page_title: Gall
section_title: Indicator insects
hierarchical_section_title: Gall / Causes of plant galls / Insects and mites / Indicator insects
caption_attribution_description: English: Round gall on stem of goldenrod (Solidago) made by the fly Eurosta solidaginis. Peace Valley Nature Center, Bucks County, Pennsylvania, USA. https://bugguide.net/node/view/275933
context_page_description: Galls or cecidia are a kind of swelling growth on the external tissues of plants, fungi, or animals. Plant galls are abnormal outgrowths of plant tissues, similar to benign tumors or warts in animals. They can be caused by various parasites, from viruses, fungi and bacteria, to other plants, insects and mites. Plant galls are often highly organized structures so that the cause of the gall can often be determined without the actual agent being identified. This applies particularly to some insect and mite plant galls. The study of plant galls is known as cecidology.
  In human pathology, a gall is a raised sore on the skin, usually caused by chafing or rubbing.
context_section_description: Gall-inducing insects include gall wasps, gall midges, gall flies (e.g., the goldenrod gall fly), Agromyzidae, aphids (such as Melaphis chinensis,  Pemphigus spyrothecae, and Pemphigus betae), scale insects, psyllids, thrips, gall moths (e.g, Epiblema scudderiana), and weevils.
  Galls produced by insects and mites include:
  Ash flower gall: this gall is caused by a small mite that causes irregular distortion of male flowers. The galls are initially green, then dry and turn brown.
  Ash midrib gall: normally 0.5 to 1 inch long, these galls are succulent and have thick walls. A small cavity within each gall contains one or more small maggots, the larval stages of very small flies called midges. Female midges lay their eggs in very young leaflets during early spring. Gall formation begins soon after the eggs are laid. Specifics of the biology of this insect are not known. The galls probably do not harm tree health.
  Elm cockscomb gall: these distinct galls, caused by an aphid, are about one inch long and about 1/4 inch high. The irregular edge of the gall and its red color at maturity account for the common name. The galls dry, harden and turn brown as they age. Aphids may be seen through a slit-like opening in the underside of the gall. This insect has a complex life cycle—it forms galls on elm in early summer, then feeds on grass roots later in the summer. The galls apparently do not cause significant harm to the tree.
  Hackberry leaf gall: this gall is caused by a small (0.1 inch long) aphid-like insect with sucking mouthparts called a jumping plant louse. The adults spend the winter under bark crevices and can invade houses in large numbers in the fall. Females lay eggs over a long period of time beginning when leaves begin to unfold from the buds in the spring. Feeding by the nymphs that hatch from these eggs causes abnormal plant growth that forms a pouch. The psyllids remain inside the galIs until they emerge as adults in late summer to early fall. There is one generation each year. Heavy infestations can result in premature leaf drop which over a series of years may affect tree health.
  Honeylocust pod gall: this gall is caused by a small fly (midge). The sunburst cultivar appears to be very susceptible to this pest. Infested leaves have globular or pod-like distortions that contain one to several small maggots (0.25 inch long). Infestations begin when females lay eggs in young leaflets. There are five or more generations each year. Infested leaves often drop prematurely and repeated damage can kill small branches. New shoots develop at the base of dead twigs. As a result, the natural shape of the tree may be lost.
  Oak gall: see Oak apple
  Petiole and stipule galls: thick globe-like galls can develop on leaf petioles and stems. Many of these are caused by insects called phylloxerans which are very similar to aphids. The hard, woody galls may remain on the tree for several years. Usually, there is one generation each year and the insects over winter on the tree in the egg stage.
  Willow shoot galls: these swellings on shoots, twigs, or leaf petioles, may be caused by small flies (midges) or small wasps (sawflies). The gall increases in size as long as the immature stages are active. They cause no significant injury. The infestation may be reduced by pruning and destroying the galled areas before the adult insect emerges, usually in late summer.
  Witchhazel gall: this gall is caused by an aphid that passes the winter in eggs laid on twigs of the plant. Feeding by the aphid causes the formation of conical galls on the upper side of the leaf. Each gall, produced by a single aphid, later becomes filled with offspring. Mature aphids with wings leave the galls in late spring and early summer and fly to birch. After several generations there, the insects return to witch hazel to lay the eggs that survive the winter. No galls are formed on the birch.
caption: Goldenrod round gall made by the fly Eurosta solidaginis
exif: {"Image ImageDescription": "OLYMPUS DIGITAL CAMERA", "Image Make": "OLYMPUS OPTICAL CO.,LTD", "Image Model": "C4100Z,C4000Z", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "v575u-75", "Image DateTime": "2004:06:29 09:19:11", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "2454", "Image XPKeywords": "[103, 0, 111, 0, 108, 0, 100, 0, 101, 0, 110, 0, 114, 0, 111, 0, 100, 0, 32, 0, 114, 0, 111, 0, 117, 0, 110, 0, 100, 0, 32, 0, 103, 0, 97, 0, 108, 0, 108, 0, 0, 0]", "Image CustomRendered": "Normal", "Image ExposureMode": "Manual Exposure", "Image WhiteBalance": "Auto", "Image DigitalZoomRatio": "0", "Image SceneCaptureType": "Standard", "Image GainControl": "Low gain up", "Image Contrast": "Normal", "Image Saturation": "Normal", "Image Sharpness": "Normal", "Image Padding": "[]", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "5778", "Thumbnail JPEGInterchangeFormatLength": "6298", "EXIF ExposureTime": "1/200", "EXIF FNumber": "11", "EXIF ExposureProgram": "Manual", "EXIF ISOSpeedRatings": "100", "EXIF ExifVersion": "0220", "EXIF DateTimeOriginal": "2004:06:29 09:19:11", "EXIF DateTimeDigitized": "2004:06:29 09:19:11", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "2", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "3", "EXIF MeteringMode": "Pattern", "EXIF LightSource": "Unknown", "EXIF Flash": "Flash fired", "EXIF FocalLength": "191/10", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "1712", "EXIF ExifImageLength": "2288", "EXIF FileSource": "Digital Camera", "EXIF SceneType": "Directly Photographed", "EXIF Padding": "[]", "EXIF OffsetSchema": "4160"}