Garden Word of the Day
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Mosquito Repelling Plants
Mosquitoes can smell the carbon dioxide you exhale from 150 feet away. They zero in, land ever so lightly, stab you, and suck your blood. In doing so, they may expose you to several fatal diseases, including chikungunya, dengue fever, encephalitis, malaria, yellow fever, West Nile Virus, and Zika virus. They can also carry canine heartworm.
Mosquitoes are responsible for 700 million illnesses and over 1 million deaths around the world each year. In 2016, there were more than 96,000 cases of mosquito-borne disease in the U.S. and California has the highest U.S. number of cases of chikungunya, a viral disease that causes debilitating muscle and joint pain and fever.
Odds are very high that mosquitoes are lurking in your garden. If you search online for “mosquito repelling plants”, you’ll find thousands of lists. With all the claims about mosquito-repelling plants, what’s true, and what’s hype? Let’s find out.
There are many claims made about plants being able to turn away these blood-sucking, disease-carrying pests. Research has shown us that many plants do, in fact, have the ability to repel mosquitoes, but probably not in the way you expect.
Pine, lemongrass, lavender, catnip, scented geranium (aka mosquito plant), jasmine, eucalyptus, chamomile, juniper, rosemary, soybean, olive, Tagetes, violets, sandalwood, thyme, peppermint, lemon, rosewood, turmeric, and sage can all repel mosquitoes. Sort of.
The essential oils contained in those plants are what provide the protection. But adding them to your landscape isn’t going to help, though they may look lovely. The problem is, you’d have to roll around in those plants every 15-30 minutes to be protected from mosquitoes. I don’t know how long your plants could tolerate that sort of treatment.
In fact, both male and female mosquitoes eat nectar, sap, and honeydew. It is only the females who need the protein found in blood for egg-laying.
Many mosquito-repelling products claim to be natural plant extracts. Keep in mind that natural does not always mean safe. Strychnine is natural but it can kill you. Extended exposure to chemically significant essential oils can lead to allergic reactions and skin sensitivities.
Mosquito predators and repellent products
Electronic mosquito repellers do not work. Either do mosquito-repelling wristbands. Bug zappers kill millions of beneficial insects and very few mosquitoes. CO2 traps are mostly ineffective. If you are sitting in one place, clip-on insecticide fans and personal propane vaporizers may provide some protection. And, as much as I enjoy candlelight, citronella candles don’t work much better than regular candles once you get a few feet away, according to a study by NIH.
Crane flies are not the mosquito-eaters we thought they were and bats do not eat as many mosquitoes as is commonly believed. Larger bats eat larger prey and smaller bats don’t seem to have a big impact on mosquito populations. Another common myth claims that setting up a Purple Martin house will reduce mosquito numbers. Local martins will be happy, but it won’t change your mosquito problem. You could always copy the Capuchin monkeys and rub yourself with millipedes, but I don’t recommend it.
Aphids spend their entire lives sucking sap from garden plants and spreading viral diseases, but those lives are fraught with danger.
Aphids are soft-bodied insects with piercing mouthparts. They insert their sharpened straws into plant tissue, where they slurp up copious amounts of sugary sap. They eat so much and so quickly, that much of what they eat simply goes in one end and out the other. This “honeydew” can be contaminated with pathogens and it attracts the attention of protective ants, but aphids have many enemies. We’re not just talking about gardeners here, either. In most cases, aphid killers are either predators or parasites, though there are exceptions.
Parasitoids are insects that lay their eggs in, on, or near host insects. Those hosts end up being the first meals for parasitic larvae. There are many parasitic wasps that bring death to aphids. They do this by laying an egg in every aphid they can. When the eggs hatch, they eat their aphid hosts from the inside out. Gruesome, right?
Eventually, the aphids die and the now-adult wasps fly away. All that’s left behind are dozens of tan or golden aphid husks known as mummies. If you use a hand lens, you may even be able to see wasp exit holes.
The most commonly seen parasitic wasps and their favorite aphids include:
Predators actively hunt and eat aphids. Aphid predators include:
Remember those exceptions I mentioned? One of them is the ichneumon wasp. Adult ichneumon wasps kill their prey outright and then lay eggs in the corpse. It’s a brutal world out there, make no mistake.
Many varieties of parasitic wasps and other aphid killers are available for purchase, but you can often attract these garden helpers to your landscape with plants that provide abundant nectar and pollen. Queen Anne’s lace, coreopsis, coneflowers, cosmos, coyote brush, dandelions, dill, goldenrod, sweet alyssum, and sunflowers look lovely and they create a natural welcome mat to insects that see aphids as the perfect meal. Hedgerows also provide beneficial insects with good hiding places.
Brood X is on the brink of clawing their way out the soil after nibbling roots for nearly 20 years. Brood X refers to this year’s cicada event.
I lived in Virginia during an emergence year. It was an experience I’d rather not repeat. While not as bad as the Midwest’s plague of locusts, there were cicadas everywhere. They coated windshields, driveways, and roads. They flew into you, clinging to your clothes and hair. The noise was enough to drive any sane person crazy. And then it was over. Like it never happened, except that there were hollowed out exoskeletons attached to every tree you saw. But finally, it was quiet.
As you may know, I use sticky barriers around the trunks of my fruit and nut trees to thwart crawling insect pests. These barriers also give me a chance to see which insects are out and about, though the sticky sheets hanging in the trees do a much better job of that. This year, I was very surprised to find cicada exoskeletons caught in the barriers protecting my navel citrus tree. I thought cicadas were only on the East Coast, but I was mistaken.
Cicadas are well known on the East Coast for their epic numbers, mind-numbing noise, and littering trees and shrubs with exoskeletons. What I didn’t know is that the West Coast has cicadas, too. And this is the year, coast to coast, when billions of cicadas come out of the ground to mate. Most of them have been underground for 17 years.
Types of cicadas
Cousin to leafhoppers, there are more than 3,000 types of cicadas around the world and more than 170 species in North American that we know of. There are two cicada families: one lives in Australia and the other lives everywhere else except Antarctica. Within that global family there are annual cicadas and periodic cicadas.
Periodic cicadas (Magicicada) are native to North America and they spend most of their lives underground as nymphs. Here, they feed on sap from tree roots. Depending on the species and location, these nymphs emerge at 13- or 17-year cycles in those mind-boggling numbers. Periodical cicadas include M. septendecim, M. cassini and M. septendecula. These synchronized mass emergences make predators less likely to rely exclusively on cicadas as food and the massive numbers ensure the survival of the species.
Most annual cicadas emerge every year, though they can spend up to 9 years underground. Unlike periodic cicadas, the emergence of annual cicadas is not synchronized. Annual cicadas are also known as jarfly or dog-day cicadas because they tend to emerge in mid-summer. In my California landscape, I probably have annual citrus cicadas (Diceroprocta apache), though there are several other cicada species present.
Cicada description and lifecycle
Cicadas are true bugs with large, wide set eyes, clear forewings, and short antennae. They are stocky, substantial bugs that are generally active during the day. While the Malaysian emperor cicada is nearly 3 inches long with a wingspan of up to 8 inches, most cicadas are significantly smaller than that and thank goodness! Most adult cicadas, or imagos, are 1 to 2 inches long with comparably smaller wingspans.
After mating, female cicadas cut slits in the bark of twigs where they lay their eggs. When the eggs hatch, the nymphs are pale and about the size of a grain of rice. They drop to the ground and use their strong front legs to burrow as much as 8 feet into the soil. Here, they will feed on xylem sap from the roots of trees and shrubs. They prefer ash, cypress, maple, oak, and willow, but will feed on other trees, as well. In the final instar, they return to the surface where they shed their skins and emerge as flying adults. Those shed skins are commonly found attached to trees.
If you’ve ever been present for a cicada emergence, you know how incredibly noisy these insects can be. At up to 120 dB, they are the noisiest insects on Earth. They are so loud that they can cause permanent hearing loss when experienced at close range.
Both males and females have membranous structures called tympana that detect sounds. When a male cicada sings, he turns his tympana off so as not to damage his own hearing. [I used to have a neighbor like that. I had to move to a new apartment.]
Male cicadas create all that noise using organs called tymbals found on their abdomen. In some species, both males and females rub their wings over a series of ridges found on the thorax. These bugs have resonating cavities and membranes that amplify the sound. While we can’t tell the difference, it ends up that each male cicada has a unique song composed of modulated clicks that sound, to us, like continuous notes. I have to assume that the female cicadas can tell the difference. To me, they’re just very noisy. I did learn that one of the Australian cicada species does not produce audible sound. Instead, they produce vibrations that are transmitted through whatever tree they are on, which sounds far more civilized to me. And some cicada songs are so high-pitched that we can’ hear them. Another interesting notes about cicada courtship is that different cicada species may be found on the same tree, but that each species uses a different height from which to operate.
Adult cicadas feed on sap from xylem tissue, but they generally do not harm mature trees and shrubs. Newly planted trees and shrubs should be protected with netting during emergence years. In most cases, cicada root feeding is not significantly destructive. Females have been known to lay eggs on asparagus, citrus trees, date palms, and grapevines.
This year’s cicadas are being called Brood X. As temperatures rise, they stop nibbling the roots of your plants and emerge from the soil to mate, lay eggs, and die. Ants, bats, birds, squirrels, spiders, and wasps will all gorge on cicadas this year. While cicadas do not bite or sting, they are also not very bright and may mistake you for a tree and try to feed.
Don’t bother spraying pesticides or insecticides for cicadas. The damage they do is minimal and you would have to use gallons of the toxic stuff to counteract a periodic emergence. If you simply cannot tolerate the noise, use it as an excuse to spend a week in Hawaii.
Cicadas are eaten in many parts of the world. In China, nymphs are deep-fried. Supposedly, they are pretty tasty when dipped in chocolate. I don’t think I’m ready for that just yet. The French onion crickets were about as far as I feel comfortable.
If you have a citrus tree, you may want to take a closer look for signs of purple scale in late spring and early summer, and again in autumn.
Purple scale insects suck the sap from citrus trees and they look like miniature mussels. Also known as mussel scale, orange scale, and comma scale, purple scale (Lepidosaphes beckii) are a type of armored scale that attach to banana, bay laurel, citrus, fig, mango, pear, and stone fruit trees and grapevines. These pests have several ornamental hosts that bear inspection:
Purple scale damage
Like other scale insects, purple scale feed predominantly on leaves and young fruit, but they will attack all parts of a tree. You may see tiny yellow halos on the leaves. Purple scale feeding can weaken branches, disfigure fruit, and reduce productivity. Heavy feeding can lead to defoliation and twig dieback. In extreme cases, the tree can die.
Populations of purple scale are usually low and found mostly in coastal regions, but mild temperatures, high humidity, and overcast skies can provide the conditions needed for a population explosion. You can find these pests in the cooler, shaded areas of trees.
Purple scale identification
Purple scale insects aren’t actually purple. That sure would make them easier to find! Instead, they are dark brown and may have tan edges with a slight purplish tinge.
Male purple scales are smaller and narrower than females. In both cases, you may see a slight bend at the narrow end of the scale. Male purple scales are 1/10” long and females are slightly larger. Male purple scale insects are tiny and they fly around in search of immobile females.
Purple scale lifecycle
Females lay 40 to 80 eggs under their protective covers. After the eggs hatch, crawlers emerge and scuttle to nearby fruit, leaves, and twigs, where they begin forming their own covers. At first, purple scale crawlers are covered with a mass of waxy threads. When they are about half-grown, their purplish-brown covers begins to develop. Once a female attaches herself, she stays put.
Purple scale management
Temperatures above 80°F are hard on purple scale, so their numbers are greatly reduced by the time summer is in full swing. But a second generation may appear in autumn and, in some years, a third generation may occur before winter cold brings them to a halt.
Natural predators, such as parasitic wasps, twicestabbed lady beetles, and Australian lady beetles, take a big bite out of these pests. We can help them do their job by avoiding broad-spectrum pesticides and applying sticky barriers to the trunks of trees. Argentine ants are known to protect scale insects, so sticky barriers remove that protection.
Purple scale prefer dusty conditions, so giving your citrus trees a quick shower with the hose can help.
Rather than steamy backseat interludes, frenching describes the way leaves can become discolored or distorted.
Frenching most commonly occurs in cotton and tobacco but may also occur in citrus, sorrel, squash, and tomatoes. Peppers seem exempt from this condition, but no one knows why.
Is frenching a disease?
Well, yes, and no, and maybe. Since botanists do not entirely understand frenching, its causes are currently called frenching factors. These factors include fungal diseases, insufficient iron, poor drainage, alkaline soils, and temperatures above 95°F. Frenching is also more likely when plants are grown in soil that stays moist during drought conditions.
In some cases, specific bacteria commonly found in the soil, such as Bacillus cereus and Macrophomina phaseolina, the latter being one cause of damping off disease, cause frenching.
How to identify frenching
Leaves can turn the wrong color or take on an abnormal shape for many reasons, but frenching has some consistent characteristics:
*Apical dominance refers to some plants’ natural tendency to have one main shoot that actively inhibits the growth of others (think Christmas tree).
These symptoms can be mistaken for aster yellows, except the roots are not involved. Frenching, regardless of its cause, starts as tiny pinheads of chlorosis. New leaves are narrower with wavy edges. As these leaves grow, only the midrib gets longer, pulling the leaves into strap-like shapes that look more like stiff strings than leaves.
Scientists have found that autoclaving soil eliminates the frenching effect, but I’ll bet none of us have that option. There isn’t much we can do about the weather, either, but there are things we can do that will reduce the chance of frenching in our home gardens. Those actions include improving drainage, monitoring and maintaining proper soil pH, and feeding and irrigating plants on a regular schedule.
Today, we are learning about the Krebs cycle. Wait! Come back!
The Krebs cycle is how plants get energy from their food. Also known as the citric acid cycle, it won’t tell you how to grow bigger tomatoes or sweeter melons, but understanding the Krebs cycle gives you a better understanding of how plants grow and what they go through to make all those delicious things we eat.
We’ve already discussed the nitrogen cycle, the carbon cycle, and the water cycle. Those cycles describe how things needed by plants become available (and unavailable) as they change forms, moving through the environment.
We’ve also discussed the Calvin cycle, which is part of photosynthesis. In the Calvin cycle, several anaerobic chemical reactions occur that allow plants to transform the carbon from CO2 into sugars. After those sugars are formed, the Krebs cycle begins.
Messrs. Krebs and Johnson
In 1937, a man by the name of Albert Szent-Györgyi was studying pigeon muscles. Stay with me, now! He received a Nobel Prize for his efforts, part of which was discovering several aspects of what later became known as the citric acid cycle. His discoveries were taken further that same year by Hans Adolf Krebs and William Arthur Johnson. [I guess they decided the Johnson cycle didn’t sound as impressive.] Krebs received the Nobel Prize for Physiology in 1953 for that work. [I don’t know why Johnson didn’t. Maybe he was too difficult to work with. I don’t know.] The Krebs cycle is part of the aerobic respiration process plants use to produce usable energy.
Respiration, not what I expected
When I hear “aerobic respiration” I start thinking about increased heart rates and time spent on a treadmill, but that’s not it. Aerobic respiration refers to one way that usable energy is formed. That usable energy is adenosine triphosphate or ADT. ATP is a molecule that carries usable energy around within a cell, taking it wherever it is needed. ADT is created when food (sugar) is burned with oxygen. I don’t know how plants burn anything but I keep getting images of little campfires in plant cells, so that’s kinda fun.
Sugar as plant food
I always thought plants simply drank sugar solution as food, but that’s not exactly how it works. Let’s put our steampunk magnifiers on and see what’s really going.
How plants convert sugar into energy is far more complex and amazing than my little campfire, especially when you think of the scale at which all of this is happening. Plant cells are 10 to 100 micrometers across, depending on the species. This means you could put approximately 180 to 1,800 plant cells across the top of an American dime. [Human and animal cells are even smaller and they do similarly impressive things.] Now back to converting sugar.
To start, you need to know that sugar molecules are relatively large. To use them, plant enzymes go to work, breaking down those acidic glucose molecules into energy and pyruvic acid. That process is called glycolysis. More reactions occur, breaking the remaining bits down into carbon dioxide. As energy is released through these processes, it is moved to other molecules, called electron carriers. Electron carriers are like tiny cargo trucks. They form a chain called the electron transport chain. This transport chain creates ATP. As a side benefit, the processes also create the building blocks of several other molecules, such as amino acids.
For perspective, let’s say that a plant cell is the size of Arizona and the mitochondria are Phoenix. Arizona has a railway system that transports food. All the food comes from Texas and is too big to be packaged. This giant-sized food is taken to Phoenix where it goes through a 9-step process that converts the giant food into snack-sized portions that are small enough to fit on the trains. The trains take the food wherever it is needed. Put simply, that’s the Krebs cycle.
But what does all this have to do with your garden?
The bottom line, healthy plants produce bigger, tastier crops with less effort on our part. They are better able to defend themselves against pests, diseases, and weather extremes. All you have to do is make sure they get the right amount of water regularly and that the soil they are growing in contains the correct range of nutrients.
Have you had your soil tested yet? What did you learn?
The water cycle is a global phenomenon, but something similar is happening in your garden.
In its simplest terms, the water cycle describes how water moves around on the Earth’s surface. It might be moving very slowly as ice in a glacier, whipping around on the wind as a gas, or flowing across the soil surface to water your bean plants. Regardless of its form, the mass of water on Earth stays relatively unchanged. The same is not true of your garden.
Unless you water exclusively from a rain barrel, the water for your garden probably comes from a spigot. If you’ve ever experienced a water main break or a flood, you know just how devastating too much water can be.
Let’s compare the basic aspects of the water cycle from global and gardening perspectives.
The water cycle starts when the sun heats the surface of the planet. Water on the surface is converted to a gas that rises into the atmosphere. The soil of your garden, your driveway and house, and the concrete of your patio also act as heat islands, absorbing the sun’s heat and causing nearby water to evaporate. This is why it’s a good idea to give potted plants sitting on a concrete patio feet that raise them off the surface enough to allow air to flow. This will keep them cooler and reduce the amount of water they need.
Water held within plants is also released. As they perform photosynthesis, gas exchanges must occur. Water is released into the atmosphere as this happens. The combination of evaporation by heat and transpiration by plants is called evapotranspiration.
As water vapor moves around in the atmosphere, the bits of water vapor bump into each other and grab on. Those clusters of water vapor keep getting bigger, condensing into fog, mist, and clouds. If you’ve ever been to Disney World in August, you know exactly what I mean.
Some plants thrive in misty, foggy, humid environments. Others end up with fungal diseases. If humidity is causing problems in your garden, be sure to prune your plants in ways that provide good airflow.
Water falling to the ground is precipitation. Globally, precipitation may mean rain, snow, hail, or sleet. In your garden, that precipitation may be rain, sprinklers, or a garden hose. As precipitation passes through the atmosphere, it collects particles along the way. Those particles are often car fumes, factory pollution, and dust, and those particles reach your soil right along with the water.
Water passing from the surface into the ground is called infiltration. Once the water has been absorbed by the soil, it is known as groundwater or soil moisture. Just because water is in the ground does not mean it is available to plants. Compacted soil has poor infiltration rates. And water does not spread smoothly through the soil in all directions.
A fellow gardener called me with a zucchini problem. Her plant kept wilting, even though she was watering it regularly. I asked her if the water was reaching the roots. The phone was silent for a moment before she replied, saying she thought so. I had her go check. She called me back a few minutes later astounded. Even though she had been laying the hose next to her zucchini plant, the water was all veering away in another direction once it infiltrated the soil. I had her create an irrigation ring around her zucchini and it grew much better.
Speaking of growing well, infiltration is how minerals get in the water that feeds our plants. As water enters the soil, some minerals are caught in solution. That solution is then absorbed by plants to provide both food and water. Now, some plant nutrients are more mobile than others. Immobile nutrients aren’t actually immobile, they just need a lot more water to move around. Blossom end rot is an example of irregular irrigation causing what looks like a nutrient deficiency. Most gardens on the West Coast of the US have abundant calcium, but may not be watered regularly enough. [Of course, a lab-based soil test is the only way you can learn what’s really in your soil.]
Water moving across the land is called runoff. Surface runoff, or urban drool, is wasteful and potentially dangerous. As suburban sprinklers miss their mark and water streets and sidewalks instead of lawns, they also carry oil, excess fertilizer, trash, and pollutants into waterways. Please make sure your sprinklers water where you think they do and adjust them as needed.
As you can see, the water cycle is a series of steps that pass through various filters along the way. I once had a student create a variety of natural filtering systems for a science project. She used rocks, leaves, sand, and soil in separate containers to filter muddy water. Which material do you think did the best job of cleaning the water?
I thought it would be the rocks, but I was wrong.
It was the soil.
You can grow a surprising amount of food in your own yard. Ask me how!
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