Distribution of a plant parasite on its host: gall distribution on the flowering dogwood, Cornus florida

Purpose To determine the distribution of midge galls on dogwood trees and to investigate some environmental factors that might influence distribution. Questions 1) How are the galls distributed spatially? Random vs. clumped vs. uniform 2) Number or distribution of galls influenced by: Size/age of the plant? Nearest neighbor distance? Sunny versus shady microhabitat?

Midge fly, Resseliella clavula

Overview (A) Parasite: Gall Midge Fly (Resseliella clavula) (B) Host: Flowering dogwood (Cornus florida) Process – Female fly lays eggs near terminal dogwood bud (late spring). – Eggs hatch & larvae enter dogwood shoots. – Abnormal swelling (galls) form on twig ends (summer). – Larvae exit galls and overwinter in soil. Pupate in early spring. Become adults in late spring. – Larvae get protection and food source. – Plants expend more energy regenerating tissue.

Random Distribution Parasites are randomly distributed within the environment. If midges flew until they “hit” a dogwood haphazardly … Predict:

Even Distribution “Regular”: Organisms are evenly spaced in the environment. Then every tree would have the same number of galls. This would suggest that the parasites are competing for the trees. Predict: 50

Clumped Distribution Most trees have few parasites, and some a lot. Typically associated with: – disease outbreaks, tree densities (“contagious”), age Predict:

Tree size Older and infirm trees might be more likely to be attacked by the midge parasite. Distance to nearest neighbor If trees are close together, they might be more likely to attract galls. Collect gall data in two habitats Some microhabitats might be better for galls than others. Environmental Factors We are Measuring

Flowers: white, four petals (spring) Fruit: bright red, in clusters (late summer, fall) Leaves: opposite, veins curved

Flower buds (reproductive) Leaf bud (somatic) Both

Bark: dark brown with squarish, scaly blocks. Young trees will be smooth and not as distinctive.

Examples of different numbers of bud scales Dogwood with two bud scales. Twigs are slender, green or purple.

Dogwood leaf scars are opposite, small, and encircle twig. Example of alternate leaf scars Opposite leaf scars Terminal bud

 Groups of 4  Go to 2 sites and survey 20 trees each 1) Record how many galls on 100 branch nodes per tree 2) Measure the DBH (cm) of each tree trunk DBH = “Diameter at breast height” 3) Measure the distance (m) of the three closest dogwood neighbors (DNN = nearest neighbor dist.)

Galls …so this branch is good for 13 out of the 100 counts you need for each tree, and the number of galls is 2