Approach to pests through plants

Press, Volume CXV, Issue 33928, 22 August 1975, Page 7

Approach to pests through plants

For a long time farmers have relied on insecticides as their main weapon in what seems to be a losing battle against pasture insect pests. Ideas being developed by scientists in the Entomology Division of the Department of Scientific and Industrial Research could now radically change the whole attitude to the problem.

Since the banning of DDT, no suitable, really effective substitute insecticides 'have been found, and as people are realising the major disadvantages of spraying toxic chemicals over the countryside, new methods of control are being sought. The insect physiology section of the Entomology Division, led by Dr Oliver Sutherland, is looking at ways of exploiting the natural resistance that some plants have to grass grub and other pasture insect pests. Indeed, farmers have suspected for some time that grass grubs and black beetle avoid certain plants, such as lucerne and Lotus pedunculatus. Investigations by Dr Sutherland’s group are aimed at determining exactly what it is in the plants that discourages the insects from feeding —

help identify the chemical factors conferring plant resistance to black beetle and also white-fringed weevil. Preliminary studies carried out by Dr J, Farrell (Entomology Division, D.5.1.R., at Lincoln) and Dr J. Mann (Applied Biochemistry Division) have shown that there is also a strong feeding deterrent for porina in the leaves of resistant Lotus species, but it has not yet been identified. Dr Farrell carried out the initial screening of pasture grasses, legumes and clovers for resistance to grass grub. Amongst these were ryegrass, clover and Phalaris, and although no appreciable levels of resistance were found, it might be possible to introduce resistance factors into their genetic material.

Once the complete characterisation of the known feeding deterrents and res i s t a n c e factors is achieved, the potential for their use is enormous. It should be practicable to breed for higher levels of the deterrents in plant roots to make them unpalatable to the pasture insects and much less damage would then be done to the plants by insect attack. This would result in a substantial increase in production together with a reduced dependence on envi-ronmentally-harmful pesticides.

which chemical substances make them resistant to attack — so that plant breeding programmes for the highest levels of insect resistance in pasture plants can be developed. The task of screening currently available pasture plant species for insect resistance is being undertaken by the Ministry of Agriculture and Fisheries insect control group at Ruakura, and D.S.I.R. entomologists at Lincoln. Plant resistance can result from a lack of attractive chemicals in the plant; from the presence of feeding deterrents making it distasteful to insects; or from the presence of substances in the plant tissue which are toxic to the insects.

In the case of lucerne, those varieties with high saponin levels in the foliage are not suitable as feed for sheep and cattle, so that future plant breeding programmes should be directed towards reducing the saponin levels in the foliage and increasing them in the roots, in order to produce plants which are both suitable for sheep and unsuitable for grass grub and black beetle.

Strong feeding deterrents for both grass grub and black bettls have already been isolated from lucerne and Lotus pedunculatus roots. In lucerne, complex chemicals called saponins have been found to be highly effective feeding deterrents, and it is suspected that lucerne resistance to grass grub attack depends largely on the level of those chemicals in the roots, although there is recent evidence that more than one compound is involved. Saponins from lucerne are also extremely effective against black beetle, and in laboratory tests concentrations of as little as 10 parts per million reduce feeding significantly.

As this sort of approach, to insect control is relatively new in New Zealand, Dr Sutherland is to make a special overseas visit this month to discuss his work and its implications with specialist scientists working in the same area of research. He will visit the Max-Planck-Institute for Behavioural Physiology in Munich; the Agricultural University at Wageningen, Netherlands; Imperial College of the University of London, at Silwood Park, and the Department ■ of Zoology, Cambridge University.

Different chemicals are involved in the resistance of Lotus pedunculatus, which has apparently developed an especially formidable defence against pasture pests. There appeal to be at least two deterrents for grass grub and three for black beetle, all of which are different chemically from each other and from those present in lucerne.

He will discuss the ways in which plant resistance can most quickly and efficiently be utilised and study the working relationship between other entomologists . and plant breeders.

Plant breeding programmes as yet are at an early stage and they involve close collaboration between the insect physiologists, M.A.F. and Grass-

Studies of plant resistance to black beetle are in their early stages, but this month, Dr Edward Block, from the University of Kansas, has joined the insect physiology section to

lands Division of D.5.1.R., Grasslands Division is currently making agronomic studies on Lotus species, lucerne and other pasture plants, with the objective of incorporating these into current farming practice as well as studying changes in farming methods which the use of resistant plants may entail. Plant breeding programmes are inevitably long term projects, but the end result could mean a big saving to the agricultural industry.