Field notes from a 2017 pepper weevil trial in a commercial greenhouse in Leamington, Ontario. What we observed, why we think it happened, and how you can test it in your own greenhouse.
In 2016-2017, pepper weevil was causing significant losses in commercial greenhouse peppers across Leamington, Ontario. During one commercial greenhouse trial, we observed something we hadn't expected.
We had begun a weekly program of Beauveria bassiana applied with MycoLiv, which had been formulated to improve the establishment and persistence of the fungus on the crop. Damage continued to climb for a short time after the program began, consistent with weevils already developing inside fruit before treatment. Then, over the following weeks, damage declined sharply.
Curious about what was changing, we began opening infested peppers. That's when we started finding Beauveria growing in an unusual place: around the oviposition plug where the female had laid her egg, and inside the fruit itself.
That raised the question this article is really about:
How did the foliar Beauveria application end up inside the fruit?
To understand, it helps to look at the pepper weevil's life cycle.
The female doesn't spend her life exposed on the plant surface. She lands on a pepper, drills a small pit with her snout, lays an egg inside, and moves on. The larva develops in the seed cavity, feeding from within. By the time the damage is visible, the insect has spent most of its life protected inside the fruit. Which is exactly why contact insecticides so often disappoints against this pest. Whatever you put on the outside, the vulnerable stages are on the inside.
So finding Beauveria established inside the fruit at the oviposition site and in the seed cavity was not what we would have predicted from a foliar spray.
Before we go further, the most useful thing you can do with an observation like this is to test it. Trial one bay. Count damaged fruit weekly. Compare it against your standard program. Let your own greenhouse decide. (We explain how to run a successful trial near the end)
The trial and the damage record
The greenhouse was under heavy pressure when we began. Weekly counts of weevil-stung fruit were climbing steeply, cresting around 400–450 damaged peppers.
We started applying Beauveria bassiana with MycoLiv on a 7-day schedule while damage was still rising. For a short period it continued to climb, consistent with larvae already developing inside fruit before treatment began then it fell, week over week, to roughly 20 stung peppers.
Figure 1. Weekly counts of weevil-damaged peppers. The marker indicates when the Beauveria + MycoLiv program began; damage crests near 410 and then declines over the following weeks toward ~20.
A single field curve doesn't prove a mechanism. What made the result worth investigating was what we found when we looked closely at the fruit and the insects.
What we found when we opened the fruit
This is the part of the story we find most compelling, so it's worth walking through carefully. For each observation, it helps to separate what we saw from what we think it means.
1. Fungal growth at the egg-laying site
Observation. We repeatedly found Beauveria growth concentrated at the oviposition plug, which is the small pit where the female deposits her egg.
What could explain this? The fungus was applied to foliage, not introduced into the fruit. For it to appear at the egg-laying site, something had to carry it there.
Most likely explanation. Females contacting treated foliage appear to pick up inoculum and transfer it to the oviposition site as they drill the pit and lay the egg.
Fungal growth is developing at the oviposition site.
2. Infected larvae inside the fruit
Observation. When we opened stung peppers, we found fungal growth in the seed cavity and larvae that had died there, rather than the healthy larvae we'd expect in untreated fruit.
What could explain this? A larva hatching into a cavity that already carries fungal inoculum would contact Beauveria almost immediately.
Most likely explanation. The evidence indicates that eggs laid into a colonized site produce larvae exposed to the fungus at hatching, before they can complete development.
Fungal growth inside the fruit, around a dead larva.
3. Infected adults inside the fruit
Observation. We also found adult weevils inside fruit showing fungal infection.
What could explain this? The initial fungal inoculum infecting newly hatched (neonate) pepper weevil larvae was low, so the infection developed slowly. As a result, the weevils completed development to adulthood but died inside the pepper before emerging.
Most likely explanation. Our observations suggest that the Beauveria kept developing in insect body passing through various life stages and killed at the end.
An adult weevil with fungal infection inside the fruit.
4. Confirming the cause of death
Observation. Dead adults were found covered in the characteristic white-and-black fruiting bodies of Beauveria bassiana.
What could explain this? That sporulation pattern is a well-recognized diagnostic sign of death caused by Beauveria not simply a dead insect with mold on it.
Most likely explanation. These insects died from Beauveria infection, which is direct confirmation that the fungus was killing weevils in the trial.
A weevil killed by Beauveria.
Putting the observations together
Our observations suggest that female pepper weevils contacting treated foliage may pick up Beauveria inoculum and transfer it to the oviposition site as they lay their eggs. If that occurs, the fungus is already present when the egg hatches, exposing the newly emerged larva before it can complete development inside the fruit.
This interpretation is consistent with what we observed throughout the trial, although additional controlled studies would be needed to fully confirm the sequence of events.
If this interpretation is correct, it changes where pepper weevil control happens. Instead of trying to reach larvae already protected inside the fruit, the opportunity becomes the moment when the female lays her egg and unintentionally delivers the fungus to the next generation.
Why not just spray Beauveria?
This is the fair question, and it's the reason MycoLiv exists.
Beauveria bassiana has long been recognized as an effective entomopathogenic fungus. Our work focused on a different question: how can we improve its establishment and persistence on the crop, so that enough viable inoculum is present when female pepper weevils arrive to lay eggs?
MycoLiv was developed to support that process; it’s a food-grade, GMO-free product that provides nutrients intended to promote rapid Beauveria growth and establishment on the foliage. The goal isn't to replace Beauveria; it's to help it perform more consistently, where and when it matters most. As a secondary effect, foliar nutrients can also support plant vigour.
Using the program successfully
If you decide to look at this approach, a few things are worth knowing. None of them are unusual for a biological program:
- It's preventive, not a rescue treatment. It works best starting at the first sign of weevil activity, not after damage has accumulated.
- It depends on regular applications — in our trial, every 7 days through the active period.
- Beauveria is temperature-sensitive and generally performs best in warm conditions (roughly 68–77°F, 20-25°C)
- Coverage matters. The fungus has to be where the weevil will contact it, so thorough coverage of foliage, flowers, and developing fruit is important; early-morning or late-afternoon applications help it establish.
- Broad-spectrum fungicides can suppress Beauveria. If you use them, separate the timing.
Test it in your own greenhouse
- We'd rather you verify this than believe it. Here's a simple way to do that:
- Choose one bay, block, or compartment with known weevil pressure. If possible, keep a comparable block on your standard program as a reference.
- Start at the first sign of weevil activity.
- Apply MycoLiv at 2 L per 100 L of water, tank-mixed with Beauveria bassiana.
- Repeat every 7 days, covering foliage, flowers, and developing fruit thoroughly.
- Scout weekly. Count stung fruit and open a few to look for fungal growth at the oviposition site, and for dead larvae or adults.
- After a few weeks, compare your treated block against your reference. Let the counts, and what you find inside the fruit, make the case.
Where this stands
Here is what we can say. In a commercial greenhouse under heavy pepper weevil pressure in 2016, weekly Beauveria bassiana + MycoLiv applications coincided with a marked decline in weevil damage, and we repeatedly observed fungal infection at the oviposition site and inside infested fruit. We believe the most likely explanation is that female weevils transfer the fungus to the egg-laying site during oviposition, exposing the next generation at the point where it is most vulnerable.
That is one commercial trial, and individual results will vary with pest pressure, timing, coverage, and conditions which is exactly why the most convincing evidence will be your own.
MycoLiv is available through Growliv Biologicals. If pepper weevil is a problem in your operation, it's worth discussing with your crop advisor and confirming the product's status for your region.