Asparagus beetle
A 6mm beetle, black with cream squares and a red border, whose adults and slate-grey grubs strip the summer fern of an asparagus bed and gnaw the skin of the stems. The damage looks trivial in July and is not, because the fern is what feeds the crown for next spring — a bed defoliated two summers running produces thin spears in the third.
Caused by: Crioceris asparagi · also called common asparagus beetle, asparagus fern beetle
What brings it on
Adults overwinter in hollow stems and plant debris around the bed and emerge as the spears do. Eggs are laid standing on end, glued along the spears and fern like rows of tiny dark pegs, which is the easiest sign to find. Two or three generations follow through the summer, so a bed left unwatched builds up over the season rather than being hit once.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Asparagus beetle
- University of Minnesota Extension — University of Minnesota: Asparagus beetles in home gardens
Checked 2026-08-13
Usually seen: May, June, July, August, September
Preventing it
Ordered by how much difference each measure actually makes, not by how often it is repeated.
- Cut the fern down to the ground in late autumn once it has yellowed and clear it away with any surrounding debris, rather than leaving it standing over winter. That removes the overwintering shelter, and it is the single measure that most reduces next year's numbers.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Asparagus beetle — non-chemical control
- University of Minnesota Extension — University of Minnesota: Asparagus beetles — cultural management
Checked 2026-08-13
- Keep the bed clear of long grass and perennial weeds at the edges, which is where the adults shelter between feeding bouts on a bed that is otherwise well managed.
source
Horticultural consensusmoderate confidence
- OSU Extension Service — Oregon State University: Asparagus — insect management
Checked 2026-08-13
Dealing with it once it is there
- Hand-pick adults and grubs into a tub of water, and rub the egg rows off the stems with a gloved finger. Going over the bed twice a week from the moment the first beetle is seen keeps a small planting clear without anything else being needed — the beetles drop when disturbed, so hold a tub underneath rather than grabbing from above.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Asparagus beetle
Checked 2026-08-13
- Do not cut the fern back in summer to remove the damage. The plant is storing the whole of next year's crop in the crown at that point, and taking the fern off in August costs more than the beetles were costing.
source
Horticultural consensusmoderate confidence
- University of Minnesota Extension — University of Minnesota: Asparagus beetles in home gardens
Checked 2026-08-13
What it affects
On sprays and treatments
GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.