Hamutahl Cohen – The Garden Professors™

Syrphid flies have immense pest control vaue

by Hamutahl Cohen

Recently an avocado grower in Fillmore sent me a photo of a wrinkled maggot-like larvae, followed by a large question mark. It looked just like a syrphid fly, what we commonly call a hoverfly or flower fly. But this fly maggot was a bright, colorful orange, not the green syrphid larvae I am accustomed to seeing. After consulting with my colleagues, I learned that syrphids actually come in a dramatic range of colors and shapes. I was excited to learn that there are over 6,000 species of these flies worldwide. There are about 300 species in California (where I’m from) including multiple species in the genera Allograpta, Metasyrphus, Paragus, Scaeva, Sphaerophoria, Syrphus, and Toxomerus. After delving into the ecology, behavior, and importance of syrphids, I’ve come to believe they are an underappreciated workhorse in citrus, avocado, and other crops. Syrphid flies not only pollinate flowers, but they also play an outsized role in biological pest control, even combatting the Asian citrus psyllid (ACP).

First, a little about the syrphid life cycle. These flies are remarkably adaptable to different conditions, and they inhabit diverse environments, including agricultural fields, grasslands, local parks, and urban gardens. Syrphids have four life stages: egg, larva, pupa, and adult. After hatching from an egg, larvae will develop through three instar stages. They then pupate on host plants or on the ground. Pupae generally require damp, humid environments to mature into adults, and likely benefit from litter, mulch, and biodiverse topsoils. The syrphid life cycle, from egg to adult, takes commonly 2-4 weeks, depending on the weather. They have 5 to 7 generations per year and are present throughout the growing season. The syrphid fly larvae vary in size from 4 to 18 mm in length, and can be green, white, brown, orange or yellow. Adult flies vary in length from to 3 to 13 mm, depending on the species. They have brown or black bodies and are marked with bright colored yellow and white stripes and spots that cover the abdomen and thorax. Because of this striping, they are sometimes mistaken for wasps or bees. This resemblance is a form of mimicry that syrphids have evolved to ward off predators. One tip for telling apart syrphids from bees is that syrphids have a single pair of wings, like all flies, whereas bees and wasps have two pairs of wings. Syprhids also tend to have shorter antennae and less hair than many bees.

Although we generally think of bees as our key pollinators, syrphid flies contribute significantly to pollination, visiting about 70% of food crop species and providing pollen transfer. Although an individual syrphid fly is unlikely to transport as much pollen as an individual bee, syrphids and other flies are so ubiquitous and abundant that en mass they play a crucial role in moving pollen between flowers. Some syrphid species also migrate over hundreds of miles by surfing on high altitude air currents, carrying pollen grains over distances which bees are not capable of flying. For some crops such as avocado, their efficiency as pollinators may rival that of bees –we just started a research trial this year to determine just exactly what their contribution to pollination is relative to honey bees.

Syrphids also play a role in controlling pests. Unlike adults, which feed on pollen and nectar, the larvael stage of many species feed on other insects, including aphids, caterpillars, thrips, mealybugs, leafhoppers, and other sap-feeding insects. Syrphid fly larvae are active year-round in Southern California, and each larvae can consume up to 400 aphids during development. When larvae are abundant, they have been shown to reduce aphid populations by up to 70% or more. Another notable example the syrphid fly’s role in pest control is its impact on the Asian citrus psyllid (ACP, Diaphorina citri), a notorious pest that poses a significant threat to citrus crops worldwide due to its ability to transmit citrus greening disease (Huanglongbing). Syrphid fly larvae are efficient predators of ACP nymphs, exerting significant mortality pressure on pest populations and reducing the spread of citrus greening disease. In the laboratory study by Dr. Mark Hoddle at UC Riverside, individual syrphid larvae consumed, on average, 421 ACP nymphs each. Syrphids, as part of an IPM program, help stem ACP outbreaks.

To attract syrphid flies, gardeners can provide them with flowers. However not all flowering resources benefit syrphids equally. UC Riverside research by Dr. Hoddle has shown that alyssum and buckwheat attract syrphids in high numbers. The small white flowers of alyssum and buckwheat are also attractive to other predators and parasitoids, making them excellent choices for insectary plantings for biological control.

Have you seen syrphid flies in your garden? What has been your experience with syrphids and pest control?

The very hungry caterpillar in my garden: Disclisioprocta stellata

The bougainvillea is my front yard looks really sad right now. The leaves exhibit scalloped, hole-y edges, something has definitely been munching on them. I suspect they are the appetizer, main course, and dessert for a very hungry bougainvillea looper, Disclisioprocta stellata.

D. stellata is a smooth-looking, yellow-green or brown caterpillar (the perfect colors to help it camouflage and evade predators). It is about an inch long. Loopers, also known as inchworms, belong to the family Geometridae. The “geometers” are earth-measurers. They move by arching and stretching, giving the impression that they are measuring their journey. This specific species is a world traveler, and can be found in sub-Saharan Africa, the islands of the Indian Ocean, eastern Canada, the United States, Mexico, Brazil, Hawaii, and other destinations.

Although D. stellata seems to really love my bougainvillea, it is not a picky eater. As a polyphagous pest, it feeds on many different hosts, including ornamentals like lantana and malvas (such as Abutilon and Hibiscus) and crop plants like guava and citrus. Polyphagous pests are often more difficult to control and manage than those pests that eat just one plant type. They are sometimes able to evade different control interventions by switching hosts. They also harbor more complex digestive systems to handle the many types of plant toxins that they naturally encounter in their diets, and can sometimes leverage their diverse metabolic pathways to breakdown and withstand pesticides. The good news about this insect is that it won’t kill my plant, which is otherwise healthy and should withstand the damage. I assume that the coming cooler weather will slow down the looper and give the plant a chance to recover.

My best bet for controlling this looper in my bougainvillea is to deal with this issue….yesterday. Perhaps in late summer when I first started noticing damage would’ve been a good time. Regularly monitoring your garden plants lets you catch issues early and head them off. But I didn’t do that, and now I have a lot of bugs and a lot of damage. Hand removal is a good choice for gardeners, so maybe on my lunch break I’ll go out with a bucket filled with soapy water and drop them in there. Or maybe I won’t – I like how the adults look, and I don’t mind a bit of aesthetic damage to my ornamentals. Maybe we can share.

For other control strategies, including organic chemical options, check out this article.

An adult bougainvillea looper. (c) Thomas Shahan – some rights reserved (CC BY-NC)

https://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/bougainloop.html

New study says that pollinators need more than a token corner of habitat

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.

Insect spotlight: the marigold fruit fly, Trupanea vicina

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. Photo taken at Elizabeth Gamble Garden in Palo Alto in 2015 by (c) selwynq some rights reserved

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.

To Neem or Not to Neem, That Is the Question

by Hamutahl Cohen

You’ve probably seen Neem oil recommended in blogs, gardening forums, and on the shelf at your local gardening store. Neem is derived from the seeds of the Azadirachta indica tree, and is one type of horticultural oil that is used by gardeners looking for alternatives to synthetic insecticides. But is it effective? Is it benign? This post explores the pros and cons behind neem and other horticultural oils.

What Are Horticultural Oils?

Horticultural oils are either plant-based (like neem, canola, or clove oil) or mineral-based (refined petroleum products), and they work mostly by smothering soft-bodied pests like aphids, scale, and whiteflies.

Some plant-based oils do contain chemical compounds that can do more than smoother —neem oil contains azadirachtin, which disrupts insect development and feeding behavior.

Horticultural oils are typically considered low-toxicity for humans, and can be used on a very wide range of plants, including vegetables, fruits, ornamentals, and houseplants.

The PROS

Broad-Spectrum Neem oil is effective against a range of insect pests and some fungal diseases, yet remains relatively safe for humans, pets, and other animals. According to the Environmental Protection Agency neem oil “has been shown to have minimal impact on non-target organisms” (EPA, 2012) such as birds and mammals.
Reduced Resistance Potential
Unlike synthetic insecticides that often target a specific physiological pathway (and thus promote resistance over time), neem oil affects multiple aspects of insect development, which makes resistance less likely to develop quickly.
Organic-compatible and easy-to-apply
Neem and other horticultural oils are generally approved for organic gardening. interventions. They are easy for home gardeners to use since you can spray from a store-bought bottle and avoid any special equipment.
Low Residual Activity
These oils break down quickly in sunlight and soil, reducing long-term environmental contamination and residue on edible crops.

The Cons

Phytotoxicity Risk
If you use oils in high temperature or direct sunlight, it can lead to leaf burn and plant damage. Apply it early in the morning or late in the day to minimize this risk.
Non-Selective Action
Neem and other oils can still harm beneficial insects if sprayed directly. Lady beetles, lacewings, and bees can be affected by fresh residues. You can time your application to avoid flowering periods, or spray during the evening when bees are less active to avoid non-target impacts.
Repeat applications sometimes required
The effects of oils can take days to manifest and may require repeated applications (e.g. every 10 days) for best results. Gardeners expecting immediate eradication may be disappointed. Oils often work best in conjuction with other control strategies (e.g. pruning out infested areas, releasing beneficials, etc.)
Storage and Shelf Life
Oils can degrade over time, especially when exposed to light and heat. They can go rancid or lose efficaciousness. Check the expiration date on your bottle and store in a cool, dark place.

Concluding Thoughts

I like horticultural oils. They can be effective tools and are safe for people to use. But they need to be used with some consideration, particularly timing to avoid non-target impacts to beneficial insects and leaf burn. Let me know your experience with oils.

References

EPA. (2012). Neem Oil; Exemption from the Requirement of a Tolerance. Federal Register. https://www.federalregister.gov/documents/2012/05/31/2012-13143/neem-oil-exemption-from-the-requirement-of-a-tolerance

Bee Hotels: A Closer Look at Unintended Consequences

Bee hotels have become popular additions to gardens, designed to support wild bees by providing them with nesting sites. Solitary bees, unlike honey bees, live in natural and man-made cavities which can be easily provided with nesting habitats. A previously published Garden Professors blog offers valuable insights into creating artificial nesting structures for these bees, emphasizing the importance of proper design and placement. However, if you’re thinking about installing a bee hotel, I’d urge you to reconsider – some studies suggest that bee hotels, if not correctly maintained, can inadvertently harm the very pollinators they’re meant to help.

While bee hotels offer nesting opportunities, there is almost no research showing that they have a positive effect on bees. Some researchers also think bee hotels can become hotspots for parasites and pathogens (MacIvor & Packer 2015). High-density nesting sites can facilitate the spread of diseases, similar to how bird feeders can become transmission points for avian illnesses. Bees are particularly vulnerable to viruses, microsporidians, and fungal agents which can spread via exposure to feces or even through pollen left behind by bee visitors. Bee hotels can also attract bee predators – nesting aggregations of wild bees that are artificially close together might be attractive to parasitic wasps which infiltrate nests, laying their eggs inside and jeopardizing the bee larvae.

Bee hotels may still have their place, especially in community gardens where they can serve as a point of conversation and provide beauty and interest as a form of garden art. However, given the risk for disease spread, here are some tips for maintaining a bee hotel.

Best Practices for Bee Hotel Maintenance

To ensure bee hotels remain beneficial:

Annual Cleaning: After bees have emerged in the spring, clean the hotel thoroughly. Remove and replace any natural reeds or paper straws. For wooden blocks, use a thin bottle brush or compressed air to clean out debris.
Use Removable Nesting Materials: Opt for bee hotels with removable tubes or liners. This design facilitates easier cleaning and reduces the risk of disease buildup.
Proper Design: Ensure that nesting holes are closed at one end to prevent parasites from accessing the nests from behind.
Limit Nest Density: Avoid overcrowding by limiting the number of nesting tubes. A lower density reduces the chances of disease and parasite spread
Create Habitat: Leave undisturbed, unmulched areas in the borders and corners of your garden so that bees can nest naturally in the ground. Some bees also nest in dead twigs and hollow stems and branches, so consider leaving some behind for them.

Do you have a bee hotel in your garden? What has been your experience with them?

References:

MacIvor, J. S., & Packer, L. (2015). ‘Bee hotels’ as tools for native pollinator conservation: a premature verdict?. PloS one, 10(3), e0122126.

Introducing Hamutahl Cohen: from bug-phobic to bug-lover

I wish I could say I grew up with an innate fascination for the insect world—that I was one of those kids who spent hours flipping over rocks to marvel at beetles and ants. But the truth is, growing up in urban Los Angeles, I rarely interacted with nature at all. And insects? I was terrified of them. I was the last person to volunteer for anything involving creepy crawlies.

That all changed in college. I enrolled in an introductory environmental science class, and one guest lecture changed the course of my life. A campus professor gave a talk on agroecology, describing how their work wove together science, practice, and social movements to promote sustainable pest management. I was captivated. Here was a discipline that tackled food production, ecological health, and community well-being—all at once. It felt like a bold, grounded approach to conservation—not one that isolates nature in national parks, but one that weaves ecological thinking into everyday landscapes like farms and gardens.

I emailed him that very day, requesting an internship—and he agreed. My first day on the job found me in a vineyard in the Central Valley, shaking grapevines over a beating sheet, pretending I wasn’t afraid as a swarm of insects fell onto the canvas. The graduate students in the lab patiently taught me how to tell wasps from bees, bugs from beetles, and moths from butterflies. They also introduced me to the foundational concepts of biological and cultural control—how pests can be managed using natural enemies and sustainable farming practices. Later that year, one of those graduate students asked if I’d be willing to host one of his beehives in my backyard. I instantly fell in love with those bees—their complex, maternal society, their admirable work ethic, and the delicious honey they produced. That hive sealed it for me: I was going to become an entomologist.

After graduating, I apprenticed at the UC Santa Cruz Center for Agroecology and Sustainable Food Systems, where I sharpened my skills in horticulture, including irrigation, nutrient management, composting, greenhouse production, and pest management. I worked across a diverse range of crops, from annual vegetables to ornamentals and perennial fruit trees. During my Ph.D. and postdoctoral work, I focused on how farm and garden management practices influence wild bees and their susceptibility to the parasites and pathogens implicated in pollinator declines. My research sits at the intersection of agriculture and ecology, and I’ve always believed that productive farming and environmental stewardship should go hand in hand.

Now, I serve as an Extension Advisor with the University of California, based in Ventura County. As an entomologist, my primarily focus is on integrated pest management of insect pests, including identification and monitoring, developing cultural and biological control methods, and evaluating new chemical control products. One of the best parts of my job is the incredible variety of crops I get to work with, including avocado, citrus, strawberries, cole crops, and greenhouse ornamentals – working with so many different commodities means that I’m constantly learning. In my work I aim to provide science-based, practical recommendations that help farmers, pest control advisors, and others in the industry to effectively manage pests while safeguarding beneficial insects and human health. For example, I’m currently leading several projects that explore how small-scale habitat enhancements—like hedgerows or cover crops—can improve pest control and support wild pollinators in citrus orchards.

I didn’t set out to become an entomologist. But through a mix of chance, community, and unexpected inspiration, I’ve found myself surrounded by insects—not in fear, but in deep fascination and appreciation.