Cecidia Plant Galls

Cecidia, or plant galls, are abnormal growths of plant tissue under the influence of a parasitic organism. Within the growing cecidium, plant cells proliferate (hyperplasy) and enlarge (hyptertrophy) into a characteristic structure specific to that particular gallmaking organism. The organism inducing cecidogenesis (gall formation) receives nourishment and shelter while the host plant seldom benefits. Plant galls are induced by a variety of organisms including bacteria, fungi, nematodes, and arthropods.

Insect gallmakers span seven orders (see Table I) within which two entire insect families (Cecidomyiidae, the gall midges, and Cynipidae, the gall wasps) are found only within

TABLE I Insect Gallmakers

Insect order

Gallmaking insects: galls

Thysanoptera (thrips)

Heteroptera (true bugs) Homoptera (aphids, hoppers, and scales)

Coleoptera (beetles) Lepidoptera (moths)

Diptera (flies)

Hymenoptera (wasps)

Thrips: roll and fold galls on leaves and buds, mostly tropical.

Tingidae (lace bugs): galls on flowers.

Adelgidae, Aphididae, Asterolecaniidae, Cercopidae, Cicadellidae, Coccidae, Diaspididae, Eriococcidae, Eriosomatidae, Kermidae, Phylloxeridae, and Psyllidae (leafhoppers, aphids, and scales): pouch and roll galls on leaf, stem, and root; mostly of woody plants.

Curculionidae (weevils): galls on Brassicaceae.

Aegeriidae, Coleophoridae, Cosmopterygidae, Gelechiidae, Heliozelidae, Lycaenidae, Orneodidae, Pterophoridae, and Tortricidae (mostly small moths): mostly fusiform galls on stems or petioles.

Agromyzidae, Anthomyzidae, Cecidomyiidae, Chloropidae, Platypezidae, and Tephritidae (gall midges, fruit flies, and leafminers): a variety of galls on woody and herbaceous dicots and monocots.

Agaonidae, Cynipidae, Eurytomidae,

Pteromalidae, and Tenthredinidae (sawflies, fig wasps, seed chalcids, and gall wasps): galls on all plant parts of mostly woody plants and a few herbaceous plants.

cecidia. While individual insect species make a characteristic gall on only one part of a single plant species, the thousands of gallmaking insects induce cecidia on nearly all plant parts of a wide variety of plant species worldwide.

CECIDOGENESIS: MECHANISMS OF GALL INDUCTION

The process of cecidogenesis (gall induction) involves increased levels of plant growth regulators (auxins, cytokinins, gibberellins, abscisic acid, etc.), the stimuli for which differ among taxa of gallmaking insect. For example, galls induced by tenthredinid sawflys (Hymenoptera) form in response to chemicals produced in the female accessory gland and placed on the plant at the time of oviposition. However, the chemical stimulus for galls induced by cynipid gall wasps (Hymenoptera) is released with larval feeding, and gall formation ceases if the larva dies. The exact mechanism by which insects induce gall structures characteristic to that insect species and markedly different from those of other gallmaking insects is still poorly understood.

The plant tissue stimulated to form a gall is always unspe-cialized parenchyma. As these plant cells undergo hyperplasy and hypertrophy, some cells may specialize to form the characteristic structures associated with that gall. However, some of the gall cells always remain unspecialized. These parenchyma cells sequester macronutrients (such as amino acids) and micronutrient minerals (such as calcium, iron, and magnesium) so that the galls act as physiological sinks in the host plant.

ECOLOGY OF GALLMAKING Biology of Gallmakers

Formation of the gall has an adaptive advantage to the gall-making insect, for nutritive gall tissues feed the growing larva and the gall structure hides it from natural enemies. These insect benefits of gallmaking are produced at a cost of photo-synthate and energy to the host plant. The majority of insect gallmakers are plant parasites with a notable exception: fig wasps in the family Agaonidae (Hymenoptera) form mutualistic associations with their host plant (in the genus Ficus) as pollinators. Agaonid wasps from male flowers are introduced into fig orchards in the ancient agricultural process of caprification to allow pollination of the female flowers necessary to produce the fruit.

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