I wanted to share a new study that came out this week in the journal Science. We generally agree how important bees, butterflies, and other pollinators are, not only for our crops but for the health of ecosystems as a whole. Yet, despite decades of awareness, pollinator numbers continue to decline worldwide. Dr. Gabriella Bishop used a meta-data approach in this study to examine why pollinators are struggling. The study concluded that current conservation targets for habitat area are simply not large enough.
The project combined datasets from 59 studies across 19 countries to measure how populations of wild bees, bumble bees, hoverflies, and butterflies respond to natural habitat around farms. I was fortunate to contribute some of my previously collected data on pollinator communities in the California Central Valley and be included as an author. Overall, by combining datasets, Dr. Bishop had information on 178,000 insects collected from 1,200 field sites, which allowed for the calculation of the minimum amount of habitat each group needs.
Many global and regional policies, like the EU Biodiversity Strategy, suggest setting aside about 10 percent of farmland as habitat for pollinators. But Dr. Bishop found that pollinators need far more space to thrive. The minimums vary group, with solitary bees needing ~16% in temperate areas and ~38% in the tropics. Butterflies needed ~37%. Hoverflies were more flexible, with the lowest threshold for habitat requirements at ~6%). Overall, this means hedgerows, woodlots, meadows, and wildflower-rich grasslands must make up a much larger share of the landscape if we want to halt pollinator declines. And critically, these areas need to be managed for the long haul. Short-term fixes such as seasonal wildflower strips can provide temporary boosts, but they do not sustain pollinator populations in the long run.
For gardeners and land stewards, the message is clear. Every patch of habitat counts, but scale does in fact matter. Planting flowers that bloom across the seasons is important, but so is maintaining semi-natural spaces for decades, not just years. If we want future generations to enjoy these insects, we must think BIGGER.
In my household, the weather is a common subject of conversation. That is only partly because I am married to a meteorologist. In fact, I have noticed that I can talk to almost anyone about the weather, and I suspect you can too. Weather is most captivating when something interesting is occurring, like liquid falling from the sky. When I give talks to master gardener groups, they are almost always consumed with how the weather is affecting their gardens. I get more questions about drought from these groups than almost any other topic, but rainfall, past or future, is also a frequent subject.
Today we will look at different types of precipitation, how they are formed, and how they affect garden plants. This is especially important as we move from summer, when rain is the most plentiful hydrometeor, into fall, when freezing rain, sleet, and snow become more frequent. Of course, this depends on where you are, and some Northern Hemisphere southern areas will experience almost no snow while for northern regions, it is the majority of what is observed.
When I started writing my post, I looked online to see how many different types of precipitation were listed. I found that there is quite a variety in the number of types listed, ranging from two to seven varieties. I am going to lump them into two basic categories: liquid and frozen. But there are subcategories within these two basic buckets, especially in frozen precipitation, which includes snow, sleet, ice pellets or graupel, freezing rain, and hail. We will see how they are related to each other and yet distinct.
Rain is essentially liquid water that is falling from the sky and hitting the surface. If the air is dry enough, rain that develops in clouds evaporates before it gets to the ground, and that is called virga. Raindrops can form in tropical clouds through a liquid process that involves the collision and coalescence of small water droplets into larger drops that get heavy enough that they cannot remain suspended in the air and fall to the ground. But you might be surprised to know that most liquid rain starts as snow high up in the clouds, where the temperature higher up in the atmosphere is below freezing. We’ve addressed some characteristics of rain and how they affect gardens in previous posts here and here.
Clouds (except those in the deep tropics) are made up of a mix of supercooled water droplets and ice crystals that float around together, but it is easier for the ice crystals to grow by sucking up water vapor than for the water droplets to grow, so the ice crystals usually dominate the process of producing precipitation and snowflakes are the result. As these snowflakes fall towards the ground, they fall into warmer air near the surface and melt into the liquid water drops that make us wet. A light rain with small water droplets may be just a drizzle, but a thunderstorm with a cloud that is 10 miles deep could have water droplets as large as 0.34 inches, although I have heard unconfirmed stories about raindrops more than half an inch across in the heaviest rain events.
What types of frozen precipitation are there?
When precipitation freezes, it can take different forms depending on the temperature of the air that it falls through. If the air that the original snowflake falls through is below freezing through the entire depth of the atmosphere, then it remains as snow all the way to the ground. The shape of the snowflake depends on the combination of temperature and humidity that is present where the snowflake forms. They can take on an amazing variety of shapes beyond the typical dendrite that we usually associate with snow. The dendrites we often see falling in winter are ideal for blanketing the ground with a carpet of white, and their shapes make them able to trap a lot of air in the snow cover, providing insulation for the soil that keeps the temperature there relatively warm by protecting it from the cold and dry air the snow is falling through. In other words, it acts as a mulch to protect the ground and the plants there.
If the snowflakes from aloft fall through air that is above freezing, they transform from snowflakes to other kinds of precipitation, as shown in the diagram below. If the warm layer is deep, all of the snow changes back to liquid raindrops. If the air is above freezing but the surface is below freezing, then freezing rain occurs. The water sticks to trees, wires, and buildings, adding to their weight and collapsing them if the accumulations are thick enough. If the warm layer is relatively deep, but the air above the surface is below freezing, the water droplets may refreeze, leading to sleet or clear ice pellets. If the layer is thinner, the snowflakes may develop a coating of liquid water that surrounds the snow crystals from supercooled water in the clouds, encasing them in ice, leading to snow pellets or graupel, which are opaque instead of clear like sleet.
Hail is another variety of frozen precipitation that forms in summer thunderstorms, which can be as much as 10 miles high. In those storms, snowflakes can circulate vertically inside the storms due to strong updrafts, gathering a new coating of ice each time they move upward through the clouds, growing larger each time they cycle up and down. The largest hailstone ever measured was 8.0 inches in diameter and weighed 1.9375 pounds. It was discovered in Vivian, South Dakota on July 23, 2010. You will often find layers of clear and cloudy ice in hailstones as they travel up and down through the thunderstorms. Hail that is much smaller, as little as 0.25 inches, can cause damage to plants and crops as the hailstones shred leaves and cause damage to fruits and vegetables, leaving them unsightly and vulnerable to mold and pests.
How do different types of precipitation affect garden plants?
The most damaging types of precipitation for gardens are those that add weight to plants or cause impact damage when they hit something. Heavy snow and freezing rain can cause tree limbs to break due to increased weight, which trees cannot withstand, especially when they are still leaf-covered in fall or when they are not well maintained and have weak points in their structure. Garden plants can also be flattened or otherwise damaged by the heavy ice or snow cover. Previous advice in the Garden Professors by Linda indicates if the snow is really heavy, it should be removed from the plants before it can do damage, although lighter amounts can remain. On the other hand, snow cover on the ground can be a benefit to your garden if it incorporates enough air to serve as an insulating layer between the plants near the ground and the colder and drier air above it.
How else can precipitation help gardeners with their gardens?
A previous post on GP noted that observing your garden after a heavy rain can be helpful in determining what the drainage patterns are and what might need to be addressed. While you might not be able to do much work in your garden right after a heavy rain event, it provides an excellent time to make future plans to make your garden more weather-proof.
Rain and snow, along with the other varieties of precipitation we experience, provide valuable moisture to our gardens as well as protection to plants in winter, but can also produce damage that can harm our garden plants. Enjoy the rain (or snow) when it comes, but be aware of the negative impacts it can have on your garden, too.
I thought it would be fun to periodically highlight some insects that are understudied or lesser-known. Today’s insect spotlight is on the marigold fruit fly, Trupanea vicina. If you grow marigolds in your garden, you might find this fruit fly or it’s larvae in your flowers. One of its most striking features is the bold, patterned wings that is has, I think the venation almost resembles shattered glass. This is a fly in the tephritid fruit fly family, a large group of flies that often specialize on flowers and seeds. There are over 4,000 species in this family of fruit flies and there are likely many more undiscovered ones. Flies in this group might be confused with kitchen fruit flies, which belong to Drosophilidae family and are usually quite small. Tephritids are larger and often have striking wing patterns which are used during courtship or to ward off predators. The group includes important agricultural pests such as Mexican fruit flies olive fruit fly.
Marigold fruit fly adults are about 4–5 mm long with banded or spotted wings. Research suggests that T. vicina primarily develops in marigolds, where the larvae feed inside flower heads (Foote et al. 1993). We don’t know to what extent it will use other host species of asters, though tephritids tend to be specialized with very close relationships to their preferred host plant. So far the species has been observed in California, Arizona, Mexico, and other parts of Central America, though its full distribution has not been systematically mapped, and I would be curious to know if you’ve seen it in any other region.
There’s a lot we don’t know about the fruit fly. While this is a pest of low concern, it’s unclear how much damage it causes to marigolds. The larvae do consume developing seeds, but we are unsure if this always reduces the quality of flower or only in cases of extreme infestation. This is the first year that I am getting reports of marigold fruit fly being an issue in home gardens in Southern California. Have you experienced it before?
References
Foote, R. H., F. L. Blanc, and A. L. Norrbom. 1993. Handbook of the Fruit Flies (Diptera: Tephritidae) of America North of Mexico. Cornell University Press, Ithaca, NY.
