Thrips are tiny winged insects that feed on a variety of plants by puncturing the tissues and sucking up the contents. They can cause serious damage to flowers, fruits, vegetables, and other crops. Some of the first signs that thrips may have infested a plant include discolored, deformed, or spotted leaves and flowers.
Thrips are difficult to spot due to their very small size, usually measuring only 1 to 2 millimeters in length. However, there are some telltale signs that can indicate the early stages of a thrips infestation. Being able to recognize these signs as soon as possible allows gardeners and farmers to take quick action to control thrips before major damage is done.
Silver or Bronze Spots on Leaves
One of the most common early indicators of thrips is the appearance of silver or bronze spots on the surface of leaves. This discoloration occurs because thrips feed by using their rasping mouthparts to puncture plant cells and suck out the fluids inside. The damaged cells collapse, lose moisture, and turn silvery or bronze.
The spots usually start out small, about 1 to 2 millimeters across. They may be isolated at first but will multiply and spread as the thrips population increases. On some plants like roses and azaleas, the tiny silver flecks eventually coalesce to form larger blotchy patches on the leaves.
Scouting regularly for these telltale spots is an effective way to detect thrips activity in the early stages before exponential population growth. Any silver or bronze flecks on leaves warrant further inspection of the plant for actual thrips insects.
Distorted Growth on Leaves and Flowers
Another early sign of thrips is distorted growth on leaves and flowers. Thrips feeding inhibits proper development of the plant cells. Leaves start curling, twisting, or growing asymmetrically. Flower petals become streaked, spotted, or deformed.
Young expanding leaves and flowers are especially vulnerable as the thrips lacerate the soft delicate tissues. The injuries interfere with the leaves’ ability to unfurl flat or the flowers to open up fully. Instead they emerge small, puckered and irregularly shaped.
This stunted distorted growth usually shows up before extensive spotting or cell death occurs. So it’s an important early clue that thrips are present and active on the plant. Careful inspection of affected leaves and flowers may also reveal the tiny winged thrips insects inside the curled up tissues.
Presence of Small Black Dots on Leaves
Another possible early sign of thrips is the appearance of numerous small black dots on leaves. These tiny black specks are actually fecal droppings or excrement from thrips.
Adult female thrips lay eggs inside of plant tissues. After hatching, the larvae feed on the plants and excrete large amounts of waste. These small black dots may be the first outward indicator of thrips larvae development inside leaves and flowers.
Thrips fecal droppings are usually concentrated near the feeding sites or hiding places of thrips larvae. If black dots show up on the underside of leaves, it’s a good indication that immature thrips are present. The larvae are very small and like to tuck into tight protected areas of plants.
Silvery-White Patches on Leaves
Silvery-white patches on plant leaves and stems can be another early sign of thrips damage. These bleached spots occur when large numbers of thrips feed on the lower surfaces of leaves.
They use their rasping-sucking mouthparts to puncture the lower epidermal cells and extract contents. As more and more cells are destroyed, silvery-white scars form on the leaves. This happens because more of the unaffected green upper epidermis shows through once the lower layer of cells collapses.
Newly formed leaves are most prone to this type of injury. Extensive silvery patches indicate a heavy infestation of thrips. However even a few small areas of bleaching warrant closer inspection for thrips presence on the undersides of leaves.
Stunted, Distorted Growth of New Leaves and Flowers
Thrips often congregate and feed heavily on newly emerging leaves and flower buds. The injuries they inflict can cause severe distorted growth of these tender developing tissues.
Leaves may be stunted, asymmetrical, and curl up or down unnaturally. Flower buds fail to open properly and petals become marked with streaks and spots. This is due to the thrips physical damage as well as enzymes they inject at feeding sites to aid in breaking down plant cells.
Careful examination of stunted leaves and flowers will often reveal colonies of active thrips larvae and adults inside. Their concentrated feeding prevents normal growth and expansion of the developing plant parts. These symptoms of distorted growth are most pronounced on new shoots and buds.
Leaf Drop
Moderate to heavy thrips infestations can lead to leaf drop fairly early on. The accumulation of feeding damage, cell death, and growth deformities puts stress on the plant. In response, the plant prematurely drops its leaves.
Leaf drop usually starts with older leaves lower down on the stems. But eventually upper younger leaves start falling off as well. Whole sections of the plant may be denuded of leaves if thrips populations spiral out of control.
Premature leaf drop depletes the plant’s ability to photosynthesize and make food. Continued loss of foliage can weaken the plant severely. Therefore, leaf drop is a troubling sign requiring immediate thrips control measures.
Wilting or Curling Leaves
Leaves that begin wilting or curling upwards can signal distress from thrips damage. The effects of thrips feeding reduce the plant’s ability to take up and transport water effectively.
As injury worsens, the leaves lose turgidity and begin to take on a limp, drooping appearance. Curling typically starts at the edges of the leaves and eventually cupping sets in across the whole leaf surface.
Plants suffering from thrips often have leaves that curl up lengthwise as well. This is due to impacted growth and cell deformation from heavy thrips populations congregating and feeding on the young leaves. Wilting and curling leaves indicate significant thrips-related impairment of the plant’s vascular system.
Yellowing Leaves
Leaves that turn yellow is another potential sign of plant stress and deprivation caused by thrips. The chlorophyll starts breaking down when the plant can’t adequately photosynthesize due to thrips damage.
Yellowing typically begins in between the veins. It spreads out from there to consume larger sections of the leaf surface. As more green fades away, the leaves take on a mottled, speckled appearance. Eventually the yellowing can encompass whole leaves as they turn entirely yellow and die off.
Any yellowing of foliage warrants inspection of the leaves’ undersides for thrips presence. The yellowing arises from sustained feeding damage that is taxing the plant’s health and vigor. Timely intervention is needed to curb further thrips multiplication.
Scarring and Scraping on Leaves and Stems
Thrips can leave behind physical scraping and scarring damage on plant tissues. This arises from the insect’s rough rasping mouthparts that are used to essentially sand down and shred the plant’s surface.
Scarring damage first shows up along the veins on the undersides of leaves, where thrips often congregate. Small scrapes may also form on leaf edges or petioles where thrips feed and crawl around. On woody stems, rough lesions and long furrows or tunnels can form.
These wounds allow entry points for bacterial and fungal infections. The injuries also compromise the plant’s circulatory flows. Excessive scarring indicates a heavy infestation of actively feeding thrips.
Excrement Buildup on Leaves
Thrips excrement can build up to excessive levels when populations surge out of control. Tiny black specks cover the leaves, stems, flowers and even the surrounding soil. This much waste product suggests an extreme level of thrips infestation.
Excrement is made up of salivary secretions and undigested contents from the thrips. It has an inky black appearance that makes it stand out starkly against green, yellow, or white colored plant tissues. Too much of it can also promote growth of sooty mold fungus.
Major accumulations of thrips waste product must be addressed quickly. It shows the thrips have reached very high reproductive levels with possibly hundreds to thousands present on the plant.
Weakened or Dying Seedlings
Thrips often inflict major damage on young vegetable and flower seedlings. The tiny vulnerable plants can become rapidly overwhelmed by feeding hordes of thrips. Entire crops of seedlings may weaken, stop growing, and eventually die off from the onslaught.
Leaves turn pale, malformed, and tattered looking. Stems collapse as vascular tissues are destroyed. Budding parts are unable to form properly. Soon the seedling topples over dead or nearly so.
Seedling death is most often seen with onions, carrots, cabbage and other vegetables. Flower annuals and perennials also frequently fall victim to thrips while still in the seedling stage. Protecting sprouting plants is key to prevent total losses.
Reduced Flower Size and Distortion
One of the first visible impacts of thrips feeding is often diminished size and deformation of flowers. This is due to thrips direct feeding damage to the fragile developing flower parts.
Petals emerge small, malformed, and streaked with silvery scars. Flowers fail to open fully and have gaps or asymmetry. Buds may blacken, shrivel up, and die off instead of blooming. Entire flower heads are stunted to a fraction of their potential size.
Roses, gladiolas, dahlias, impatiens and other ornamental flowers are most prone to thrips-induced deformation. But any plant grown for its flowers can show impaired development and opening. It takes only a small thrips population to substantially distort flowers.
Slowed Growth Rates
Plants infested with thrips often exhibit noticeably slowed growth. This is due to the combination of direct tissue damage and the overall stress thrips inflict.
Leaves, stems, and runners increase less in size and at a more sluggish pace compared to normal growth speed. New shoots and leaves emerge erratically. Flowering and fruiting are delayed and reduced. The plant struggles to put out new growth and expand normally.
Slow growth can have many causes, but thrips must be ruled out in affected plants. Examine the newest leaves closely to see if thrips larvae or adults are present and accounting for the stunted growth. Thrips removal is needed to get growth back on track.
Increased Number of Predatory Insects
Some predatory insects like minute pirate bugs, lacewings, and lady beetles prey on thrips. An abundance of these “good bugs” on plants may indicate a thrips infestation they are feeding on.
While the predators help control thrips, large numbers of the predators reflect expansive thrips populations providing them food. So an influx of beneficials like lady beetles may be an indirect early sign that thrips are present and available as prey.
Look for congregations of the predators on plants and inspect carefully for thrips near those areas. The “bad bugs” and “good bugs” often coexist in the same spots where thrips are plentiful.
Conclusion
Thrips infestations start small but can quickly grow out of control. Being able to recognize the earliest signs of thrips allows prompt action before major damage occurs. Subtle symptoms like silver spots, distorted growth, and small scrapes signal that thrips are present and active on the plant. Identifying these early indicators is key to getting on top of thrips before they ravage flowers, fruits, vegetables, and other important crops. Keeping a watchful eye allows gardeners to maintain plant health and vigor.
