Phytopathology

D. Andrew White M.Sc. 15/05/2011



Emerald Ash Borer

The emerald ash-borer (Agrilus planipennis) is a beetle of Asiatic origin that has recently begun to infest North America. The long bodied metallic beetle especially seeks out ash trees (Fraxinus americana & F. pennsylvanica). Though, the beetle does not much harm the rarer blue ash (Fraxinus quadrangulata). It also can infect other species besides ash. Between May and July the female lays her eggs on the bark. In about one week the eggs hatch and the pale white larvae bores through the bark, to feed on the cambium tissue. They tend to carve out long S-shaped galleries. If too prolific, the larval galleries can choke off the sapwood in a tree resulting in its death.

There are several native North american species of Agrilus that feed on a number of broadleaf species. For example, the bronze birch borer (A. anxius) feeds on birch, and the chestnut borer (A. bilineatus) feeds on chestnuts and oaks. These borers usually do not cause mortal injury - except in trees already stressed by other disorders.

In 2002 the emerald borer was found to have become established in Michigan. The emerald ash-borer does not have well established native enemies. The innate resistances of the non-Asian ash are insufficient to check the spread of this Asian borer. The native Asian species tolerate the borer quite well. But North American white ash and green ash are often mortally injured by the beetles. Consequently, the emerald ash borer has started to become a serious problem in the Americas. The ash borer epidemic has so far caused the death of millions of ash trees. The pest has spread into Canada, having gained entrance near Windsor Ontario.

Control

Borers are difficult to control. To apply contact insecticides just at the right time so as to counter the egg laying adults, is nearly impossible. Once the larvae are in the sapwood, systemic insecticides tend not to be effective. The Canadian Food Inspection Agency (CFIA) had originally planned to halt the spread of the borer by creating a 'quarantine'. This planned quarantine was to consist of an ash-free corridor 30 kilometres wide between Essex and Kent counties. Unfortunately, budgetary and logistic problems prevented this project from being implemented in early 2003. By the end of the year the quarentine option was re-opened. The recommendation for home owners is that any emerald borer infected ash tree should be removed. As half-expected, the beetle did 'jump' the quarantine line. By 2007 the beetle had reached as far as the Mississauga and Toronto area.

Asian Longhorned Beetle

A longhorned beetle (Anoplophora glabripennis) of Asiatic origin has recently taken up residence in Ontario. This 'Asian longhorned beetle' often feeds on maple, but it feeds also on poplar, birch, ash, elm and horse-chestnut. This longhorned beetle one of many longhorned beetle species. Most native longhorned beetles do not cause any great consternation among foresters. However, the Asian longhorned beetle has created quite a stir. Its damage tends to exceed that of the native longhorned beetles.

The adult of this Asian beetle is quite long bodied (3.5 cm). Its antennae are even longer than its body (4 cm). It is dark black with rows of white spots on its elytra (shield wings). The adult beetle lays its eggs under the bark of trees from spring to late summer. The eggs hatch into a borer larvae which tunnel into the sapwood. These tunnels can damage conductive sapwood, and can even kill a tree within one year's time. A complete generation takes from 12 to 18 months, the larval beetle over-winters inside its tunnel. When the pupa forms into an adult, the mature beetle exits through a rather large hole in the bark. The adult takes off, mates and repeats the cycle.

The Asian longhorned beetle has apparently been in Canada and the north-eastern U.S.A. since the late 1990s. In 2003 the beetle was detected in Toronto. Probably the beetle was introduced accidentally in some shipment of a wood product from China. The Asian longhorned has few natural controls in the Americas. There are very real fears that the Asian longhorned beetle shall cause extensive damage.

Control

In theory injected insecticides could control the borer. However, such insecticides are poor at controlling wood-borers. Canadian forestry officials are hoping that the outbreak can be contained by limiting the spread of the beetle. This means, that infected trees will require removal, chipping and thorough disposal (Strauss 2003).

Tent Caterpillars

There are three main kinds of 'tent caterpillars' in North America. They are moths which feed on leaves as larvae (caterpillars). Each species has a distinct type of tent, and the caterpillars are easy to distinguish. The caterpillars feed in colonies, and they retreat to their silken 'tents' in inclement weather. They can cause extensive defoliation. However, healthy trees usually recover from these attacks.

Eastern Tent Caterpillar

Eastern tent caterpillars (Malacostoma americanum) make their tents in the axils or forks of branches, not on twig tips. The caterpillars are hairy and have a white stripe along the centre of their back. They over-winter in the egg stage. Egg masses are arranged in bands around small twigs. They feed on many tree species, but cherry trees are the most common host.

Since the tents are made in larger forks, pruning off each tent is not always practical. Surprisingly, manually crushing the caterpillars along with their tent is a very effective means of diminishing their numbers. (Seems too simple!) Egg masses, which are visible in the winter, can also be crushed.

Forest Tent Caterpillar

Forest tent caterpillars (Malacostoma disstria) make 'tents' that are more like silken mats on large branches or on the trunk. The caterpillars are hairy and brownish. They have a row of white spots along their back, and pale blue stripes on their sides. They over-winter in the egg stage. Egg masses are quite large and are located on twigs. They feed on many tree species. Poplar, maple, oak, ash and birch are common hosts.

Crushing the silken mats is not as easy as for the eastern tent caterpillar. Egg masses, easily spotted in the winter, and can be crushed, if they are in accessible places.

Fall Webworm

Fall webworn (Hyphantria cunea) create large tents on branch tips. Webs are largest in the late summer and autumn. Caterpillars are hairy, and pale yellow or green. Manitoba maple and crab-apples are the common hosts, they infest other tree species also.

Fall webworm tents are the easiest of the tent caterpillars to prune out of trees. This pruning should be done as soon as possible after they appear.

Winter Moth

The winter moth, fall inchworm or late inchworm (Operophtera brumata) is a 'geometer', or an 'arpenteuse'. The caterpillar has six thoracic forelegs, but it only has four prolegs on the far end of its abdomen. It moves by looping - the inchworm gait. This grey-greenish caterpillar feeds on the leaves of apples, linden, maples and many other trees. Winter moth can cause significant 'defoliation', which can be serious on fruit trees. It tends to chew large holes in leaves, seldom devouring the entire leaf blade. The caterpillar descends to the earth on a long silk thread in June or August. It pupates under the forest duff in the soil. Late in the autumn or early winter the adults emerge. The male is tan-grey with softy striped wings a few centimetres wide. The females are wingless, they look like fatter versions of the male sans wings. After mating the eggs are laid in bark crevasses. In the spring the tiny caterpillars crawl up to feed of the burgeoning buds.

Winter moths were introduced to the Americas accidentally, in the late 1940s. Originally they were from Eurasia. In Ontario, just before or after the first snows, one can enjoy the spectacle of hundreds of tan-grey moths fluttering about a metre off the ground between the trees. November and December are the best months to see the adult males. The wingless females are often well camouflaged on the tree trunks. (If you want to see the females, this can be done by following the males.) Males are apparently directed to the females by scent.

Control

Orchard and city trees often lack the protection of shrews, and other insect predators, which are present in the wild. In Orchards winter moth inchworms can be a problem. To control them grease bands can be placed on the trunks of fruit trees in October. These bands stop most of the females from crawling up to lay their eggs. Tar winter oil can be used to kill the over-wintering eggs. The most direct control is with contact insecticides. Since the caterpillars start to feed in the early spring, the insecticide must be applied as soon as the buds open

Giant Caterpillars

Sometimes gigantic caterpillars are found feeding on tree leaves. Most of these 'bait-worms' are the larvae of moths. (Although, the swallowtail butterflies, Pterourus spp., also have big greenish caterpillars.) The later instars of these caterpillars can be as large as one's thumb, squat, greenish, bald or scantily 'haired', and they often sport 'knobs' or 'horns' on their bodies. Generally they do not feed in the winged adult stage, rather they live off stored reserves. Adult males track down females by smell, and males are distinguished from the females by their larger more feathery antennae.

In some cases giant caterpillars devour significant quantities of leaves. Usually, however, their populations are relatively low. Infestations seldom kill trees.

The cecropia moth (Hyalophora cecropia) is a large brown moth with orange-rust bands and tan eyespots on its wings. It has up to a 150 millimetre wingspan. The large green caterpillar has a double row of yellow knobs along its back. It is especially fond of Manitoba maple leaves.

The luna moth (Actias luna) is spectacular blue-green moth with long trailing 'swallowtails' and transluscent eyespots. Its wingspan is up to 100 millimetres. The big green caterpillar has small haired knobs along its back. It feeds on the leaves of birch, sweetgum, walnut, persimmon, and on other Carolinian broadleaf species.

The polyphemus moth (Antheraea polyphemus) is a large golden-tan moth with big blue and yellow eyespots on its wings. Its wingspan is about 125 millimetres. The large squat green caterpillars feed on birch, maple and other broadleaf trees.

The Twin-spot sphinx moth (Smerinthus jamaicensis) is a large brown moth with elongated narrow wings of about 85 millimetre span. The hind-wings have red eyespots. The long green caterpillar has a 'horn' on its hind end. It devours mostly poplar and willow leaves, but apple, elm, ash and catalpa leaves are also eaten. There are other sphinx moth species, in several genera, that feed on a wide variety of tree species. Some of them make a barely audible buzzing noise when disturbed, and most have noxious exudes to dissuade birds from eating them.

Control

There is a widespread agreement among arborists that these giant moths are worthy of preservation. Generally the defoliation they cause is tolerable. Only in special cases do they pose a significant hazard to trees. Cecropia, luna, polyphemus, sphinx and other giant moths are seemingly becoming ever more rare. Artificial lights, especially in cities, throw off the navigation of these moths. Often they circle light-bulbs uselessly. This distraction can cause them to delay or miss mating, it can even fatigue them to death. Furthermore, the forest trees in which they feed have been much reduced in extent. If you have giant caterpillars on your tree, I recommend that you desist from spraying the tree for any reason. Let the tree host these nocturnal beauties.

Codling Moth

Codling apple-worm (Laspeyresia pomonella) is the caterpillar of a moth. The adult moth is little over one centimetre long. Its scaly grey-tan wings with a darker blotch on the forewing tips. The adult is most active in June and July. The larva feeds on the fruits of apples, pears and crab-apples. The larva seeks out the flower-end (calyx) of the fruit. There it burrows into the pulp causing, potentially, a lot of damage. After several months of feeding the larva exists, then hides under loose bark, or similar cover, to pupate inside a cocoon. The pupa usually over-winters before metamorphosis. Although, sometimes there is a second generation in the early autumn.

Codling moth is one of the most destructive orchard pests. Its burrows can make apples and pears unpalatable. Originally from Eurasia, the moth has long ago become naturalised in the Americas. The codling moth has native relatives. A related species, (Laspeyresia caryana) feeds in the husks of walnuts and hickories.

Control

In wild settings insects are usually kept in check by birds, shrews and other predators. However, orchards often lack sufficient populations of these natural controls. Chemical controls are the most effective means of controlling codling moth in commercial orchards. Contact insecticides are effective, if the spraying is well timed. Trees should be sprayed just after the last blossoms fall, in early to mid June. A second spraying three weeks later is recommended. It is difficult to absolutely rid an orchard of codling moth. For ornamental crab-apples and Bradford pears, controlling the codling moth is not really necessary.

Note: Chemical insecticides should never be applied during flower bloom. This can endanger honeybees.

Honeybees, Leafcutters & other Bees


leafcutter bee & bumble bee

Honeybees are protected by law in Ontario. This is to protect the beekeepers' trade, as well as to protect the bees' role as pollinators. Although, other types of bees also pollinate plants. Which is why the Americas had pollination long before the honeybee came along. Bees collect pollen sometimes as food, and sometimes pollen clings to them incidentally as they search for nectar. Either way, they can transfer pollen between flowers. Native pollinators include: carpenter bees (family Anthoporidae), sweat bees (family Halictidae), burrowing bees (family Andrenidae), leafcutter bees (family Megachilidae),and the bumble bees (family Apidae). Some of these bees pollinate very specific plants, others are less specialised.

Honeybees (Apis mellifera), once called 'white man's flies', are not native to the Americas. (Book of Mormon not withstanding.) Since their introduction from Eurasia, honeybees have spread west faster than the pioneers. They are Apidae bees, as are the bumble bees. Honeybees are social insects with a complex queen dominated caste system. One African sub-species, and its introduced hybrids, are known as 'killer bees'. Killer bees are more aggressive nest defenders than Eurasian honeybees. (Most people who have been stung by these bees do not die! Actually, these bees are called 'killer bees' only when they sting gringoes!) Honeybees usually cannot pull their barbed stinger out of thick skinned animals. Therefore, when they sting humans the sting organs usually rips out of their body. The hapless honeybee usually dies after one sting.

The honeycombs honeybees produce are beneficial to human beings. However, the honeycombs are often hidden in hollow trees, and may not be noticed until the hive is disturbed, and someone gets stung. Honeybee inhabitation of hollow trees has reduced the access of native fauna to hollow tree trunks. There is evidence that these bees have ousted some native birds that nest in hollow trees. The Carolina parakeet (Conuropsis carolinensis) once nested in such hollows. Honeybees, along with human hunting, have almost surely contributed to the extinction of this parakeet.

Bumble bees (Bombus spp.) are a group of large bees. There are at least 50 North American native bumble bee species, and many Old World species also. They are squat and hairy, like carpenter bees. These bees are excellent pollinators. They nest underground in burrows. They are social insects, with small colonies of workers and queens. Only the gravid queen survives the winter. They can sting repeatedly, unlike honeybees. Although, generally they are less aggressive than honeybees.

Bumble bees are quite large. If they cannot reach into the nectary of a flower, they chew it open to expose the nectar. Thus it is that bumble bees often wreck small or narrow flowers. Nevertheless, the fact that they pollinate larger flowers outweighs the demerits of their destructive behaviour. They are more of a pest to gardeners than to gardens. They sometimes sting people when their nest portals are disturbed. Care is needed when cultivating or weeding in the vicinity of their nest sites. Avoid destroying the tunnels of bumble bees.

Leafcutter bees (Megachile spp.) are small dark coloured bees, of several species. The queen is said to be 'solitary', because she alone takes care of a nest of larvae. These bees do have one drawback, from a human point of view, they cut-apart leaves! They cut out discs of leaves with which they use to fashion their larval cells. One group of related bees, the mason bees (Osmia spp.), line their larval cells with mud, not leaves. Both bees tend to nest in holes that they bore in deadwood. All of these leafcutter and mason bees are pollinators of flowers.

Leafcutter bees are a 'pest', after a fashion, as they often slice apart leaves. Plants in the lilac, ash and rose family are often the targets of these bees. The damage looks like arc shaped cuts, or discs, of missing leaf blade. The bore-hole nests of certain species can disfigure exposed wood surfaces. The benefits of these leafcutter bees, especially pollination, far outweigh their destructive features.

Gypsy Moth

Gypsy* moth or spongieuse (Lymantria dispar) is a serious pest. The adult moth is about two centimetres in wing-span. Males are brown and thin, and females are grey winged and plump. The female, although winged, does not seem to fly very often. In the spring the female crawls up the trunk, attracts a male, mates, and then she lays a large egg mass covered by a silken tent. Eggs over-winter, and hatch in the spring. The caterpillars climb into the crown to feed. The caterpillar is dark, 'hairy', and has blue spots along its thorax, and red spots on its abdomen. It feeds voraciously. It will consume whole leaves, not just making holes as winter moths do. The cocoon covered pupa matures into an adult moth in the same season. By June or July, the adults of the next generation are out and about.

Gypsy moth is one of the most serious pests in eastern North America. Originally it was an endemic of Eurasia. It was introduced accidentally in 1868, and has since become widespread. It feeds on a variety of trees including apple, pear, aspen, mountain-ash, oak, and willow. It can also feed on conifers, such as spruce and pine! Few insects consume such a variety of foodstuffs.

In the late 1980s a new strain of gypsy moth was found on the west coast. It has been repeatedly eradicated in spots. Apparently this strain is Siberian in origin, and differs a little from the familiar kind. In particular, in this subspecies both males and females fly. It is much more winter hardy, and it is even more likely to eat conifer needles. With diligence, this subspecies shall remain under control.

Control

Gypsy moth caterpillars can be controlled by broad-spectrum insecticides. Spraying is most effective just after egg hatching in April. Luckily, the moth can be very well controlled with the biological control agent Bacillus thuringiensis. On very small trees the caterpillars can be controlled by shaking or plucking them off manually.

* Note: Obviously, the current English name of this moth is politically incorrect. (The Romany people, I am sure, do not appreciate the slight.) As far as I know, there is no other common English name for the moth.

Sawflies

Sawflies are a group of thick-waisted wasps. The larvae look a lot like small bald caterpillars with dark heads. The female adult wasp has a saw-like ovipositor that cuts plant tissues, to inject the eggs. The eggs hatch and the larvae crawl out to seek fruits, leaves or whatever organ they specialise in eating. Some species feed en masse, others are more spread out. Some species skeletonise leaves. Some devour leaves entirely from tip to petiole. Some burrow into fruits, as the apple sawfly (Hoplocampa testudinea) does. Most when disturbed rear-up and regurgitate their stomach contents. Probably the issue of this foul-tasting vomit is meant to discourage birds and other insectivores.

There are many species of sawfly wasps, in several taxonomic families. Most feed on rather specific plant taxa peculiar to the sawfly species. Two species of special note are:

Pear sawfly (Caliroa cerasi) feeds on pear, apple and cotoneaster leaves. The larva feeds by eating the surface of the leaves, leaving a skeleton of veins. It over-winters in the larva stage, the pupa and adult phases occur in the spring.

European sawfly (Neodiprion sertifer) feeds on Scots, Austrian and mugo pine. The larva's feeding leaves behind the rachis of each needle it eats. Strangely, they prefer to eat year-old needles, the young needles are generally left. It can infest en masse large tracts of Scots pine. Adults emerge in the autumn to mate and lay eggs inside pine leaves (needles).

Control

Sawfly larvae are not caterpillars, thus they cannot be controlled with Bacillus thuringiensis. Broad-spectrum insecticides are somewhat effective if applied when the larvae first appear. Luckily, a natural virus(es?) can control sawfly outbreaks. Sometimes a sawfly infestation will suddenly end in a mass die-off caused by the virus.

Whiteflies

Whiteflies are not actually flies, but Hemiptera (bugs) in the Aleyrodidae family. Adult whiteflies are tiny with white moth-like scaly wings. They often appear to the human eye to be as small as gnats. They lay their eggs on the undersides of leaves, and these hatch into crawling larvae. The larvae move around, find a feeding place, and then excrete a scale cover, like a scale-insect. Like most 'bugs' they feed off plant sap, which they obtain via their piercing ‘beak‘.

There are many species of whitefly. Only a few attack trees and shrubs in Canada.

Azalea whitefly (Pealius azaleae) is a European whitefly. The adults emerge in mid-summer. The larval scales are flat and green, and are mostly on the undersides of the leaves. Generally one generation per year occurs.

Mulberry whitefly (Tetraleurodes mori) is a native whitefly that infests mulberries. Sometimes great numbers of adults congregate on leaf undersides in the summer. The larvae are black and white, and quite visible.

Glasshouse whitefly (Trialeurodes vaporariorum) is a tropical whitefly. It does not over-winter outdoors in Ontario, but only indoors or in glasshouses / greenhouses. It infests many perennials and even annuals. Hibiscus and poinsettias are among the shrubs and trees it can infect. Populations outdoors are often larger than those indoors.

Control

Whiteflies can be controlled with insecticidal soaps when the adults appear. However, some have recently evolved resistance to many insecticides. Spraying and washing plants before taking them in for the winter is a good idea. Adults are attracted to yellow, so yellow sticky-traps can be used to trap them. It is possible to control whitefly with a biological control agent, the parasitic wasp Encarsia formosa.

Apple Maggot

The apple maggot fly (Rhagoletis pomonella) feeds on the fruit of apples, pears and hawthorns. It is a housefly-like fly with black zigzag markings on its wings. It lays its eggs on immature fruits. Its larva is a small legless 'maggot'. The maggot tunnels about inside the fruit. It pupates into an adult fly in the soil. Significant damage to cultivated fruits can be caused by these maggots.

A few centuries ago apple maggot flies fed mostly on the pomes of hawthorns (Crataegus spp.). Since then, a new set of races has evolved. These subspecies, each genetically suited to their host, feed on the relatives of the haws: the apples (Malus spp.), the pears (Pyrus spp.) and the roses (Rosa spp.). It can also feed on Prunus species. The large cultivated fruits the pioneers brought were a bonanza for the native hawthorn maggot. It evolved rapidly to fill the new niches.

Control

In orchards the apple maggot is controlled by insecticides, such as Malathion, applied after the flower bloom. Home owners are sometimes taken by surprise when infestations afflict their backyard fruit trees. Generally these infestations are not equally severe every year. Household insecticide application in the spring can be effective if this is a recurring problem. For those who wish to use organic pesticides, Rotenone is somewhat effective.

Honeylocust Plantbugs

Honeylocust plantbugs (Diaphnocoris chlorionis) are a true hemipteran 'bug'. They feed mostly on honey locust. They also occur on black locust, which they tend to harm less. These small bugs resemble leafhoppers, but the plantbug nymphs have more prominent pre-wing buds. The late nymphs and adults are about 3 millimetres long. The nymphs are like the adults, squat, with wide spaced eyes, stubby pre-wing buds, and piercing mouth parts. This plantbug has a cockroach-like flattened body. It is the feeding of these nymphs that can weaken developing leaves such that some branches may be virtually defoliated, even before they fully flush out. Often the more severely defoliated branches die in the following spring. By about mid June these nymphs moult into winged adults. The adults fly off to search for new locust trees, to lay the eggs for the next generation. Eggs are inserted into the bark in midsummer, where they over winter. There is only one generation per year.

Some combination of factors caused these sap sucking insects became a major problem in the spring of 2004, in the environs of Toronto.

Leafhoppers

Leafhoppers are bugs in the homopteran clade. There are many species of leafhoppers, which feed on a variety of hosts. One common species infests locust trees. The honeylocust leafhopper (Macropsis fumipennis) is a sap-feeding bug with folded-back wings. The small nymphs of leafhoppers are paler than the plantbug nymphs. Leafhoppers have broad heads, wide set eyes, and they have a lateral flattening and a cicada-like body. Leafhoppers often have several generations per year But the leafhoppers are not as damaging as are honeylocust plantbugs. Honeylocust plantbugs often share their habitat with honeylocust leafhoppers.

Control

Plantbugs can be somewhat controlled with insecticidal soaps or contact insecticides. Like all sucking insects, these controls are not always fully effective. Usually, by the time the withered leaves are noticed, much of the damage has already been done. Leafhoppers and plantbugs can, in their early stages, be partly controlled by knocking them off by hosing them with water. When they are adults, blasting them off is not as effective, as the adults can fly back to their host tree.

Chafers & White Grubs

white grub & chafer

A number of beetles cause damage to plants in both their adults and larval stages. The large pale beetle larvae, with bodies bent head-to-tail, which live in the upper soil layers, are often called ‘white grubs’. These white grubs are one of the food items that skunks are seeking when they dig-up lawns. Most white grubs are members of the Scarabaeidae family. Many white grubs feed on grass roots, although broadleaf roots may also be attacked. Most white grubs live for about one year underground, pupate in the earth, and emerge briefly as breeding adults in the summer. The adults feed on leaves, often on different host plants than the larvae. Scarbaeid beetles are distinguished by their shiny metallic-looking forewings (elytra), thick rounded bodies, and ‘L’ shaped antennae with lamellate tips. They are very similar to the scarabs (dung beetles), which were a popular motif in ancient Egyptian artwork.

The Japanese beetle (Popillea japonica) is about a centimetre long, with ametallic-green head, thorax and legs. The elytra are shiny and copper-coloured. It has distinctive tufts of white bristles around the rim of its abdomen. The white grub feeds mostly on grass roots. Unlike most white grubs, the larvae can cause significant damage to turf grass. (Anything that kills turf grass can't be all bad.) Adult feeding can skeletonise both leaves and petals. The adult phase feeds on many species including: apple, birch, horsechestnut, linden, maple, oak, rose, and the prunus species. This introduced species can be very damaging to garden plants.

The rose chafer (Macrodactylus subspinsus) is less than a centimetre long, narrower than most other scarabs, and has golden-coloured elytra. The larval form feeds on a variety of grass and weed roots, causing little damage of human concern. The adult phase skeletonises many plant leaves, including: rose, apple, mountain-ash and other members of the rose family. Grape, peony and birch are sometimes targeted. The adult beetle is toxic, and birds learn to avoid it. The rose chafer is native to the Carolinian region, and has close relatives in other regions.

June beetles, May beetles or ‘Junebugs’ (Phyllophaga spp. ) are a set of scarabaeids with bodies over a centimetre long with chestnut-brown elytra. The larva forms generally feeds on grass roots. Some species, like the P. crinita of Texas, are a turf pest. In the north, the larval forms may spend up to three years underground, and do not significantly damage turf grass. The adult beetle feeds on a wide variety of plant hosts. Oak leaves are the preferred food source of many June beetle species.

Control

White grubs often attract skunks and raccoons, which dig-up turf to catch the grubs, and in the process they ruin the lawn. Often the animal damage is worse than the grubs themselves. Mothballs can be used to repel these digging animals. Some white grubs can kill turf grass outright. Most white grubs can be reduced in number with various commercially available poisons. An application of insecticidal dusts around valued plants can reduce local white grub populations, and thus reduce the number of adult beetles. The adult beetles are of greater significance as garden pests. The feeding beetles can be knocked down with a strong contact insecticide. It is best to apply the contact insecticide when the beetles actually appear on the leaves.

Ambrosia Beetles & Fungi

Ambrosia beetles are a peculiar group of borers. The adult beetle is small, a few millimetres long, black and elongated. It is only the female beetle which bores, as an adult, into sapwood or heartwood to form brood chambers. These chambers are several centimetres deep, with several short side galleries. The eggs inside these chambers hatch to form little grubs. The larvae feed on a particular fungus, and do not themselves bore into wood. In the spring the adults emerge, the females seeking their preferred host species.

Ambrosia beetle larvae live on a fungus. One common species is Ambrosiella brunnea. The imperfect, or asexual, form of the fungus used to be called Monilia brunnea. The fungus, as known, is mostly asexual in its reproduction. The fungus forms a dark coloured mycelium in the wood around the brood chambers. The fungus and the beetle have a mutualistic relationship, as it is mostly the beetle which spreads the conidial spores.

There are many species of ambrosia beetle, in the genera Monothrum, Trypodendron, Xyleborus, Xylosandrus and Xyleborinus. The introduced Asian ambrosia beetle (Xylosandrus crassiusculus) is a problem for many tree species, including sweetgum, hickories, elms, oaks and magnolias. The western ambrosia borer (Monothrum scutellare) is especially a problem for coast live oak. Research has shown that, as long suspected, various decay agents can penetrate into a tree along the brood chamber tunnels. Since both the beetle and its fungus garden need oxygen, the beetle clears out her tunnel of frass. The well aired tube allows fusarium, hypoxylon, inonotus and other fungi to instigate rot deeper into the tree. Even the sudden oak death oomycete can spread more rapidly along an ambrosia tunnel (Švihra & Kelly 2004).

Control

The ambrosia beetle can be controlled with insecticides. Only if the timing of spraying is opportune is this effective. Spraying for borers has always been problematic. Controlling the beetle could reduce the rate of heartwood decay.

Beetle-Fungus Symbiosis

Beetles and fungi often have symbiotic relationships with each other. The Scolytus beetles are often said to have a symbiotic relationship with the elm wilt fungus. The beetle more easily feeds on weak elms, and it seems to spread the fungus - as if to weaken its host. But some beetles are even more obviously symbiotic than that. These beetles can spread fungi about, and the fungi in turn feed the beetles’ larvae. This kind of symbiosis occurs in the aforesaid ambrosia beetles. Symbiosis also occurs in the beetle genera: Dendroctonus and Ips. In some species, the beetle has a special pouch (mycangium) that carries the spores or mycelial fragments. The same beetles may also carry tiny mites. The mites in turn also feed on the fungi. Mites and beetles are more-or-less competitors. The mites using the beetles mostly as mere vectors.

Aphids

Aphids are homopteran bugs in the Aphididae family. They have small squat bodies, often they have horn-like siphunculi on their abdomens, they can have four wings, and they have piercing mouth parts. They have a gradual metamorphosis from nymph to adult form. During the height of their feeding-season, usually summer, the aphids are often wingless. During this feeding-season they tend to reproduce via parthenogenesis or ‘virgin birth’. That is, the mother’s unfertilised egg hatches inside her. The nymphs thus ‘live-born’ are all females. Sometimes these female nymphs are born already pregnant! Cycles of parthenogenesis may buff-up aphid populations to incredible numbers. At the end of the feeding-season, male aphids are born. Then the winged adults, both male and female, take flight to seek mates.

Like most ‘true bugs’ aphids feed on plant sap. In fact, they feed so voraciously that their excreta can contain plenty of undigested sugar. This ‘honeydew’ then becomes food for ants, fungi, and other creatures. Indeed, aphids themselves are preyed upon by many other animals.

There are many species of aphids, most are rather specialised as to the host plant species they can feed upon. Aphids do not always build into outrageous populations. This is because predators keep them in check, and also because plant sap contains toxins to deter sap-feeders. Nevertheless, sometimes aphid populations become so high that plants wilt, and even die, from loss of sap. Furthermore, aphids can spread viruses, and their honeydew can instigate fungal growth.

Ecologic conditions that lead to aphid plagues are rather particular, they depend on rain, temperature, predator populations, and other factors. It is difficult to predict when aphid populations will explode. Aphids often proliferate on plants that have been over-fertilised. With excessive nutrients, plants often produce new shoot growth out of proportion to their defensive toxin production. If the plants lacks sufficient natural toxins, aphids can more easily proliferate. Luckily, aphids are one of the few insects that can be controlled by relatively mild insecticides, including insecticidal soaps. Aphids can be checked in very early spring by dormant oil application. If a few aphids appear early in the season, it is best not to over-react. Too much insecticide, applied too soon, can kill-off natural aphid predators.

References

Anulewicz, A., McCullough, D.G. and Cappaert, D.L. 2007. Emerald Ash Borer (Agrilus planipennis) Density and Canopy Dieback in Three North American Ash Species. Arboriculture & Urban Forestry. 33(5): 338-349.

Cranshaw, Whitney. 2004. Garden Insects of North America. Princeton University Press. Princeton.

Grierson, D. and Covey, S.N. 1988. Plant Molecular Biology. 2nd Edition. Blackie. London.

Klepzig, K.D.; Moser, J.C.; Lombardero, F.J.; Hofstetter, R.W. and Ayres, M.P. 2001. Symbiosis and Competition: complex interactions among beetles, fungi and mites. Symbiosis. 30: 83-96.

Rose, A.H. et Lindquist, O.H. 1982. Insectes des feuillus de l'est du Canada. Ministère de l'Environnement Service canadien des foréts. Ottawa.

Smith, D.C. 1988. Heritable divergence of Rhagoletis pomonella host races by seasonal asynchrony. Nature (336): 66-67.

Strauss, S. 2003. Voracious Asian beetle discovered in city maple trees, officials say. Globe & Mail. Saturday September 13. A17.

Švihra, Pavel and Kelly, Maggi. 2004. Importance of oak ambrosia beetles in predisposing coast live oak trees to wood decay. Journal of Arboriculture. 30(6): 371-376.

Wilson, I.M., Haack, R.A. and Poland, T.M.. 2002. New wood-boring insect kills ash trees. Arborist News. 11:(5): 13-14.

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