INSECT CONTROL OF NOXIOUS WEEDS.

New Zealand Journal of Agriculture, Volume XXXIV, Issue 2, 21 February 1927, Page 84

INSECT CONTROL OF NOXIOUS WEEDS.

JOINT SCHEME INITIATED AGAINST BLACKBERRY AND OTHER SPECIES. A visit was made last year to America and Europe by Dr. R. J. Tillyard, F.R.S., head of the biological branch of the Cawthron Institute, on various scientific business. One of the most important investigations undertaken was that of the biological control of pest weeds, with special reference to blackberry (Rubus), with which New Zealand is particularly concerned. The line of biological control proposed by Dr. Tillyard is that of the introduction of insect natural enemies of the respective weeds —a method which involves considerable contingent danger, and which therefore had hitherto been deprecated by several leading entomological authorities. Before Dr. Tillyard’s departure from New Zealand an agreement was made between him and the Director-General of Agriculture in connection with any experiments to be undertaken for the control of blackberry by insect agency, with the object of safeguarding against the introduction of species which might also attack economic plants. The conditions of the agreement were as follows : (i.) No species to be forwarded from any country to New Zealand except such as are known to feed on species of the genus Rubus only. (2.) All shipments on arrival in New Zealand to be taken charge of by an officer' of the Department of Agriculture, who shall examine the cages to see that they are intact and that no insects can escape from them while being forwarded to Nelson. (This would allow of broken or damaged consignments being either destroyed or their cages repaired before forwarding.) (3.) Dr. Tillyard to furnish to the Director-General of Agriculture an account of the life-history of each species selected for study. (4.) The permits granted for introduction of all Aw&ws-feeding species to be permits restricting the study and rearing of such insects to closed insectaria and cages in the Cawthron Institute grounds and laboratories. (5.) All such insects to be thoroughly tested within such insectaria or cages on all important economic plants, particularly introduced Rosaceae, such as apples, pears, stone-fruits, roses, &c. (6.) If considered necessary similar tests to be made in country of origin before shipment. During his visit Dr. Tillyard discussed his plans very fully with Dr. L. O. Howard at Washington, Dr. G. A. K. Marshall in London, and Dr. D. A. Imms at Rothams ted, with the result that all these representative authorities signified their approval of them. When in England negotiations were initiated by Dr. Tillyard for the organization of research into the problem of control of noxious weeds by their natural enemies. The result was the acceptance of a scheme by the Empire Marketing Board, the New Zealand Government, and the Cawthron Trust. For the purpose of this work a grant of £4,000 per annum for five years has been made, one-half to be contributed by the Board, one-fourth by the Government, and onefourth by the Trust. The work is to be carried out under the control of Dr. Tillyard, with the approval of the New Zealand Research Council. The annual expenditure is allocated to cover the salaries and expenses of a field entomologist and assistant in New Zealand,

and an entomologist and assistant in England; -salary of Dr. Tillyard ; supplies from Europe and America; insectaries and apparatus ; travelling-expenses; and miscellaneous. A report on his tour and investigations, dated 7th January, 1926, has been furnished to the Government by Dr. Tillyard. . The first section of the report, dealing with the present subject, is as follows : —■ SUMMARY OF THE PRESENT POSITION AS REGARDS BIOLOGICAL CONTROL OF NOXIOUS WEEDS.

There is at present only one completed piece of research along these linesviz., the attempt to control lantana in the Hawaiian Islands by the introduction of its natural enemies. In this case the line of attack was to try to prevent the plant from seeding. This attempt, though carried out some years ago without such safeguards as we should now deem necessary, was entirely successful. The plants left on the islands, failing to seed, gradually died out, and some blocks were burnt and cleared also. At the present time lantana is no longer a menace in the Hawaiian Islands. A much more ambitious attempt on a larger scale is that being now carried out by the Federal Government in Australia for the control of prickly pear by insect enemies. It is perhaps too early yet to prophesy complete success for this vast enterprise, where no less than thirty millions of acres of land are involved, with an increase of the pest engulfing another million acres annually; but the latest' reports indicate that a number of valuable insects have been acclimatized, and can do very fine work in destroying the pear. As a result of my studies and inquiries in America and Europe I feel able to state definitely that there is a very good prospect of either partial or complete control of a number of pest- weeds in New Zealand without exposing the country to any serious danger in other directions. The following is a short summary of the principal insects which, in my opinion, ought to be studies in connection with biological control of pest weeds :- Blackberry. (i.) Insects which attack the Crown and Stem by Boring or Gall-forming. Coroebus rubi Linn. : A Buprestid beetle. The larva does great damage to blackberry in southern Europe, up to 60 per cent, of the new stems being destroyed in some years. It does not touch raspberry or any other plant, except that occasionally it is known to attack, more or less accidentally, the long runners of the Frau Karl Druschki and similar roses. This habit would bar this insect from entry into New Zealand under clause i of the agreement given.* As, however, the beetle is common right through Hungary, where roses of all sorts are grown in enormous numbers for perfume, and has never been known to attack roses anywhere else except at Grasse, in France, I am of opinion that it should be given a trial. It would be one of the exceptional cases where, it seems to me, a permit might be granted for the testing of the insect under New Zealand conditions, especially

as it is one of the few blackberry insects which does not destroy the raspberry, and in view of its great potential value in destroying blackberry. Agrilus ruficollis Fabr. : A Buprestid beetle found in North America. Smaller and less destructive than Coro ebus rubi, but does considerable damage at times. Attacks blackberry, raspberry, dewberry, but not roses or any other plants. A provisional permit has been granted for this insect, and the first consignment has been forwarded by Dr. Howard to Panama, where it was transhipped for New Zealand last month. Bembecia marginata Harris : A rather large clear-wing moth from North America whose larva damages the crowns of blackberry and raspberry, forming galls which kill the stems. It can be controlled in raspberry by careful cutting out of the infected canes. Does not attack roses or any other plants. A vigorous insect of great potential value. A provisional permit has been granted for this insect, and the first consignment has been forwarded by Dr. Howard with that of Agrilus ruficoUis mentioned above. Bembecia hylaeiformis Lasp. : The smaller European representative of this genus, having a smaller larva which feeds in the same manner, but is only known to attack blackberry and dewberry. A most valuable insect, for which a provisional permit has been granted. Not found in England. Supplies will have to be obtained probably from Austria. (2.) Insects which attack the Twigs. . ' • Diastrophus rubi Htg. : A small Cynipid whose larvae form galls ranging from 2 in. to 8 in. in length on the twigs of blackberry, dewberry, and raspberry. Not known to attack any other plants, with the exception of a single record from bracken —probably an erroneous determination. Can- be easily controlled on raspberry by cutting out the galls. A provisional permit has been granted for 'this insect. As it only occurs very locally in Europe and is not found in England, supplies will not be easy to obtain. A large number of parasites are known from this insect. When these are eliminated the damage which it is capable of inflicting on blackberry should be very considerable. The galls usually completely prevent the fruiting of the twig attacked. There are also large galls on blackberry in Europe formed by the Cecidomyiid fly Lasioptera rubi; but, as far as I can find out, these galls rather tend to stimulate than check the growth of the plant, the insect acting as a natural pruner. For this reason it will not be considered further at present. (3.) Insects which attack the Leaves and Shoots. The attack on these parts is to be regarded as auxiliary to the main attack on the stems. The leaves being the parts where the foodsupply is formed, much weakening of the plant can be caused by a strong attack on them but destruction of the leaves alone is not likely to succeed in controlling so vigorous a plant as blackberry. Thyatira batis Linn, (peach-blossom moth) : The name is given owing to the forewings having oval marks coloured white and pink, just like the petals of the peach -blossom. It is now known that

these markings are a natural. protection to the moth while resting on blackberry, and really imitate, not peach-blossom, but blackberryblossom. The caterpillar feeds on the leaves of the blackberry and raspberry. It can be easily controlled on the latter by a single arsenical spray, applied early in the season .so as not to affect the fruit. It is common in England and all over Europe. A provisional permit has been granted for the insect, and two consignments of pupae have been sent from England. Broods are now being reared at the Cawthron Institute. Habrosyne derasa Linn, (buff arches moth) : Allied to Thyativa batis, but the larva has a different mode of feeding, and the pupa goes underground for the winter. The caterpillar feeds normally on blackberry, very rarely touching raspberry. Common in England and Europe. Owing to published but somewhat doubtful records of this moth having been found eating hawthorn and hazel, the supplies obtained by me in England have been left behind at Rothamsted, where Mr. Davies* has instructions to test the larva on these and other plants fully before sending it out to New Zealand. Cidaria albicillata Linn, (beautiful carpet moth) : This is a smaller geometrid moth whose larva feeds on blackberry and raspberry ; also recorded from strawberry (wild), alder, and clematis by one writer (Scorer), but these records need verification. Supplies obtained by me in England this year, but left behind at Rothamsted for testing. Tischeria margined H.W. : A small Tineoid whose larva makes mines and blotches on the leaves. Only known from blackberry. Common in England and Europe. A provisional permit has. been issued for this insect. ' Notocelia uddmanniana Linn.: The larva, common in England and Europe, is a leaf-roller which attacks blackberry and raspberry. Easily controlled by arsenical spray. A provisional permit has been issued for this insect.' Schreckensteinia festaliella Hb. : A small Tineoid found in Europe whose larva feeds on the leaves of the Rubus. A provisional permit has been issued ‘for this insect. Typhlocyba tenerrima H.S. : A small leaf-hopper, very rare in England and Europe, but perhaps capable of doing considerable damage. Only known to feed on blackberry. A provisional permit has been issued for this insect, but I have instructed Mr. Davies to carry out very full tests at Rothamsted before forwarding supplies. Monophadnoides rubi Harris : A sawfly found in North America whose larva does considerable damage to the leaves of raspberry and blackberry. Easily controlled by a single arsenical spray. A provisional permit has been issued for this insect, which should prove of considerable value against blackberry. Metallus rubi Forbes : A small sawfly from North America whose larva forms mines and blotches in the leaves of the blackberry only. Probably of considerable value. A provisional permit has been issued for this insect.

(4.) Insects which attack the Flowers and Fruit. I have not been able to find any insect so far which is effective in attacking these parts of the blackberry. Various species of By turns attack the fleshy receptacle of the fruit of species of Rubus, but they all seem to prefer raspberry to blackberry, and in any case they do not prevent seeding, but only make the fruit unpleasant to eat. The Anthomyiid fly, Phorbia rubivora Coq., known in America as the raspberry-cane maggot, is very deadly on raspberries, but prefers them to blackberries. There may be other species of this genus which will only attack blackberry, but I have no record of them so far. The bl ackberry Pterandrus rubivorus Coq., found in South Africa, is stated never to attack raspberry or any other fruit. But in my opinion it will be necessary to have very full tests made with this insect in its country of origin before considering its use against blackberry in New Zealand. General Summary. From the above it will be seen that a z very strong attack can be developed' by means of insects against the blackberry. The main weight of the attack must be directed towards destruction of the crowns and stems, with a subsidiary attack on the food-supply of the plant by destruction of the leaves. The weak spot of the attack is the absence of reliable species to attack the flowers and young fruit, but quite probably future research may remedy this. Allowing for the admitted toughness and rapidity of spread of this very vigorous plant, I am. still of opinion that, under the favourable conditions available in New Zealand, control of blackberry can be obtained through its insect enemies, provided it is understood that such control carries with it a menace to raspberry requiring a single spring spraying with arsenic, and a certain amount of watchfulness in cutting out infected stems each winter when pruning. Even if we admitted Coroebus rubi, which I regard as the most promising of all blackberry insects, the menace to roses would be infinitesimal, and would probably be confined to occasional attacksA>n\ the water-shoots of Frau Karl Druschki. Gorse.’j The problem of controlling 'gorse is ajvery special.^'one. It is admitted that this plant is of value in supplying nitrogen to the soil, and also in providing, when young, good fodder for sheep. Consequently it appeared to me necessary to find some insect which, while not destroying the plant, would as far as possible prevent it from seeding. The explosive scattering of the seeds from the ripened pods appears to be the only important method of spread of the plant. Consequently an insect which will eat out the. pods without previously destroying the blossom is highly desirable. Such an insect occurs in many areas in England, chiefly on commons. It is Apion ulicis, a tiny weevil which feeds on gorse without doing much damage, but whose larva feeds inside the green pod and destroys a very large percentage of the seeds. So successful has this insect been in its attacks during the last three or four years on Harpenden

Common, alongside the Rothamsted Experimental Station, that the botanist there has not been able to obtain supplies of seeds during that period. Apion ulicis Forst, is well known throughout Britain, and feeds only on gorse, though there is a record of its having been found on broom once, which needs confirmation. The mode of feeding of .both larva and beetle indicated that it would not attack any of the softer leguminous plants. ' At Harpenden the beetle has been common for many years without ever touching the clover, lucerne, peas, beans, or any other Leguminosae. Dr. Imms has given it as his opinion, and I agree, that any further testing of this insect in England is a mere waste of time, as it is already quite clear that it will not attack any plants of economic value. - I think it should be tested out on blue and yellow lupins under New Zealand conditions, but do not anticipate that it will be able to damage either. A provisional permit has been granted for a single consignment of these insects. In my opinion this one insect should suffice for the complete control of gorse in New .Zealand. I regard it as one of the most valuable and also one of the safest insects so far discovered in connection with, the control of pest weeds. Tortrix ulicitana attacks the flowers of gorse, and might prove of value, but I should not recommend a study of this insect except in the improbable event of Apion ulicis proving a failure. St. John’s Wort. This weed is more especially an Australian problem, but thepossibility of its becoming a serious pest in the North Island of New Zealand and elsewhere in the Empire must not be lost sight of. In the Ovens Valley, in Victoria, it has greatly increased in size and vigour, and has put a very large area of land out of cultivation. - The plant presents a special problem owing to the general lack of the knowledge of the insects which feed on it. Chrysomela varians and Chr. hyperici attack the leaves, both as larvae and as beetles ; neither species is known to feed on anything else. Anaitis plagiata (treble-bar moth) and its close ally A. effumata are geometrid moths whose caterpillars attack both plants, but are so abundant in places where St. John's wort is rare that it seems highly probable that they do actually feed on other plants also. Several species of Perrisia form galls on the plant, especially P. hyperici and P. serotina. A number of Tineoid moths attack the leaves and shoots, especially Depressaria hypericella, Cracilaria auro guttata, Epinotia hypericana, and Aristotelia atrella. ■ Thus it will be seen that, provided the introduction of these insects can be made with safety to other plants of economic value, there is a strong probability of the insect enemies gaining complete control. The policy of attempting to control this weed biologically is a sound one, provided that sufficient work is done first in England to ensure the safety of the species later imported into Australia. . Ragwort. This weed is spreading very greatly over large areas of the Empire, including New Zealand. As it is poisonous to both cattle and horses, and takes possession of large areas of both good and waste land, the problem of its future control is an important one.

There are two insects which attack the weed vigorously and to a large extent successfully in England—viz., Tyria jacobaeae Linn, (the cinnabar-moth) and Homoeosoma cretacella Rsl. The former has a voracious larva which often eats the plant completely to the ground, and must be considered a most valuable species for control; the latter has a smaller larva which mines in the stems and damages the flower-heads. The combination of these two species should ensure adequate control. The main difficulty would appear to be the acclimatizing of these moths in New Zealand. This may prove unexpectedly difficult, especially as the pupa of Tyria jacobaeae seems to be subject to a severe fungoid disease. Tyria jacobaeae feeds also on groundsel, and has once been recorded on coltsfoot (this needs confirmation) ; both these plants are weeds. Homoeosoma cretacella is only known on ragwort. a A provisional permit has been issued for both insects, and large supplies of. Tyria jacobaeae have already been sent to New Zealand. Late frosts destroyed Homoeosoma in England in 1926, but supplies should be available in 1927. Tests will be very carefully carried out with these moths to find out whether they will damage any of the native species of Senecio or allied genera. Foxglove. This is a very poisonous plant, which is likely to become a bad pest in hilly country in various parts of the Empire, including portions of New Zealand. Scarcely any insects are known to feed upon it. A small moth, Eupithoecia pulchellata Steph., attacks the flowers ; the larva later bores into the seed-capsule and prevents seeding to a large extent. The heath fritillary, Melitaea athalia, is generally reared by breeders on foxglove, but its normal host plant appears to be the parasitic Melampyrum, another pest fortunately absent so far from New Zealand. Provisional permits have been issued for both these insects, and a supply of hibernating larvae of Melitaea athalia has been shipped in cool storage to New Zealand. ■ The attempt to rear a race of this insect which shall normally feed on foxglove is of the greatest scientific interest. Normally a female moth or butterfly will only lay its eggs on the food plant on which its larva originally fed. Thus the problem is to rear a brood artifically on foxglove in captivity, and then select those females which show the greatest, tendency to return to this plant for egg-laying. As Melampyrum is absent from New Zealand, nonsuccess of this attempt will only mean that the insect will die out. Convolvulus. It appears to be the general opinion of botanists in England and New Zealand that this plant (Calystegia Sepium) will become a very serious menace in the near future. While at present there seems to be no prospect of control by means of insect enemies, the subject is by no means exhausted, and every effort will continue to be made to find a species which will prevent it from seeding or will destroy it in some other manner.

* As quoted on opposite page.—Ed,

* Mr. W. Maldwyn Davies, M.Sc., appointed entomologist at Rothamsted under the scheme of research.