INSECT PESTS

Press, Volume LXIX, Issue 20989, 18 October 1933, Page 14

INSECT PESTS

NUMBERS AND WEATHER KOTHAMSTED RESEARCH iraois oua own cokksspondiht.) LONDON, September ID. The chief line of work in the entomological department at Rothamsted Experimental Station (says the annual report of the institution) is the Study of the factors determining the size of insect populations. Insect pects are always with us, but so long os their numbers are small they are comparatively harmless. Sometimes, however, one species begins to multiply, and its power to increase is so enormous that the harmless few rpeedily become a serious pest causing great loss of crops. Hitherto the factors responsible for this rapid multiplication have been but little known, and consequently it has not been possible to take preventive steps beforehand or even to warn farmers of the probability of attack. This subject is now under full investigation at Rothamsted. Soon after Dr. C. B. Williams entered, en his duties as head of the department on July 1, 1932, he began an investigation into the relation of insect numbers to weather conditions. The great difficulty has hitherto, been to find some numerical expression of the abundance of insects; Dr. Williams is trying to overcome this by taking daily samples of all flying insects under definite standard conditions, and identifying and counting them. He does this by means cf a light trap, operating from sunset to sunrise and fitted with a mechanism for dividing its period of operation into eight sub-periods, so as to show the actual hours during which each catch of insects is obtained. The trap is near to the meteorological enclosure, so that the precise meteorological conditions during each subperiod are known. All the working condtions, including the intensity of the light, are standardised, so that the catches of each season may be comparable with those of any other. It is hoped in time to obtain data from which relations between weather conditions and rate of multiplication of insect populations may be worked cut. ; Migration of Insects. It does not necessarily follow that a large catch of insects means a large multiplication of the local population. Insect migrations are-known to occur and steps are now being taken to follow them. A migration of small cabbage white butterflies was observed at Rothamsted in mid-August, 1932; the horde was traced to the Norfolk coast, where it had arrived presumably from the Continent; it had travelled westwards passing over Rothamsted and the resulting larvte did a good deal of damage to cabbages in September. Another factor affecting the size of the insect population is the degree of

parasitism: this is being studied by Dr. Barnes, using certain of the jnidges as the test insect. Attractiveness of Plants. An interesting observation was I made on two of the grass plots by Dr.! Sharga in studying one of the thrips j infesting the grasses. Where the i grassland had been treated with lime about 12 to 20 per cent, of the thrips were parasitised by a nemstode Tyienchus aptini, Sharga; where. : however, the grassland had received no lime, the thrips were free from parasites. This is now being further investigated. Another subject of investigation in the department is to find how the insects are attracted to the host plant. Apparently they have some sense of smell, but among different varieties of the same plant some are attractive and others are not. The property is transmissible to the offspring, and Dr. Barnes has tested willows supplied from Long Ashton. Thus some willows are resistant to the attack of a willow midge that ordinarily does much damage; these are being studied by Dr. Barnes. Mr Newton is endeavouring to find what difference in the willow accounts for the difference in attractiveness to the zmdge. At a meeting of the Addington Burgesses' Association on Monday evening Mr E. Parlane stated that he had been asked by several residents of Burke street to bring before the City Council a request for tar-sealing the street. He did not, aS reported, de«cnb?,the condition of the street as wretched" but said that as it was Btaost a main thoroughfare residents nad considered it should be tar-sealed. I^^™, °nd cattle aie marketed for slaughter by the Home Counties in Britain every year Reptile skins are now being so widely used that more than 2,500,000 lizard, crocodile, and pythan skins alone exported last ear from India