Patterns of leaf and stomatal development explain ovipositional patterns by the gall midge Cystiphora sonchi (Diptera: Cecidomyiidae) on perennial sow-thistle (Sonchus arvensis)

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Abstract
The gall midge Cystiphora sonchi oviposits through the stomata of perennial sow-thistle (Sonchus arvensis) leaves, and requires tissues that are conducive to gall formation. To determine if these requirements affect ovipositional choice, egg distribution patterns among and within sow-thistle leaves were related to patterns of leaf and stomatal development. Our study showed that C. sonchi prefers to oviposit into leaves that are in the final stages of growth, when mature stomata are available and tissues are still responsive to the gall-forming stimulus. The youngest and oldest leaves within plants were avoided by ovipositing females. Cystiphora sonchi egg distribution within leaves generally reflected the availability of mature stomata, which develop from tip to base in young S. arvensis leaves. Hence, mature stomata, and C. sonchi eggs, were more frequent at the tip of young leaves. By the later stages of leaf development, the basipetal pattern of growth was no longer apparent, and mature stomata and C. sonchi eggs were more evenly distributed within leaves.