What causes misshapen strawberries?

Recently, I've gotten a few questions about misshapen strawberries.  Misshapen or "wonky" strawberries can be caused by several things, but the two most common causes likely to occur in North Carolina are poor pollination and lygus bug injury.  It's actually fairly straightforward to distinguish between these two types of injury.

Poor pollination
Strawberries do not require, but benefit from, insect pollination, so weather conditions that limit insect activity and pollen movement typically precede when this type of misshapen fruit appears.

Misshapen strawberry fruit due to poor pollination (above) and a normal appearing fruit (below). Photo: HJB

Poor pollination causes misshapen fruit because unfertilized seeds remain small and the fruit around them does not grow at the same rate as the fruit around fertilized seeds.  On the image above, the top fruit has seeds of different sizes next to each other while the seeds on the bottom fruit are uniform.

Lygus bug injury
North Carolina Department of Agricultural (NCDA) regional agronomist David Dycus mentioned via email today that growers in Mississippi had recently expressed concern about misshapen fruit due to lygus bug feeding.  Lygus bugs (also known as tarnished plant bugs) feed on strawberry seeds and fruit and can also cause misshapen fruit.  Lygus feeding does not result in variable seed size and can, therefore, be distinguished from poor pollination in strawberries.  Typically, North Carolina growers do not experience extensive damage from lygus bugs, because they are active later in the year.  This year, however, they may be active earlier (although, our warm spring has turned a little chillier), and it's worth keeping an eye for their damage.  Lygus can be difficult to control, so if grower suspect they have damaging populations of lygus present, they should contact their county agent or myself for recommendations.

Strawberries damaged by lygus bugs. Photo: UC IPM Program

Other, non pest, insects can look like lygus bugs, include beneficial insects like big-eyed bugs and non strawberry pests insects like false chinch bugs, and it's important to correctly identify pests.  BugGuide has a nice identification guide for Lygus lineolaris (the east coast species which feeds on strawberries).

Other causes of misshapen berries
Insects and poor pollination are not the only causes of misshapen fruit in strawberries. Pathogens can damage fruit, mechanical injury can scar it, and abiotic factors can change fruit shape.

Update, 27 April 2013
An update to this post has been added to the NCSU Strawberry Growers Information Portal.