346 



THE AGRICULTURAL NEWS. 



OCTOBEK 21, 1916. 



INSECT NOTES. 



A SCALE INSECT PEST OF CITRUS AND 

 ITS CONTROL. 



A serious pest of lemons in Sioily is the scale 

 insect Chrysomphalus cUctyospermi. This w;is record- 

 ed in the West Indies as early as 1899, and has more 

 recently been identified on West Indian material 

 sent to Dr. Beilese at Florence. This pest appears to 

 be fairly wide-spread in the West Indies, but it does 

 not occasion any serious damage as it does in Sicily. 

 It is interesting to record the fact that Dr. Berlese 

 has found parasites on the soale sent from the West 

 Indies, but these have not yet been fully identified. 

 Perhaps the most important part of the investigation 

 by Del ( Juercio and Malenotte in connexion with the 

 pest, an account of which is reproduced below from 

 the Review of Ajtplied E-ntoviology (Vol. IV, Ser. A, 

 Part 4), is that concerning methods of control. These 

 possess an importance as regards the treatment of scale 

 insects generally. 



0. dictyospermi wa.s first discovered in Italy in 1895 by 



Berlese on Pandanas grananifolia in the Botanical Garden 



at Florence, but the present severe infestations are due to 



importations from France along the Riviera, whence they have 



spread to southern Italy. The various stages of this scale 



are described in detail. The number of males is twice that 



of the females, or even more, especially on the fruit. Tables 



are given to show the influence of the seasons on infestation. 



The temperature has a great influence on the number of 



larvae which emerge from the shields at the period of 



maximum emergence, an abrupt fall diminishing the number 



and an abrupt rise increasing it. A very high temperature 



in July, when the temperature rose to 104° F. at 1 p.m., 



hastened the dessieation and the fall of a large number of 



adults which had been sprayed with a .5 per cent, solution 



of colloidal polysulphide of calcium. In autumn the varying 



frequency and quantity of rain explains the different results 



yielded by colloidal polysulphide sprays. In the months 



of February, March, and nearly the whole of April, no larvae 



were seen to emerge even on heavily infested plants. This 



observation was in apparent conflict with tke presence of 



young larvae, but the.se were all born in the previous 



November and December. Larvae which attach themselves 



from mid-November onwards develop so slowly that the first 



moult has not taken place after a month, whereas only 



twelve to fifteen days are necessary with June larvae. 



Oviposition begins from late in April to early in May, but 



proceeds very slowly and the clusters never contain much 



more than about fifteen eggs, and may therefore often pass 



unnoticed. This species is certainly oviparous, but if the 



shield of a female be raised, it will be seen that most of the 



larvae have already hatched. The fact that more larvae than 



eggs are usually found beneath the shield accounts for the 



belief that the species is viviparous. Observations on 



Coccids of genera allied to Chrysomphalus show, however, 



that a species which is usually oviparous may occasionally 



produce living young, especially towards the end of the 



oviposition period. Observations on the speed with which 



the larvae of C. dictyospermi spread, show that they do not 



move far from the centre of infestation; they have a tendency 



to climb, especially if they happen to be on the trunk and 



larger branches, and if the lower portion of the foliage is 



infested, this is largely due to larvae falling from infested 



parts above; larvae which fall to the ground perish. External 



causes are therefore responsible for the spread of this pest 

 from plant to plant. Infestation is strongest on the leaves 

 and fruit, and also occurs on young shoots and on various 

 parts of the flower, the larger branches and the trunk being 

 immune. The principal causes of the spread of the insect 

 are pruning, gatheiing the crop, irrigation, rain and wind. 

 Pruning carried out when the scale is very active is more 

 dangerous than in winter. The best plan is to prune 

 in winter, either letting the prunings dry where they fall, 

 or removing them to a distance of several yards from 

 plants still uninfested. In the case of lemons, which 

 are also pruned 3,t seasons other than winter, the 

 prunings must be left where they fall, so that infestation 

 may be restricted to the plants already attacked Gather- 

 ing the fruit is usually done in winter, except in the 

 case of lemons, limes and some oranges. The danger 

 is therefore chiefly present in these latter cases. Irri- 

 gation of the citrus plantations is less dangerous than 

 appears at first sight, and it is only in cases where a larva 

 la carried by the water and thrown on a trunk around 

 which there is no ring of foam that it is able to 

 ascend the plant; infested leaves may, however, be carried 

 by the water. Gentle rain falling on mobile larvae washes 

 them down to the lower portions of the plant. A heavy 

 rain reduces the number of larvae by washing them down to 

 the ground, where they perish, but may carry infested leaves 

 to con.sideiable distances. Wind must however be held to be 

 the most important agent in difl'usion. Branches isolated by 

 rings of cotton from the infested portion of a plant were 

 found to be infested ''after a certain time if the wind was 

 blowing. The importance of wind carriage is evident from 

 the fact that plants sheltered from the wind are the most 

 strongly infested, as a larger number of larvae are left on them. 

 Strong wind and rain together act by carrying the larvae in 

 the rain drops, as has previously been noticed by Del Guercio 

 in the case of Ceropla's'tes sinensis. D.G 



A list of fifty-six food-plants is given, including the 

 following: — Acacia longifolia, A. retinoides, Arbutus 

 dndrdchno, Callistemon sp., Citrus auranfiuni, C. deliciosa, 

 C. Liiiionuvi, C . sinensis, Cycas revoluta Eriobotrya Japonica, 

 Euoiiytnus europijtuf^, E. japonicus, Ficus stipulacea, Hedera 

 helix, Kentia sp., Melalewa sp., Metrosideros sp., Muhlen- 

 beckia platpclados, various species of palms, Pandanus 

 graminifolia, Phm'tuium sp., Strelitxia re(/inae, and Zea 

 mays. Observations on these plants show that C. dictyospermi 

 can develop and attain maturity on them. The Coccid was 

 seen on some other species, but though the larvae became 

 attached on them, they died at a certain stage probably 

 because the juices were not suitable. This was noticed on 

 the grape vine, peach and apricot, which in Sicily are 

 cultivated promiscuously with citrus plants. C dictyospe>mi 

 was never ob.served on thfem when they were at a considerable 

 distance frem tlie citrus trees. They are therefore not true 

 hosts and may be transported without any risk of spreading 

 infestation. Nor is there any risk in the transport in winter 

 of deciduous plants of which the leaves alone are subject to 

 attack. It was foui'd that the natural mortality among 

 larvae in favourable conditions amounted to .5 per cent, 

 before fixation. In late autumn, however, especially in 

 rainy weather, eggs which had dried up and dead larvae 

 were observed under the mother's shield. After fixation 

 early in June, larvae fifteen to twenty days old showed 

 a natural mortality of 5 to 10 per cent. Later on, the adult 

 female mortality was 2 per cent., increasing to 15 per cent, 

 in October and 2-") per cent, late in November, at which time 

 the mortality of adult males was 40 per cent, and that of 

 larvae and nymphs 1 9 per cent. These figures were allowed 

 for in estimating the value of insecticides. 



