Spotted Tentiform Leafminer
Excerpt from Publication 310, Integrated Pest Management
Table of Contents
The spotted tentiform leafminer, Phyllonorycter blancardella (Fabr.), is a pest that infests apple foliage throughout North America. The larvae mine between layers of apple leaves, reducing photosynthetic area. Heavy infestations of leafminer affect fruit sizing, reduce vegetative growth and/or cause premature fruit drop. This insect primarily attacks apples and has also been found on crab apples.
Tentiform leafminer was introduced from Europe in the 1930s. Populations in commercial orchards increased dramatically in the 1970s and 80s as the insect became resistant to organophosphate insecticides, pyrethroids and methomyl (Lannate). This pest was a major indirect pest of apples during this time, however IPM practices (using pesticides with less impact on beneficials), suboptimal weather conditions (cool wet springs), and the registration of new reduced-risk pesticides from many chemical families has significantly reduced tentiform leafminer populations in Ontario orchards.
Tentiform leafminer eggs are translucent, yellow-green in colour and approximately 0.35 mm in diameter (Figure 4-31). Eggs are round, flattened and a pale yellow colour. They are laid individually and randomly on the underside of leaves, making them difficult to detect, even with a hand lens. Eggs are most easily observed under a microscope.
Figure 4-31. Tentiform leafminer eggs
There are five larval instars and two distinct forms of larvae during development. First through third instars are referred to as sap-feeders. Sap-feeders are white to yellow in colour with a flattened head capsule and wedge-shaped body reaching a length of 1-2 mm in size (Figure 4-32). Fourth and fifth instar larvae are called tissue-feeders and are more cylindrical and have more of an oval head (Figure 4-33). They are 3.9 mm in length when mature. Pupae are brown and tapered at the hind end and approximately 3.3 mm in length (Figure 4-34).
Figure 4-32. Tentiform leafminer sap-feeder
Figure 4-33. Tentiform leafminer tissue-feeder
Figure 4-34. Pupa
The adult tentiform leafminer is a small moth measuring 4-5 mm with a wingspan of 7-8 mm (Figure 4-35). Patterns of gold, white and black on the wings appear iridescent in direct sunlight. Adults rest on leaf undersides during the day and are active at night.
Figure 4-35. Adult tentiform leafminer (Dr. Art Agnello, Cornell University)
There are three generations of tentiform leafminer a year. The insect overwinters as a pupa in leaves on the orchard floor. Adult moths begin to emerge when apple buds begin to break in late April and continue to emerge throughout May. Peak emergence is from pink through bloom. Moths remain on the orchard floor until a calm, warm day when they fly up into the lower scaffold limbs. Individual insects are inconspicuous, but walking through an orchard in late April often results in clouds of moths flying up from the orchard floor when disturbed.
Mating occurs soon after emergence and eggs are laid on the underside of developing spur leaves on lower scaffolds and limbs. Eggs are laid singly on the undersides of leaves, usually in the evening. Females lay up to 152 eggs in their life time. Egg laying does not occur if temperatures are below 9°C. First generation eggs hatch in two to three weeks, depending on temperature.
Newly emerged larvae are called sap-feeders and they enter the leaf where to suck sap from the mesophyll layer. Sap-feeding mines appear as a light green line visible on the undersurface of the leaf, and are best seen under a microscope. By the end of the fourth instar, larvae begin to feed on parenchymal and epidermal cells in the leaf. These later instar larvae are called tissue-feeders - removing patches of epidermis from the upper leaf surface and creating a spotted appearance on the upper surface of the leaf. Tissue-feeders are more elongated then sap-feeders. Tissue-feeding mines are easily seen with the naked eye, and opened mines reveal the yellowish, flattened larvae. As they grow, they attach silk-like threads to the sides of the mine.
The fifth and final instar pupates inside the leaf. Second generation adults emerge in mid to late June. Second generation larvae continue to mine leaves through mid July. The third generation is active in August and into September. Pupae from this generation fall to the ground in senescing leaves and spend the winter this way.
Sap-feeders cut through the leaf surface and feed on the leaf juices only, and are only detectable from observing the undersides of leaves (Figure 4-36). Later instars are tissue-feeders, feeding in the enclosed tissue until the entire area is light green. Tissue-feeding mines are visible on the upper leaf surface (Figure 4-37). The mines appear as blotchy, skeletonized areas 10-12 mm long and 4-5 mm wide on the leaf surface. Each mine takes up about 4% of the leaf surface. Apple tree tolerance of this pest varies with time of year, tree health, variety and environmental conditions (drought). Healthy apple trees tolerate considerable levels of injury, but stressed trees are much more prone to respond to leafminer pressure. The McIntosh cultivar is particularly susceptible to premature fruit drop. Leafminer mines, accompanied by nutrient deficiencies (particularly boron and magnesium), poor vigour, heavy crop load or drought conditions, increase the risk of early drop.
Figure 4-36. Sap-feeding mines (Dr. Art Agnello, Cornell University)
Figure 4-37. Tissue-feeding mines
Tentiform leafminer adult activity has traditionally been monitored by placing pheromone traps in orchards, however more recently consultants just collect leaf samples to monitor for this pest. When using pheromone traps place four diamond traps at each site around mid April. Place a pheromone on a sticky plastic grid at the bottom of the sticky trap, with traps about 15-20 m apart in sheltered parts of the orchard in early April. Tie traps close to the trunk - on the lowest limbs available - to ensure moths are intercepted flying up from the orchard floor. Monitor traps twice weekly and change the sticky grids when saturated. When emergence of overwintering population is complete, change the pheromone wick and move traps to low-positioned limbs about 1 m off the ground. In recent years, leafminer populations have been so low in orchards that trapping is rarely used to monitor adult populations. Where trapping is used, remember pheromone trap counts are not an indicator of damage potential - they indicate first moth emergence and peak activity for each generation. This information helps crop scouts know when they will start to see eggs and larvae in orchards. Monitor tentiform leafminer stages (i.e. eggs, sap-feeders) weekly by collecting fruit spurs/leaves, and counting the number of egg and larvae on the leaves by viewing with a 25-50X microscope. Preferred cultivars to sample are McIntosh or any predominant cultivar in the block.
Prior to egg hatch for first generation (pink to tight cluster) collect five spurs from each of five trees from the lowest limbs on each tree. Count and record the number of eggs on the underside of the leaves. Calculate the number of eggs per spur.
After detecting first generation egg hatch, (around petal fall), collect 5 leaves from each of 10 trees from the lowest limbs. Using a microscope, count all eggs, sap feeders and tissue feeders on the undersides of leaves. Calculate the number of eggs per leaf and the number of sap feeders per leaf.
Begin sampling for second generation eggs and larvae after second generation moth flight begins in mid to late June. Collect a minimum of 50 shoot leaves (5 leaves from each of 10 trees) from lower limbs (below 1.5 m). Sample leaves from middle aged leaves on extension/shoot growth. Using a microscope, count and record all eggs and sap feeders on the undersides of the 50 leaves collected. Total all eggs and sap feeders on the 50 leaf sample and calculate number of eggs per leaf and number of sap feeders per leaf.
Note: Discontinue sampling when tissue-feeding mines become predominant - insecticides do not control this stage.
Table 1.Thresholds for tentiform leafminer on apples
Beneficial insects are important in managing tentiform leafminer, so conduct surveys during the year to evaluate the presence of parasitoids. Conduct parasitoid surveys after the majority of tissue feeders have pupated but prior to any moth emergence (usually in early June). Collect 100 leaves with visible mines (2 from 50 trees). Using a microscope, gently open each mine with a sharp probe and record presence of leafminer tissue feeders, pupae, parasitoid pupae, or dead or diseased leafminer tissue feeders. British Columbia reports 30% parasitism of first generation tissue-feeder larvae in the spring provides biological control for the rest of the season.
Consider management only if pest populations are above threshold. Sprays to control leafminer are often unnecessary as they are controlled by natural predators or insecticides used to manage other orchard pests.
Predators such as spiders and ground beetles are important predators as they feed on overwintering pupae on the orchard floor. Other predators such as lacewings feed on leafminer larvae, however research suggests the most important natural enemies are parasitic wasps. In Ontario the parasitoids Pholetesor ornigis and Sympiesis marylandensis are the most important natural enemies of tentiform leafminer. In some orchards, levels of parasitism are as high as 80% from these parasitoids. Higher levels of parasitism are generally present in orchards where growers are using IPM practices and minimizing use of harsh, broad-spectrum insecticides.
For more information on natural enemies, see beneficial insects.
Removing fallen leaves is a cultural method for reducing leafminer populations in orchards. Use leaf-blowers - attached to tractors and PTO-driven - to mulch leaves. Mulching leaves or applying urea (to enhance decomposition) to fallen leaves in the late fall also reduces numbers of overwintering leafminers. This practice is not scientifically proven in Ontario, but the theory is smaller leaf pieces decompose quicker and are easier for earthworms to pull into their burrows.
Apple growers have relied on a pre-bloom pyrethroid to manage tentiform leafminer, but pyrethroids are disruptive to IPM programs because of high toxicity to predators and parasitoids. Pyrethroids also cause increased dispersal, feeding activity and egg laying in phytophagous mites, resulting in mite outbreaks.
In recent years, several reduced-risk chemistries - including neonicotinoids and insect growth regulators - have been registered to control this pest and are considered to have less impact on beneficial insects. These are preferred products for growers in an IPM program. For a list of products available to manage tentiform leafminer, see OMAFRA Publication 360, Fruit Production Recommendations.
Chemical controls are not recommended for the third generation leafminer whose damage is usually seen in the orchard in August and September. Controls are generally unnecessary because it is too late in the season for damage to be a concern, and avoiding pesticide use against this generation helps encourage establishment of beneficial insect populations.
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