10 PROCEEDINGS OF THE NEW ZEALAND ECOLOGtCAL SOCtETY, VOL. 15. 1968

ENTOMOLOGICAL PROBLEMS IN NEW ZEALAND FORESTS

R.ZONDAG

Forest Research Institute, Rotorua

INTRODUCTION

The devastation of the indigenous forests of NewZealand since the arrival of white settlers hasreduced them to only about 14.5 million acres(22% of the total land area). On the other hand,1.04 million acres have been planted in exoticforest, of which 870,000 are in plantations largerthan 50 acres. Pinus radiata, our main ,~xotic foresttree, occupies over 562,000 acres (65%) of theseholdings. Pinus ponderosa (including the un-promising variety scopulorum), occupies 86,000acres, Pinus nigra 69,000 acres, Pseudotsuga men-ziesii 62,000 acres and Pinus contorla 26,000 acres.The remainder is occupied by less importantspecies of Pinus. various species of Larix, Eucalyp-.{us, Cupressus, Chaemaecyparis, Abies, Picea andother genera.

The largest areas of exotic plantations are situ-ated in the Rotorua-Tokoroa-Taupo area whereP. radiata has been planted in extensive monocul-tures. Overseas entomologists have emphasisedthe vulnerability of these forests to attack byinsects, but so far their fears have proved un~founded. With the exception of the heavy mor-tality associated with attack by Sirex noctilio (F.)and its associated fungus, no epidemics haveoccurred. However, epidemics of a defoliator haveoccurred in the Eyrewell and Balmoral StateForests of the eastern South Island.

Epidemics of fungal diseases have been morespectacular. In the 1930s Phomopsis strobi killedand severely damaged many acres of P. radiata.At present the large monocultures in the NorthIsland and some forests in the South Island areaffected by Dothistroma pini, a needle cast fungus,which attacks P. radiata up to IS years old, butnot, apparently, older trees. P. nigra and P. pon-derosa are attacked at any age. The ultimateimpact of this disease cannot be predicted atpresent. A copper fungicide was aerially sprayedover 80,000 acres in 1967.

ENTOMOLOGICAL PROBLEMS IN NZ. FORESTS

When considering the entomological problemsin New Zealand forests a comparison should bemade first with forests in the northern hemisphere.

There, the number of species causing losses is fargreater than here. To mention a few examples:Species of Scolytidae (bark beetles) have causedmortality over thousands of acres and the losseshave been calculated in several thousand millionboard feet. Species of Tenthredinidae (sawflies)and Lepidoptera frequently cause severe defolia-tion; the spruce bud worm epidemic in Canadaand the U.S. has continued for several decadesand has caused heavy economic losses. and Dutchelm disease transmitted by insects has been respon-sible for widespread mortality.

New Zealand's forests have few insects capableof causing appreciable damage and in the last 15years only a few species have reached epidemiclevels. Scolytidae, both native and introduced, arepresent, but fortunately no potentially dangerouskillers have become established. Defoliating saw-flies capable of attacking forest trees are notknown here. Of the wood borers, Sirex noctilio,with its associated fungus, is regarded as the mostpotentially harmful insect to the pine forests.

Most of the endemic insects of the indigenousforests are usually kept under control by bioticand environmental factors, though epidemics dooccur at intervals.

It is vital to the future economy of New Zealandthat, with the increased speed of travel and greatertrade with other countries, timber.attacking insects,.,

especially Scolytidae, should be prevented fromentering. The first line of defence, the QuarantineService, has intercepted insects in articles ranging

. from swizzle sticks to heavy timbers, and althoughit is unlikely that all of these species would havefound New Zealand suitable for survival, thearticles attacked by them have been treated ordestroyed. A second line of defence is providedby the Forest Biology Survey, whose province isthe living forests. Unfortunately, its inspections arelimited to the exotic forests.

Three questions remain:(i) How good will our quarantine service be

in the future?(ii) How long will it take for new pathogen

to be detected after establishment?(iii) What action will we then take?

ZONDAG: ENTOMOLOGICAL PROBLEMS IN NZ. FORESTS

THE INDIGENOUS FORESTS

The value of the indigenous forests, especiallythe protection forests, cannot be readily measuredin terms of money. Loss of trees and shrubs in thehigh country has resulted in erosion and floodingin the lower reaches of our rivers. Browsinganimals not only prevent regeneration but causeinestimable damage by removing soil and initiatingbiotic changes.

Cutover areas in millable native forests areusually left depleted and derelict and are not likelyto produce a timber crop in the next century.Although enrichment plantings are carried out insuch areas, these can, as yet, be regarded only astrials because only a small part of the yearly cut(between 25,000 and 30,000 acres) is beingreafforested. Establishment of exotic forests ;nscrub and fern country is found to be easier.

Protection against insects has never beenattempted in native forests because of the beliefthat biotic factors soon bring sudden increases inpopulation under control. Besides, the nativeforests are regarded as uneconomical once thestanding crop has been sold. Consequently, theirrelevant insect fauna has received scant attention.Some indigenous insects are said to kill trees, buttheir importance has not been critically evaluated.

For example, the beech buprestid, Nascioidesenysii (Sharp) is regarded by some as the mainkiller of Nothofagus, but is it primarily responsi-ble? The health of trees before attack and thenature of the damage caused by the insect need tobe taken into account. Unpublished current workhas shown that a species of wilt fungus (Cerato-cystis sp.) is introduced by pinhole borers (Platy-pus spp.) and evidence has been found that theseborers will attack apparently healthy, as well asweakened and recently dead, Nothofagus. So far, itis known that the fungus may kill the cambium,and that the borer attack is often concurrent withthat of the beech buprestid. Such ;nvestigations, ifcontinued, could lead to a different view of the roleof Nascioides.

After the epidemic of Proteodes carnifex (Butl.)in 1954 which extended over 50,000 acres in thesouthern part of the South Island, mortality ofNothofagus solandri var. clifjortioides was attri.buted to mass attack by Nascioides. In this instancethe role of insects other than these two has notbeen considered.

11

Mortality asssociated with Nascioides usuallyappears a few years after beech has been felledand the canopy opened up. The question iswhether, with better hygiene after felling (removalof dead material), such mortality would occur.With poor hygiene, insects breeding in the slashcould kill the trees by mass attack after .omergence.Exposure of trunks to direct sunlight may ad-versely affect the cambium layer and encouragelocalised attack. These are simple possibilities, butit would need a team of plant physiologists, silvi-culturists and entomologists to investigate them.However, such a study may demonstrate a systemof management suitable for our native beeches.

Aenetus virenscens (Dbld.), the ghost moth, isresponsible for considerable damage not only tobeeches in the North Island, but also to other hard-woods. The holes it makes extend high up thetrunk and branches and beech regeneration isattacked while the trees are still young. The holespermit the entry of wood-rotting fungi whichfurther damage the timber. There is no knowncure.

The dying of Metrosideros (rata) and Wein-mannia (kamahi) have not received the attentionthey warrant, perhaps because the insects involvedare regarded as of minor importance. And insectsappear to be unimportant in the degeneration ofPodocarpus totara and P. hal/ii (totara). Bei/-schmiedia (tawa) and Dacrydium cupressinum(rimu) in the North Island. Whether in these fourspecies faunal climax has been reached is con-jectural at present.

In podocarp forest and the kauri forest remnantsof Northland little insect damage occurs.

THE EXOTIC PINE FORESTS

Forest nurseries

Damage caused by grassgrubs (mainly species ofCostelytra and Odontria) and weevils, such asGraphognathus leucoloma (Boh.) and Xyrostyg-nus binodulus Broun, - have been reported on

~;everal occasions. A few larvae per square footwill result in heavy losses, so control should aimat complete eradication. Regular checks for thepresence of larvae should be made and, if found,an insecticide should be applied. That recom-mended is dieldrin, at a minimum rate of 2 lb. ofactive ingredient per acre per year. The rate ishigh but the cost is. relatively low per 1,000 seed-lings. It is sometimes necessary to give a secondapplication at the same rate in autumn.

12 PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. IS, 1968

Originally, DDT was used at a rate of 5 lb. peracre and it is still used in some nurseries at a rateof 2 lb. per acre; but an amount of 15 lb. appliedin one nursery did not give immediate control.hence the change to dieldrin. Little work has beendone during the past five years with other insecti-cides to control soil-inhabiting insects in forestnursenes.

Establishment of planted stock

In 1967 failures in establishing Pinus radiataand Pseudotsuga menziesii were reported in clear-felled pine forest from many parts of the NorthIsland. The losses were attributed to Hvlastes ater.(Payk.) and grassgrub larvae.

The importance of the part played by Hylastesater is doubtful, as it usually selects weakenedseedlings for attack. Whether, in the circum-stances, mass attack would have killed the seed-lings is a controversial point. Trials withinsecticides are being made to eliminate the insectssuspected of causing the mortality.

Much remains to be explained, however, sincein the Karioi Forest no mortality has occurred inP. contorta plantings and, in the high risk areas ofthe Eyrewell State Forest, mortality associatedwith H. ater in planting trials has been as low as0.2 per cent.

Factors involved in other places may be thecondition of planting stock, the method of sitepreparation and the weather during planting.

The establishment of P. radiata in scrub- andgrassland has on several occasions resulted inheavy losses which have been attributed to grass-grubs. Protection with 21b. of DDT or dieldrinper 1,000 plants, applied as a spray, has givengood results. Such applications are not used regu-larly as they would increase the cost of establish-ment.

Regeneration of Pinus radiata

Regeneration of P. radiata in Kaingaroa StateForest after summer felling is generally good,though it varies. A considerable reduction in thenumber of seedlings occurs during the followingtwo years. Frost, fungal diseases, and intraspecificcompetition render the seedlings susceptible toattack by H. ater. In better sites, where regenera-tion is high, the reduction may be beneficial, butin poorer sites supplementary planting to replacelosses is often necessary.

Little or no regeneration occurs after winterfelling because birds, mainly chaffinches, and miceeat the seeds. The extermination of the birds andmice would, of course, be the answer. However,control of birds may result in wide repercussionsand possibly upset the existing biotic balance.Aerial seeding with 21b. thiram-coated seed, hasresulted in good strikes and reduction in cost ofestablishment. Thiram acts both as a fungicide anda bird-repellent.

Lepidopterous defoliators

Several species belonging to the family Geo-metridae (8 spp.) , Tortricidae (4 spp.), andPsychidae (l sp.) attack the foliage of exoticforest trees. Only one of these, Selidosema suavis(But!.) has, on occasions, reached epidemic levelsand caused serious defoliation. These outbreakshave occurred only in Eyrewell and Balmoral StateForests.

The first occurred in 1951-52 and in September1952 the total area infected was approximately8,000 acres. After a trial spraying over 500 acresin February 1952 to establish techniques, approxi-mately 7,000 acres were sprayed from the air with.DDT in September-October of that year, althoughthe entomologists doubted its desirability. Thespraying was most successful, but two weeks latera polyhedral virus disease was evident amongstthe caterpillars, which after another two weeks,reduced the population to normal in the 1,000acre control area. In some localities in bothforests defoliation after the end of the epidemicwas severe, only the last three or four branchwhorls remajning green. Epidemics occ1:lrred againin 1956-57 and in 1960-62.

The 1957 epidemic was not as severe as that of1951-52 "and was shorter. The 1960-62 epidemicwas as serious as that in 1951-52; both were inEyrewell Forest only. During these epidemics awatch was kept on the population levels of thecaterpillar, parasite and predator complex, andespecially on the increase of the polyhedral virusin the caterpillars. From these studies the time atwhich the epidemic would end was predicted andso no chemical was applied. .

Eyrewell Forest suffered from severe windblowin 1964 which resulted in the destruction of it asa living laboratory for the study of S. sua vis. Dur-ing the salvaging of the area H. ater created aproblem by infesting export logs. There was fearthat Hylastes would also affect re-establishment,but it did not.

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LONIJAG: IONTOMOLOUIl'AL I'RUHLbMS tN j~.L. I'URbStS111.1

Woodborers

The only woodborer responsible for killing pinetrees in New Zealand is the introduced Sirexnoctilio. It is regarded as the most potentiallyharmful insect for our pine stands and. with its

symbiotic fungus, usually kills trees weakened byother causes. The heaviest mortality occurredbetween 1946-51 when, in the Rotorua districl,25-30 per cent of the trees were killed over300,000 acres. In one area the stocking of 15-year-old stands was reduced from 700 to 85 trees peracre. It was fortunate not only that the lossesoccurred when the radiata pine was not being usedon a large scale but also that mortality was gener-ally confined to suppressed and malformed trees.The resultant thinning could be regarded as bene-ficial in that it permitted greater increments in thesurvivors. The mortality started after the droughtof 1946 had weakened the trees, the numbers ofthe insects increased and, over the years. byrepeated and mass attack, caused serious losses.Scattered mortality still occurs, but this is oftenassociated with incorrectly applied silviculturalpractices such as thinning and pruning just priorto and during the flight season and excessivedoses of arboricide in thinning by poison. Thenumbers of the wood wasp are being considerablyreduced by the presence of newer, effective bio-logical control agents and the more intensivethinning schedules of recent years have reducedits habitat.

Mortality in older P. radiata stands in theKaingaroa Forest has been very low during thepast five years and total basal areas' of stems peracre have risen above those which are regardedas safe. Although little is known about the resist-anceof the otder trees to Sirex, it may be that,when adverse conditions such as drought occur,losses could become high. In the younger standsa close watch is being kept on the mortality afterdefoliation by Dothistroma pini.

Although four species of parasites of Sirex werealready established before 1962, several other para-sites have been introduced since and are beingbred in the laboratories of the Forest ResearchInstitute. The breeding has met with varyingsuccess: three species have now been released; theoutcome of the breeding of two is still in doubtand two are known to have :cailed.

A nematode infection of Sirex. discovered in1962, is an additional important factor in its bio-logical control. Infection, which results in com.

plete sterilisation of the adults, has been found in

up to 95 per cent of the population in one locality.Such a high degree of infection has never beenrecorded for the combined insect-parasite-complexof the wood wasp in New Zealand. Introduction ofthis nematode to areas where it does not occur are

being made.

Although recently-introduced insect parasites

from Europe, India and Japan have been found tobe infected with nematodes, no such infection hasbeen found in the two parasites established in NewZealand, lbalia leucospoide.l' (Hochenw.) andRhyssa persuasoria (L.). Such precautions aretaken in breeding the recently introduced parasitesthat only adults free from nematodes are released.

Termites

The incidence of native dry- and wet-woodtermites, Kalotermes brouni Frogg. and Stolo-termes ruficeps Brauer, in farm shelterbelts andwoodlots of pine and larch has on several occa-sions made necessary the fumigation of logsintended for export. The attack by wet-woodtermites usually extends no deeper than threeinches into the branch stubs. The attack by thedry-wood termite extends into the heartwood andmay render the logs useless for sawn timber.Attack is usually commoner in overmature trees.especially in farm woodlots and shelterbelts, whereunhygienic conditions prevail. The problem maybe considerably reduced by early pruning and byremoving all dead logs and stumps. where theinsects breed. Only on one occasion has the wet-wood termite been found infecting living pines(P. taeda) in forest areas.

Miscellaneous

Logs in stock-pile are often attacked under thebark by Hylastes ater. These will not cause anydamage. but logs exported to other countries mustbe free from insects. Log stockpiles left too longin the forest have, on occasion, been attacked bv

-pinhole borers, causing damage to the timber.Stacked sawn timber is often attacked by huhubeetles (Prionoplus reticu/aris White) but thedamage is usually slight. About 10 inquiries a yearare received about its attack on treated P. radiatatimber. The young larvae hatch from the eggs andbore into the wood.

* Basal area: area of cross section of stem. usuallymeasured at breast height. When applied to a crop, thesum of all the stems or the total basal area per unitarea.

14 PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. 15, 1968

Problems in the less important species of intro-duced pines are often greater than in P. radiata. Inthe Cupressaceae, wood borers such as huhu, two-toothed longhorn (Ambeodontus tristis Fabr.) andtermites are often found breeding in the livingtrees, causing serious damage to the timber. Attackon spruces (Picea alba and P. sitchensis) by thespruce aphid (Elatobium abietinum (Walk.» andthe conifer spinning mite (Paratetranychus unun-guis (Jacobi)) results in poor growth, so that largescale plantings of these species have been practi-cally abandoned.

THE EUCALYPTS

Opinion differs widely on the future of eucalyptsas major forest trees for New Zealand. In myopinion several species of this genus may be usedto advantage to restock derelict cutover areas, andplantings could add considerably to our hardwoodresources. Some species are attacked more heavilythan others. The main insect problem for someyears has been Paropsis charybdis (Stal) whichcauses severe defoliation of several species. It isfortunate that some of the more promising are notattacked by this insect. Since the initial invasionof the North Island by Paropsis about 1955 defoli-ation has been heavy but, at present, its numbersare on the decline, probably as a result of adisease. The answer to the Paropsis problem isbiological control. The introduction of parasiteshas been considered for several years. The maindifficulty in obtaining them is that P. charybdis iscomparatively uncommon in Australia, so para-sites of closely related species which are moreeasily collected may have to be tried.

Not only would biological control be beneficialin deciding on the future plantings of suitableeucalypt species, but the aesthetic value of thelandscape would also be enhanced if defoliationcould be considerably reduced. For that reasonalone biological control should be encouraged.

The attack of pinhole borer (Platypus spp.) oneucalypts used in enrichment plantings in indigen-ous forests is being investigated. The result is notdeath of the trees, but damage to the timber, thesignificance of which will not be known until millstudies have been made on logs from mature trees.But because few eucalypt enrichment plantings aremore than 15 years old such material is not yetavailable.

Other introduced insects such as Gonipterusscutellatus (Gyll.), Eriococcus coriaceus Mask"psyllids, etc" are usually well controlled by theparasites and predators already introduced.

CONCLUSIONS

The approach to insect control in forests is moreor less biological and relies, for both native andintroduced insects. on appropriate biological con-trol agents. As far as possible the use of insecH-cides should be limited and chemical controlshould be undertaken only after careful study hasshown that biological controls are inadequate.

The problem would, in principle, not be anydifferent even should one of the potentially moreharmful insects become established. In that eventchanges in existing silvicultural practices mightha ve to be devised.

SUMMARY

The, main entomological problems of the indigenous,the exotic and coniferous forests of New Zealand arediscussed. Insects which could cause serious mortality orgrowth losses should be prevented from entering thiscountry. The control of forest insects already establishedin New Zealand depends mainly on natural and intro-duced biological agents and upon silvicultural practices.The use of insecticides is advocated in forest nurseriesand when biological controls fail.