Spotted Tentiform Leafminer

Table of Contents

1. Introduction
2. Description
3. Biology
4. Damage
5. Monitoring and Management

Introduction

The spotted tentiform leafminer, Phyllonorycter blancardella (Fabr.), is a minute moth in the Gracillariidae family. The larvae mine between layers of apple leaves thereby reducing the photosynthetic area. Heavy infestations of leafminer affect fruit sizing, and may result in reduction of vegetative growth and/or premature fruit drop, which can affect the hardiness of trees going into winter.

The tentiform leafminer was introduced from Europe in the 1930s. Populations in commercial orchards increased dramatically in the 1970s as the insect became resistant to organophosphate insecticides, earning it a major indirect pest status. For a number of years, pyrethroids and methomyl (Lannate) were used to try to control tentiform leafminer in Ontario apple orchards. By the mid 1980s, levels of resistance to these chemicals had also developed.

The status of tentiform leafminer as a major pest of apple has slowly decreased in the 1990s, partly as a result of the continued adoption of IPM practices by Ontario growers, and partly due to spring weather conditions. Cool, wet springs limit activity of the first generation of adults, resulting in lower seasonal pressure. Increased emphasis on biological and cultural controls and the registration of newer, narrow-spectrum insecticides offer new hope in controlling this indirect (leaf-feeding) pest in the future.

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Description

The adult tentiform leafminer is a small moth measuring 4-5 mm with a wingspan of 7-8 mm. Patterns of gold, white and black on the wings appear iridescent in direct sunlight (Figure 1).

The eggs, laid on the underside of leaves, are round, flattened and a pale yellow colour (Figure 2), making them difficult to detect, even with a hand lens or microscope.

Larvae, which mine between cell walls on the leaf, go through five instars (Figure 3). The first three instars are referred to a sap feeders (Figure 4), feeding on leaf juices only and are detectable only from observing the under sides of leaves (Figure 5). The final two instars are referred to as tissue feeders (Figure 6), since they chew the tissue layers between the cell walls. The mines of tissue feeders are visible on the upper leaf surface as blotchy, skeletonized areas 10-12 mm long and 4-5 mm wide (Figure 7). Each mine takes up roughly 4% of the leaf surface. The pupa found inside the mine is 3-4 mm long, elongated, brown and pointed at the tail end (Figure 8).

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Biology

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 the month of may. Moths will remain on the orchard floor until a calm, warm day when they fly up into the lower scaffolds. 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 developing spur leaves on lower scaffolds and limbs. The eggs are laid singly on the undersides of leaves, usually in the evening. These first generation eggs hatch in two too three weeks, depending on ambient temperatures.

Newly emerged sap-feeding larvae enter the leaf where they begin to suck sap from the mesophyll layer. By the end of the third month (fourth instar), the larvae begin to feed on parenchymal and epidermal cells (tissue feeder stage). Tissue feeders are easily distinguished from sap feeders in the orchard since only tissue feeder stage mines are visible on the upper surface of leaves. Mines can be opened to reveal the yellowish, flattened larvae. As they grow, they attach silk-like threads to the sides of the mine. When dry the threads pull the mine into a tent shape.

The fifth and final instar eventually 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.

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Damage

Apple trees can tolerate considerable levels of tentiform injury when healthy, from seven to ten mines per leaf. However, 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. Severely stressed trees are more susceptible to winter injury.

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Monitoring and Management

In Ontario, OMAF monitors tentiform leafminer activity by placing pheromone traps in representative regional orchard sites in major apple-growing areas in the province. The trap consists of a one-litre-round cardboard ice-cream carton with a semi-circle cut out of each end. A pheromone wick is placed on a sticky plastic grid attached flat tot he inside bottom of the carton with a paper clip.

Two traps are placed at each regional site, about 15-20 metres apart, in sheltered parts of the orchard in early April. Tying the traps close to the trunk, on the lowest limbs available, i.e., 0.5, ensures greatest interception of moths flying up front he orchard floor.

Counts of adult moths are conducted twice weekly. The sticky grids are changed each time due to quick saturation with the tiny moths.

When emergence of the overwintering population is complete, the pheromone wick is changed and the traps are moved on to low-positioned limbs approximately one metre off the ground.

Pheromone trap counts are not an indicator of damage potential. They are, instead, an indicator of first moth emergence peak activity, and the tail-end activity of each generation. Trap counts are also inputted by OMAF into a computer prediction model called Bugwatch to predict first and peak egg hatch.

Tentiform leafminer life stages (i.e., eggs, sap feeders) are monitored by collecting fruit spurs and leaves and performing hard counts. Preferred cultivars to sample are McIntosh or any predominant cultivar in the block. Although with several years of experience and a sharp eye, this procedure can be performed in the orchard with a good hard lens, it is strongly recommended that a dissecting microscope with a 25x to 50x magnification be used. General guidelines are described below. See Appendix III of Publication 310: Integrated Pest Management for Ontario Apple Orchards for sample record keeping charts.

Note: Leaf sampling is conducted once or twice weekly.

First generation (prior to egg hatch)

1. Begin sampling once moth activity has begun.
2. Collect a minimum of 25 fruit spurs (five from each of five trees) from the lowest limbs on each tree.
3. Using a microscope, count all eggs (on the undersides of each leaf on a spur) and record.
4. Total the number of all eggs found on the 25 spurs and calculate number of eggs per spur.

First generation (after first egg hatch detected)

1. Collect a minimum of 50 spur/cluster leaves (five leaves from each of 10 trees) from lowest limbs.
2. Using a microscope, count all eggs and sap feeders on the undersides of the 50 leaves collected and record.
3. Total all eggs and all sap feeders on the 50 leaf sample and calculate number of eggs per leaf and number of sap feeders per leaf.

Second generation

1. Begin sampling once 2nd generation moth flight begins.
2. Collect a minimum of 50 shoot leaves (5 leaves from each of 10 trees) from lower limbs, i.e., below 1.5 metres. Sample leaves from middle aged leaves on extension/shoot growth (Figure 9).
3. Using a microscope, count all eggs and sap feeders on the undersides of the 50 leaves collected and record.
4. Total all eggs and all sap feeders on the 50 leaf sample and calculate number of eggs per leaf and number of sap feeders per leaf.

Figure 9. Shoot leaves to collect for monitoring second generation leafminer.

Parasitoid Surveys (first and second generations)

1. Conduct survey when the majority of tissue feeders have pupated but prior to any significant moth emergence.
2. Collect 100 leaves with visible mines (two from each of 50 trees).
3. 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.
4. Calculate and record all of the above as "per leaf" numbers, see the record keeping form in Appendix III of Publication 310: Integrated Pest Management for Ontario Apple Orchards

A second monitoring technique, called sequential sampling, can also be used to calculate tentiform leafminer number. This method does not provide actual number of life stages, but gives an estimate of population densities based on estimates on whether leafminer populations are under or over threshold (Figures 10, 11, & 12). This sampling procedure is useful when large numbers of orchard blocks are being monitored and time and/or labour is limited.

Action thresholds have been established for tentiform leafminer in Ontario apple orchards as follows:

• First generation: Prior to egg hatch (pre-bloom and petal fall) - three or more eggs per spur. After egg hatch begins (petal fall) - one or more sap-feeding mines per leaf.
• Second generation: Healthy, unstressed trees - four or more sap-feeding mines per leaf. Stressed trees - two or more sap-feeding mines per leaf.

A number of pesticides are available to control the first and second generation of tentiform leafminer. See the Pest Management Decision Guidelines in Chapter Five of Publication 310: Integrated Pest Management for Ontario Apple Orchards for further information. Chemical controls are not recommended for the third leafminer generation.

The use of pyrethroids is strongly discouraged due to their negative impact on IPM programs. These impacts include high toxicity to predators and parasitoids, and behavioural changes in mite pests, including increased dispersal, feeding activity and egg laying, which will result in mite outbreak.

There are several special of parasitic wasps that attack leafminer larvae. In Ontario, parasitoid surveys indicate levels of parasitism as high as 80% in some apple orchards. Generally, higher levels of parasitism are present in orchards where growers are using IPM practices and minimizing use of harsh, broad-spectrum insecticides.

The most important parasitoid is Pholetesor ornigis, a tiny black wasp (Figure 13), which searches out and parasitizes sap-feeding larvae in mines. Single eggs are inserted into the leafminer's body with a long ovipositor. The wasp larva that hatches out of the egg feeds internally on the sap-feeding larva (endoparasite) and eventually kills it. The parasitoid then pupates inside the mine. When torn open, the mine revels a small, white, pill-shaped cocoon attached at either side to the mine wall by a strand of soil (Figure 14).

The life cycle of the tentiform leafminer and P. ornigis are highly synchronized. Adult wasps, which overwinter as pupae in leaf litter, emerge just as the first sap feeders appear. This even occurs just after petal fall at a time when organophosphate applications are being applied to control plum curculio. A delayed spray at this time, or a border spray when plum curculio is not a major problem, will ensure greater survival of P. ornigis.

A second group of parasitoids are chalcid wasps (Chalcidae) (Figure 15), which in some orchards are the predominant parasites attacking leafminers. Chalcid wasps also parasitize the sap-feeding stages but are ectoparasites, meaning the chalcid larva dies externally on the leafminer larvae (Figure 16). When mines are opened, the pupa of the chalcid is easily recognizable but its mummy-like appearance (Figure 17).

A cultural method of reducing leafminer populations in the orchard is the removal of fallen leaves. of course, this is not economical to do manually, but brisk winds or man-made leaf-blowers, which attach onto tractors and are PTO-driven, can accomplish this job. Mulching of leaves or application of urea (to enhance decomposition) to fallen leaves in the late fall, may also reduce the number of overwintering leafminers. This has not yet been scientifically proven in Ontario, but the theory is that the smaller pieces of leaf decompose quicker and are more easily pulled by earthworms into their burrows.

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