Raspberry fruitworms, also known as raspberry beetles (or raspberry fruit worms), are members of a small family of beetles known as the Butyridae.
The damage caused by these beetles can range from the occasional worm in your raspberry to major destruction of the leaves and severe infestation of the berries.
In addition, heavy damage to the leaves can result in a weaker plant that will produce fewer fruits.
Such tiny beetles and so much damage!
The degree of infestation will dictate whether you will need to implement control measures, and we will guide you through the process of monitoring these pests and deciding on the appropriate course of action.
What You’ll Learn
- Identification, Biology, and Distribution
- Life Cycle
- Raspberry Symptoms
- How to Monitor for Raspberry Fruit Worms
- Choosing a Tolerance Level
- How to Control Raspberry Fruitworms
Identification, Biology, and Distribution
There is some confusion about the name of these pesky worms that can ruin your delightful raspberries.
While many people still call them raspberry worms, some experts call them “raspberry beetles” and consider “western raspberry fruitworm” to be an outdated name.
In the US, the scientific name of the fruitworms has changed over time. Originally, there was Butyrus rubi on the East Coast and B. bakeri on the West Coast.
However, both species have now been merged into one – B. unicolor.
Its range extends from California and Arizona up through Canada to the Arctic.
Europe and Asia have their own species – B. tomentosus. It is particularly prevalent in the United Kingdom, and frequently causes major damage to raspberries, blackberries, and hybrid berries such as loganberries and tayberries.
This pest can destroy up to half of the harvest.
The North American species also infests a range of brambles, including many wild species.
Thimbleberry (Rubrus parviflorus) appears to be the favorite host among the wild Rubus species – probably because it flowers at the same time as the cultivated red raspberries.
Summer crops suffer the most severe damage, due to the life cycle of the insects. Fall crops escape most of the damage.
The brown to reddish-brown adult beetles are oval-shaped, somewhat hairy, and tiny – about 4 millimeters long.
The larvae are whitish yellow and range in size from 6 to 8 millimeters long. They look like grubs and their bodies are segmented.
The adults overwinter in the soil, so they are poised to strike when plants start producing leaves in the spring.
When they emerge from the ground, the beetles start flying and continue to do so until August.
B. tomentosum flies to alternate hosts, including pear, hawthorn, and apple, returning to the raspberry plants once the flowers start blooming.
In contrast, once B. unicolor beetles start flying, they begin feasting on the midribs of partially folded leaves. They are the most active in the evening.
When the flower buds start to form, the beetles eat into them and then move inside the buds once they separate. They leave behind a large entrance hole.
The beetles either lay their eggs on buds that have not opened or inside the flowers and developing fruit. Each female beetle can lay 100 or more eggs.
When the larvae hatch, they typically feed on the flowers before making their way into the center of the developing fruit.
This often damages the fruit or causes it to drop prematurely.
The larval stage continues for about a month until the insects drop to the ground in mid-summer. Then, they pupate, and the adults spend the winter in the soil.
Fall crops typically aren’t damaged by fruitworms because the insects are buried in the soil.
The initial signs that your raspberries are infested by these beetles are elliptical holes in the foliage.
If there are a lot of beetles, you will see slits where the adults ate around the veins of the leaves.
If the infestation is heavy, the adults can severely damage the canes.
Blossoms injured by these pests often develop into distorted berries.
The need for control will vary depending on how you want to use the berries. For example, the presence of these pests is less critical if the fruit will be made into juice or puree.
However, if you were planning to sell your harvest, the presence of fruitworms can significantly impact your bottom line. This is particularly true for fruits that will be “individually quick frozen.”
That’s why it is so important to monitor the level of infestation to determine whether or not you need to take measures to control them.
How to Monitor for Raspberry Fruit Worms
The pre-bloom phase in the spring is the critical time to start checking for the presence of raspberry beetles. The most effective way to limit an infestation is to kill the adults before they produce the larvae.
Once the larvae have infested the fruit, you will not be able to kill them with insecticides. You will have to manually remove the infested berries and destroy them.
Washington State University’s Whatcom County Extension provides a detailed document on integrated pest management (IPM) for raspberry fruitworms and describes the monitoring processes in detail.
You can use the old fashioned way of looking for the insects or their damage. For example, if there is a lot of damage to the leaves, that could indicate that you have high populations of the adult beetles.
White Sticky Traps
If you want to get more precise, you can attract the adults with white sticky traps made from cardboard that doesn’t reflect UV light. One brand of them is Rebell® Bianco.
The reason why these traps work so well is that the fruitworms think they are raspberry flowers.
A good place to set them up is around the edges of your bramble patch, particularly if you have any wild berries there.
There is a good chance that the wild berries will have a higher population of adult beetles, so your odds of catching them are greater in those locations.
Place the trap on the top wire and keep a map of where you put them.
You should check your traps weekly and this will help you decide on your course of action for treatments (or not).
Note the date, the number of beetles, and the location of the traps.
You can use beating trays to dislodge insects from the canopy. The idea is that they will fall into your tray, and you can count them.
You should hold the tray one foot below the trellis wire in the canopy. You have two choices to dislodge the insects:
- Grab the wire and shake the foliage
- Take a rubber sprayer hose and hit the top wire three times
Choosing a Tolerance Level
Now you have to decide what numbers of adult raspberry worms justify taking action.
The need for action will depend a great deal on the final destination of your fruit. Raspberries used for juice can have more larvae than ones that are destined for immediate consumption.
You can get away with more beetles if you will be hand harvesting your fruit compared to using a machine harvester. Humans understand the need to discard any damaged fruit.
The highest standards of control are required if your fruit are destined for immediate quick freezing.
You have a choice of not treating, treating part of your field, or treating your whole field.
It is not uncommon to have patchy results with some parts of the field having greater numbers of adult beetles than others. Growers in this situation have had some success just treating the areas that have more beetles.
The presence of just a few beetles does not necessarily mean that your fruit are contaminated with larvae.
You should consider treating your plants if you find adult beetles at most of the sites you check.
One rule of thumb is to consider treatment if you find more than five adult beetles in the beating tray samples.
Think toward the future, too. If you find a lot of beetles, that is probably a good indication that you will need to treat your raspberry plants next year.
How to Control Raspberry Fruitworms
You have a choice of control options depending on how severe the infestation is on your raspberry plants.
Unfortunately, beneficial insects have not been found to be effective in controlling raspberry fruitworms – mostly because the larvae hide inside the raspberry fruit and are inaccessible to them.
Till the Ground
It may sound old school, but hoeing or tilling in the late spring and early summer can be surprisingly effective. It rousts the adults or pupae out of the soil and onto the surface where the birds can eat them.
This is a nice change of pace considering that crows, starlings, and blackbirds alone cause more than $150 million in damage to fruit, berry, and grain crops in the US each year.
Clean Up Weeds
Maintaining good weed control can be surprisingly helpful. The adult beetles spend a significant amount of time eating the pollen of flowering weeds – especially composite flowers like dandelions.
This can have a noticeable effect on your raspberry plants, since the damage from raspberry fruitworms tends to be more severe in weedy areas.
Host Plant Odor Traps
The Royal Horticultural Society of the UK recommends using a volatile compound that mimics the odor of the host plant (a karimone) to lure the flying beetles.
The smell will draw both male and female raspberry beetles into a water trap – an effective way of eliminating them.
Set the traps 4-6 weeks before the first flowers appear.
One of the advantages of this type of trap is that it attracts the beetles as they exit the soil before they attack your raspberry plants.
However, if you are trapping more than 5-10 beetles per week before the flowers appear, you should also consider using an insecticide.
You have several options for organic insecticides to spray when the flower buds are visible and again before the flowers open.
This will limit the populations of the adult beetles and therefore reduce the amount of larvae that will end up in your fruit.
You should avoid spraying when your plants are blooming, so you won’t kill bees or other pollinating insects. This is true for both organic sprays and synthetic insecticides.
This pesticide is based on insecticidal compounds produced by a naturally occurring soil bacterium.
The Entrust version of spinosad is OMRI-labeled, so it is certified organic in contrast to the Success product.
For home gardeners, I would suggest using Captain Jack’s Deadbug Flower & Vegetable Garden Dust from Bonide which includes spinosad as its active ingredient. It can be picked up from Arbico Organics.
Spinosad is also available in liquid form from Arbico Organics in spray bottles of various sizes.
These chrysanthemum-based compounds don’t persist for long on the plants, so you will need to apply them more frequently than a conventional insecticide.
Brands that are labeled as controlling raspberry fruitworms include PyGanic EC 1.4 II and EC 5.0 II.
You can pick up PyGanic via Arbico Organics.
Be sure and wait at least one day after spraying to harvest your fruit.
Neem seeds contain the insecticidal compound azadirachtin, which has been incorporated into a number of formulations that state they are effective against raspberry fruitforms: Aza-Direct, AzaGuard, AzaMax, AzaSol, Azatrol-EC, Ecozin Plus 1.2% ME, and Neemix 4.5. Arbico Organics carries many of these products.
For home gardeners, Bonide® Neem Oil is often available locally or you can order it from Arbico Organics.
You can apply synthetic insecticides to prevent infestation of your plants before flowering or when 5% of the flowers have opened. Spraying at each time point should give you greater control.
Classic insecticides that are effective against the raspberry beetles include methoxychlor, malathion, carbaryl, or diazinon.
You also have the option of synthetic pyrethroids:
You may get better control using these synthetic pyrethroids than the more natural compounds.
Tiny Beetles Can Cause Large Damage on Raspberries
Beetles in the Butyrus genus are tiny insects that can cause up to a 50% reduction in the yield of summer raspberry crops.
These pests are also known as raspberry fruitworms because of their tendency to infest the fruit and greet unwary consumers with an unpleasant surprise.
You have some leeway in the amount of fruitworms you can have in your fruit depending on whether they will be eaten directly or made into juice.
Monitoring the populations of beetles will help you plan your strategy of controlling them, and you can choose from a number of measures ranging from cultural controls to organic or synthetic insecticides.
Have you dealt with raspberry fruitworms in your plants? If so, let us know how it went in the comments.
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About Helga George, PhD
One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the knowledge that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.