were dead. Many had that shriveled and black appearance like a banana set in the freezer for a while (bananas do not supercool). While we can count on our cold winters to kill a lot of larvae, it is not cold enough, long enough to stop the numbers from growing each year. The population will grow each year, along with ash mortality, just slower than for more southern communities. We will be sampling the emerald ash borer larva population in two weeks to see if the cold had any effect on survival.

Did my perennial flowers get frozen?

Amongst gardeners and plant people that grow or work with perennials, both woody and herbaceous, we sometimes say that cold like that make it a good “test winter”. What we mean is that it is a chance to see if some of our plants are really as hardy as they are claimed to be. As you recall, we usually use the USDA Zone map as a guide to give us an idea as to which plants are most likely to survive for us over the winter. It is based on the average lowest cold temperature for a given region. Most of SD is in Zone 4 with some in Zone 5. You can check what your actual zone is by using this interactive map

Just type in your zip code and it will zoom in on your area and tell you just which zone you are in. When you buy a perennial plant, it will usually come with a plant tag that will tell you the hardiness zones it is adapted to. If you select plants that are rated to your exact zone, then usually you should have pretty good success with winter hardiness for that plant.

Of course, there are more factors than just how cold it gets during a given winter or even more specifically, how cold it gets during a particular few days during the winter. Other factors also come into play. One of the most important, especially for herbaceous perennials whose top growth generally dies back to the ground each year, is depth of snow cover when it gets really cold. Snow, particularly if it fell naturally on a particular spot, is an excellent insulator. The snow crystals are usually loosely packed together providing lots of little insulating air gaps that help to hold in warmth from the soil. You may have heard meteorologists say that the ration of snow to water is usually 10 to 1, meaning that for each 10 inches of snow that fall, there is really only 1” of water. Essentially this means that snow is usually about 10% water and 90% air. Snow is usually not going to insulate as well if it is packed down or piled up by a snow plow or snow blower, because it tends to get rid of a lot of those little air pockets. But snow cover of any kind is generally going to be better than no snow cover at all. It doesn’t take a lot of snow to provide protection. Just a 12” covering of loose snow can be an excellent insulator and moderate temperatures significantly. While the actual air temperature above the snow might be -20°F, at the area where the snow meets the soil it might be a relatively balmy 30°F.

Winter snow cover also provides protection from soil temperature fluctuation. When the sun comes out and air temperatures start to rise, bare soil starts to warm up fairly quickly. But, then when the sun goes down or it gets cloudy, the temperature can drop again too. These temperature fluctuations can be quite hard on our plants. If it is dramatic enough, we can sometimes see plant damage by what is called frost heaving. As the soil warms up a little bit, some of the snow or surface ice melts and can get down between a plant’s roots, crown and the soil. Then when it freezes again, the ice that forms expands and can actually push plants up out of the soil. This is particularly common for plants that have a tap-root or if plants were not well-rooted before the ground froze.

Another important aspect of a good snow cover is that it protects plants from desiccation. This is in particularly helpful to protect plants that are partially evergreen or have crowns that are close to the soil surface. If that plant tissue is exposed to drying winter winds, it can suffer damage or be killed.

So, that was a very long answer to what was a fairly simple question. The simplest answer would be “it depends”. We will just have to wait until spring and see how our plants survived. If you are like me, I usually push the winter hardiness zone envelope a bit and try plants are not quite rated as hardy to my area. It is fun to try new things and if they survive for a few years and I got the chance to enjoy something that most gardeners in my area were not able to grow or did not try, then I consider myself lucky.

Bergenia has semi-evergreen winter foliage.
Bleeding heart with freeze damage.
EAB feeding gallery with larva.
Sedums often have some evergreen shoot tips that could suffer winter damage.
Snow drifted perennial bed.
USDA Plant Hardiness Zone Map.