Mirid damage to strawberry flowers

When mirid or lygus bugs feed in strawberry flowers, they damage the flower with their piercing/sucking mouthparts.  The damaged ovules do not develop into viable seeds, and the resulting fruit grows with a “catface” damage pattern.

When looking at the ripe fruit, it’s not at all easy to determine whether the damage was originally caused by lack of pollination, or by bug feeding (see https://berryworldnz.wordpress.com/2025/03/17/from-mirids-to-pollination/).

A Canterbury strawberry grower had a big infestation of mirid bugs late this summer (mid February), after a neighbour mowed and bailed some lucerne.  While very unfortunate for the grower, it has given us an opportunity to observe carefully what mirid bug feeding in the flowers looks like.  The experience has also led to the observation that the bug population can happily stay on lucerne nearby a strawberry crop until something causes them to move on.  This could be useful in a trap cropping strategy. 

The mirids at the Canterbury farm were all in the flowers.  Fruit that had set prior to mirids moving in had developed normally—either the bugs didn’t feed on the developing fruit or their feeding didn’t damage the developing fruit.

At the same time that all these high quality ripe fruit were being harvested, the bugs were in high numbers in the flowers, and nearly every developing green fruit was damaged.  This is important--when you first get mirids into the crop, the resulting fruit damage only shows up in the harvest after 3-4 weeks, when the damaged flowers mature to ripe fruit.

A closer look at the flowers showed both blackened stigmas (the bit where the pollen lands before it grows down to the ovary to fertilize the egg), and blackened achenes, presumably where bugs had fed.

We observed small nymph stage bugs even in flowers that had not opened yet.  Another observation was damage or missing anthers.  Perhaps the bugs feeding in the developing flower also decreases pollination by their feeding on the pollen source.

While traveling in the Bay of Plenty in early April, we observed mirid bugs on strawberry farms.  Growers don’t always notice them; the nymphs are tiny and green, and they run fast but don’t fly.  They resemble large aphids.  Adults (several species) are often a brownish colour, and fly quickly, but adults aren’t doing most of the damage, the nymphs are.

Both mirid feeding and lack of pollination make “catfacing” damage to strawberry fruit, and determining the cause is essential when formulating a management strategy.

Mirids are hard to see, but an easy assessment method is to hold a tray under strawberry plants and whack them three times, causing the bugs to fall out of the foliage.  A plastic cricket bat makes an excellent whacker.  It’s easier to do in the cool of the morning when bugs are slower, but should be done after the dew has evaporated.  Coating the tray with a light material makes bugs easier to see.

In our recent assessment, we were getting 2-4 bugs per 50 cm of plants whacked (4 plants).  This is a really high population; at these populations nearly every developing fruit was damaged.  Much lower populations still lead to fruit damage.

What can we do about mirids once they have infested the crop? 

They’re one of the pests where we don’t have a good selective insecticide that won’t kill the beneficial predators, so we’re stuck with a broad spectrum sledge hammer.  Lannate is no longer available, leaving us with carbaryl (Sevin Flo), one of the products for which Worksafe has now set a 12 day REI, or a pyrethrin such as ZetaPy.  At our Canterbury farm, we trialled fipronil (Recoil), which is off label and broad spectrum, and are doing residue testing to work out the withholding period.  The efficacy was good, and the WHP is a work in progress. Remember that for all off-label pesticide use, it is the responsibility of the grower to assure residues are compliant. 

Given the non-ideal spray options, we are still motivated to work out an effective trap-crop/repellent system.

For growers with autumn fruit, now is a good time to make a close observation of your plants to determine if mirid bugs are impacting your crop.

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