Author: Hamutahl Cohen

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.

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.

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.

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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

1. 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.
2. 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.
3. 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.
4. Low Residual Activity
   These oils break down quickly in sunlight and soil, reducing long-term environmental contamination and residue on edible crops.

The Cons

1. 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.
2. 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.
3. 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.)
4. 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.

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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