About three weeks ago a friend contacted me from Brentwood Bay (on Vancouver Island.) He sent a picture of beetles destroying his dahlias. Not only were the beetles destroying the dahlias, but they were stripping-plants-clean seemingly over-night.
A week after that, I noticed mention of similar, striped beetles around the South Island, with similar damage. Within a few days, gardeners were posting on Facebook, emailing me, and phoning our offices to see if there was anything we could do. Gardeners were experiencing a horrific invasion of coreopsis beetle.
How to ID the coreopsis beetle:
Fortunately the ID is easy. These are small beetles – two to three times larger than a big ladybug – and similar in shape. However, they have a beautiful striped colouration. While there are many beetles that might look like this or do similar damage, the coreopsis beetle has a limited host range and is likely only to be found on specific plants.
If you’re reading this and have these beetles in your yard – stop reading. Go outside, put gloves, fill a bucket with water and a bit of dish soap and start removing the beetles from your plant and into the soapy water. Let the beetles drown.
The next step is to recognize two things: 1. You will have missed some, or more will show up; 2. Eggs have been laid. And now is the time to prevent this damage from occurring on your plants in the future:
If you don’t already have it, your garden needs protection in the soil. You should have either Stratiolaelaps scimitus or Gaeolaelaps gillespiei (both in-soil predatory mites) in your yard. These mites are such a standard bio-control agent that most growers put them in every pot, every year and home growers put them in indoor pots or apply them outside. They live permanently in the soil and eat a variety of pests, including the larva of beetles – while they are small.
The larva of the coreopsis beetles start underground, or in leaf litter, but quickly become foliar pests. At this point BtK can be used (a bacterial spray that infects leaf-eating caterpillars and larva). Keep in mind, if you are seeing adults in late May and early June, it means the time to spray BtK is in the spring well before you see adults. BtK only affects caterpillars and larva that are feeding on plant tissue infected with BtK.
A control I would urge people to try is the rove beetle, Dalotia coriaria. It is more expensive than the soil mites, and more likely to spread-out, but it’s host range includes larger pests and those both above and below the soil. While it’ll eat most things the soil mites will, it is also known to eat spider mites, thrips, scale, and other soft bodied insects. However, eating them doesn’t alway mean controlling them. I only suggest using them because the coreopsis beetles are showing up in huge numbers (literally hundreds per plant), and that means a lot of eggs are being laid. The rove beetle may be the dramatic reaction needed.
Of course, chemicals can be used, but I never recommend it. As with all invasive species, the knee-jerk reaction has been to spray and eradicate the pest. This has never once worked. Instead, ultimate control of all invasive species has been achieved when natural native predators adapted to feeding on them (or in some cases when their natural predators also arrived). So, allowing birds to discover and feed on the beetles, or parasitoids to try a few generations inside their larva will more quickly bring the invasive pest to a control level than if we simply try to “nuke” them. This is so true of all pests, that a Government Regulator I know once ironically said: “We’ve now eradicated the greenhouse whitefly 27 times.”
But, with that being said. If your plants have a commercial value and you and your family depend on that income, you are in luck: the coreopsis beetle has a limited host range. While Dahlia’s are not considered one of them, we have proof that they are. Otherwise, ragweed, and tickweed (also knowns as coreopsis) are their known hosts. So you could apply chemicals to only those specific plants in order to get control down to your economic threshold, but be prepared to do the same next year, since you will have deterred natural predators from finding them and the chemical residue washed into the soil mayl prevent predation of the eggs and early larva. At least these chemical applications would be confined to specific plants and not general areas. For us home gardeners – leave the chemicals alone. Nothing is worth deterring their potential predators from discovering them.
Fortunately, here in BC the common practice for dahlia growers is to remove the tubers to overwinter elsewhere. This provides the opportunity to place the tubers in a new garden the following year which will also prevent the beetles from cycling. If the new area is far enough from the previous area, the young larva will have difficulty getting to new host plants. They may starve or be eaten trying to get to the new dahlias.
But, if you’re like me and you just leave dahlias in the ground then you must assume eggs are in the leaf-litter around the plant. As part of fall or spring-clean up, this layer should be removed, and composted, with aged compost being placed instead. And of course, use the predatory mites, no soil should be without them.
Names to remember (and where to get them):
Straiolaelaps scimitus : in-soil predatory mite. Available through distributors here.
Gaeolaelaps gillespiei : in-soil predatory mite. Available through distributors here.
Dalotia coriaria : predatory rove beetle. Available through distributors here.
It is also important to remind growers that plant diversity welcomes invertebrate diversity (both pest and predator), and the more diversity there is the less opportunity for any one pest to take over. This is the essence of truly organic growing and companion planting. For more information on growing with bugs, check out some of my other articles:
2021 Garden update (see what I’m doing).