Asparagus beetle
Crioceris asparagi
A checkered orange-and-black beetle whose grubs defoliate asparagus fern, weakening next year’s spears. Hand-pick from late spring and burn cut fern at season’s end.
How to know it
A neat beetle 6–7 mm long, black with cream squares and orange margins on the wing cases. The larvae are slug-like, gray-green with a black head, and hang from the fern; both leave black eggs standing on end like tiny pegs along the needles.
What you'll see
- Black eggs standing upright in rows along the fern and on the spears.
- Fern browning and dying back in patches from midsummer.
- Gray grubs clinging to the needles, dropping when disturbed.
- Spears scarred and bent where beetles have grazed them in spring.
- A weaker, thinner crop of spears the following season.
Where it comes from
Adults overwinter in hollow stems, leaf litter and soil, and emerge as the first spears rise. Eggs hatch in a week or so; the grubs feed for a fortnight on the fern, drop to pupate in the soil, and two or three generations follow through the summer.
Established asparagus beds, particularly those where old fern is left standing over winter. Damage to the fern in late summer is what steals next year’s crop — the plant needs those weeks to build the crowns.
Dealing with it
- Hand-pick adults from the spears in spring, morning and evening, while numbers are still low.
- Rub the standing black eggs off spears and fern whenever you cut.
- Knock grubs off the fern into a tray of soapy water; they do not climb back.
- Cut and burn the fern at the end of the season rather than composting it.
- Protect the fern, not just the spears — the fern is next year’s crop.
Keeping it out
- Clear all old fern and debris from the bed each winter, and keep the edges mown.
- Cut spears hard through the picking season, which removes eggs with them.
- Do not mulch a bed with material harboring last year’s beetles.
- Encourage ladybirds and parasitic wasps, both of which take the eggs and grubs.
- Keep the crowns strong with a spring feed so the fern can carry some damage.
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com