One can hardly glance at the news recently without noticing an item about the health of bees and other pollinators. We can all agree on the importance pollinators play in the health of our planet and the critical role honey bees and bumble bees play in agriculture. There is no doubt that populations of honey bees in particular have been in decline over the last several years. The multiple reasons for the decline are not as clear. This article will address the role that fungicides may play in bee health.
There are many possible reasons for the decline of bee populations. Pesticides have been implicated in bee declines. Most experts would agree pesticides may play a role in bee population declines. The type of pesticide that is most often implicated in bee declines are the insecticides. This makes sense: bees are insects. There is less known about the role that fungicides may play in bee health. Below I will discuss what is known about fungicides and bee health and what cucurbit growers may do to mitigate the role fungicides might play in bee declines.
In Table 1, select pesticides that may be used in cucurbit production are listed along with possible toxicity to honey bees. Precautions are listed for honey bees based on a table from the University of California. Each pesticide is ranked as either a I, II or III. The meaning of these designations is listed below.
I=Do not apply or allow to drift to plants that are flowering
II=Do not apply or allow to drift to plants that are flowering, except when the application is made between sunset and midnight if allowed by the pesticide label and regulations.
III=No bee precaution, except when required by the pesticide label or regulations.
Other precautions listed are: may be toxic to honey bee broods (young) and may be toxic to other bee species.
Note that most insecticides are rated as more toxic to honey bees than most fungicides. Insecticides are evenly split between I’s and II’s. Fungicides are split between II’s and III’s. All but two of the fungicides that are ranked II have interactions with insecticides that may cause health problems to bees. Below are listed examples of pesticides that may have toxicity to bees as described in Table 1.
- Pristine® is listed as having possible toxicity to bees if applied with Lorsban® (chlorpyrifos). However, Lorsban® is not labeled for cucurbit production.
- Chlorothalonil maybe toxic to bees if applied with pyrethroid insecticides. These insecticides include Pounce® and Warrior®. Therefore, do not apply a chlorothalonil product (e.g., Bravo® Equus®, Echo®, Initiate®) with a pyrethroid product except between sunset and midnight.
- Fungicides in FRAC group 3, may increase toxicity of certain insecticides in the groups listed. See the mode of action column to find examples of fungicides in FRAC group 3. Examples of insecticides common to cucurbit production that may have increased bee toxicity when applied with FRAC group 3 fungicides include Admire®, Platinum® and Assail®.
- Copper products with the active ingredients copper hydroxide or copper oxychloride may be toxic to bees and have a II restriction presumably because of the innate toxicity of copper.
In addition to the synergistic effects described above, fungicides have been shown to be harmful on their own, especially chlorothalonil. For example, honey bees culture the pollen they collect and bring back to the hive, with help from a variety of microbes, and turn it into a substance called bee bread. The presence of fungicide residues in the hive decrease the nutritional quality of the bee bread and impair the ability of bees to absorb nutrients from their food source. Chlorothalonil has also been associated with reduced colony growth in the commercially available bumble bee, Bombus impatiens, and increases the prevalence of the parasites Nosema ceranae in honeybees and N. bombi in multiple species of bumble bees.
The effect of fungicides and bee health is still not well understood. The table and discussion presented here indicate that the fungicides are more likely to be toxic when applied in a mixture with certain insecticides. This information should help cucurbit growers avoid practices that may negatively impact bee health and choose active ingredients that may be less detrimental to these communities. Additional measures that may help to maintain bee health are listed below.
- Read the label of each pesticide carefully before application. Some labels include specific information about bee safety.
- Scout fields for insect pests and apply an insecticide only when the economic threshold has been reached.
- Before making a fungicide application in response to a specific disease symptom, make certain of the identification of the disease, with an official diagnosis if necessary.
- Use disease forecasting programs such as MELCAST when appropriate.
The health of pollinators, whether bumble bees and honey bees or native bees, is everyone’s business. Although it is not always easy to know what to do to help bee health, this article will hopefully provide some answers.
Table 1: Possible toxicity of select fungicides and insecticides used in cucurbit production taken from the University of California < http://www2.ipm.ucanr.edu/beeprecaution/#>
| Trade name | Common name | Pesticide type | Mode of actionz | Ratingy |
| Actigard | acibenzolar-s-methyl | fungicide | P01 | III |
| Quadris | azoxystrobin | fungicide | 11 | III |
| Pristinex, w | boscalid/pyraclostrobin | fungicide | 7/11 | II |
| Bravo, Echo, Equus, Initiatev, w, u | chlorothalonil | fungicide | M5 | II |
| Champ, Kocide | copper hydroxide | fungicide | M1 | II |
| COCu | copper oxychloride | fungicide | M1 | II |
| Multiple | copper sulfate | fungicide | M1 | III |
| Torino | cyflufenamid | fungicide | U6 | III |
| Inspire Superu, t | difenoconazole/cyprodinil | fungicide | 3/9 | II |
| Forumu | dimethomorph | fungicide | 40 | II |
| Luna Experiencet | fluopyram/tebuconazole | fungicide | 7/3 | II |
| Luna Sensation | fluopyram/trifloxystrobin | fungicide | 7/11 | III |
| Dithane | mancozeb | fungicide | M3 | III |
| Revus | mandipropamid | fungicide | 40 | III |
| Rallyt | myclobutanil | fungicide | 3 | II |
| Fontelis | penthiopyrad | fungicide | 7 | III |
| Cabrio | pyraclostrobin | fungicide | 11 | III |
| Quintec | quinoxyfen | fungicide | 13 | III |
| Monsoon, Onset, Toledo, Vibet | tebuconazole | fungicide | 3 | II |
| Procuret | triflumizole | fungicide | 3 | II |
| Agri-Mekt | abamectin | insecticide | 6 | I |
| Assails, u | acetamiprid | insecticide | 4a | II |
| Acramitet | bifenazate | insecticide | UN | |
| Admire, Provado w, u, s | imidacloprid | insecticide | 4a | I |
| Oberon w | spiromesife | insecticide | 23 | II |
| Actara, Platinumu, s | thiamethoxam | insecticide | 4a | I |
zMode of action is taken from the Fungicide Resistance Action Committee or the Insecticide Resistance Action Committee.
y Ranking of pesticides according to whether (I) the product should not be applied to flowers, (II) the product should not be applied to flowers except between sunset and midnight, or (III) no bee precaution except what is listed on label. More explanation is provided in text.
xMaybe more toxic if applied with an insecticide from IRAC group 1B
wToxic to honey bee brood
vMaybe more toxic if applied with an insecticide from IRAC group 3A
uToxic to other bee species
tMaybe more toxic if applied with an insecticide from IRAC group IRAC3A, IRAC4A, IRAC4D, IRAC15
sMaybe more toxic if applied with an insecticide from FRAC group 3