A Biological Study of scale insects on Chalk Grassland

Abstract

Attached

Five species of grass-infeating scale insects, representing five general and three families were collected at Rough Common, wytham Wood, Berkshire, and the biology of two of them (Greenisca placida and Eriopeltis sp.) was studied: Famiy – Coccidae Subfamily – Filippinae I. Geus Eriopeltis signoret, 1872. 1. Eriopeltis sp. II. Genus Parafairmairia Cockerell, 1899. 2. Parafairmairia gracilis Green, 1916 Family – Eriococcidae III. Genus Greenisca Borchsenius, 1948. 3. Greenisca placida (Green, 1921) IV Genus Rhizococcus Signoret, 1875. 4. Rhizococcus pseudinsignis (Green, 1921) Family – Pseudococcidae V Genus Dysmicoccus Ferris, 1950. 5. Dysmicoccus walker (News tead, 1891)

In all the casea, except Parafairmairia gracilis, the favourite food plant was found to be Brachypodium pinnatum (L.) Beauv.. Both Greenisca and Eriopeltis were found to be host-plant specific from laboratory experiments. No experiments were carried out to determine the food plants of either. Rhizococcus or Dysmicoccus. However, at wytham, both have also been observed on Agrostia sp., and the latter occasionally on Nardus stricta and Zerna (Bromus) eracta Huds. as well. The hosts of Parafairmairia gracilis were determined (by Dr. Wells) to be carex flacca Schreb. and Zerna erecta Huds..

Detailed studies were carried out on Greenisca placida (Green) and Eriopeltis sp., Greenisca placida (Green, 1921) was recorded from Britain and USSR (Leningrad) only, and was first reported and described by Green, as Eriococcus placidus Green, 1921. The species of Eriopeltis that occurred at wythans, is defferent from the other two species known to occur in Britain. Eriopeltis festucae (Fonsc., 1834) on Festuca is on the Britain list and the other species known at present as Species A (on Agrostis at Silwood Park) and Eriopeltis sp. on Brachypodium pinnatum from Wytham, are new to Britain.

Data on the life histories of both Greenisca and Eriopeltis on Brachypodium pinnatum, at Wytham, showed that in this wood each of them has only one generation a year, consisting of four stages in the female, namely, egg, two nymphal instars and adult, but six stages in the male: egg, two nymphal, one prepupal, one pupal and the adult male. The egg stage lasts from August to May; first instar from mid May to mid June; second instar from early June to late July and the adult females from late July to late September. Apparently, Eriopeltis eggs hatch about a week later than those of Greenisca and its life cycle is of longer duration. All the stages of the female Greenisca including the adult before the secretion of its ovisac are mobile whereas in the case of Eriopeltis all the stages except the crawler (early first instar nymph) are unable to move. Males of both species emerge from mid July to early August and have a very short life of 1-4 days.

Morphological characters of each stage were studied. Detailed descriptions and figures for each stage of the two species (on Brachypodium pinnatum ) are presented. The adult female Greenisca placida is redescribed and briefly compared with Greenisca inermis (Green, 1916) on Deschampsia flexuosa. The males of both Greenisca and Eriopeltis belong to the `lecanoid type’ and the description of the male of the genus Greenisca is given for the first time.

At Wytham, in 1969, female Eriopeltis commenced oviposition in the second week of September, by which time Greenisca had already finished laying their eggs and shrivelled up. Eriopeltis females being about four times the size of Greenisca females usually produced four times as many eggs or more. When both species were bred in the greenhouse it was found that Greenisca could tolerate greenhouse temperatures (about 260C) and reproduce whereas adult female Eriopeltis could not do so. Perhaps the higher temperatures did have an injurious effect on the reproductive system of female Eriopeltis.

Although male were very rare in the field the sex ratio was found to be 3… : 2… for Greenisca (by laboratory experiments). In the laboratory, males and females occurred in equal proportions in Eriopeltis, with a tendency for more males to occur when warm and dry conditions prevailed during development. Crawlers of both Greenisca and Eriopeltis were found to move away from direct and bright light. This may account for the crowded occurrence of adult females of both the species on the basal third to half of the grass blades. Their preference of a more basal position may be due to three reasons: firstly, to protect themselves from being dislodged and blown away by the wind, especially during the crawler stage when they are vigorously searching for a suitable host (food) plant; secondly, to avoid direct bright light, as mentioned earlier, and higher temperatures during the summer; thirdly, to get the maximum amout of food and protection, as the grass usually starts br browning and drying off from the tip downwards.

Dispersion by emigration, wind and animals can occur at any stage of the life cycle in Greenisca, but not after the ovisac has been secreted, unless mechanically transported. In Eriopeltis natural dispersion can occur only during the crawler stage, the other instars having lost their powers of movement. Both Greenisca and Eriopeltis were seen to be attacked by a number of chalcid parasites. Trichomasthus frontalis Alam, was the most important but it was in turn attacked by a pteromalid, Pachyneuron concolor Forster which was equally abundant. The other entomophagous parasites of Greenisca included Microterys zarina (Walker), Metaphycus ? piceus Hoffer, Protyndarichus comara (Walker) and Rhopus (Rhopus) piso (Walker). Eusemion, cornigerum (Walker) was a hyper parasite of Greenisca placida.

Metaphycus zebratus Mercet, Subprionomitus cantabricus Mercet and Cerapterocerus nirabilis Westwood were bred from parasitised material of Eriopeltis out of which C mirablis was a hyperparasite. Eunotucus cretaceous walker is an egg predator which was seen to be closely tied ecologically to Eriopeltis sp. at wytham, not attacking other species of scale insects. Cheiloneurua paralis (Walker) was bred from both Eriopeltis sp. and Greenisca. The Chamaemylid fly. Leucopis silesiaca Egger was an egg predator of both these scale insects at Wytham. None of the parasites were seen to attack the male instars and the first nymphal instar. The parasite larvae, specially those of Pachyneuron suffered a very high mortality in the field during winter. The mean percentage parasitism tended to increase from 1968 to 1970 (1968 – 7.2%; 1969 – 15.6%; 1970- 29%). Out of the parasitised females of Greenisca the majority deposited approximately half their full complement of eggs, so that the mean number of eggs laid by a parasitised Greenisca was eleven.

The period in which suitable Greenisca were available to the parasites was 2-3 weeks. Although parasites were available in the field during this period they did not appear to respond to the presence of Greenisca but to the period when Eriopeltis had been available in the same locality in the past few years, about two weeks later than Greenisca. Judging by Prof. Varley’s field collections at Rough Common, Wytham, from 1964 onward, there had been a heavy infestation of B. pinnatum by Eriopeltis which by 1969 had been drastically reduced in numbers, probably due to the action of parasites and predators which later moved on to Greenisca.

Two species of Chalcidoid parasites, both Encyrtids, were reared from Dysmicoccus walkeri, namely, Leptomastix epona (Walker), and Anagyrus sp.. No parasites were reared from Parafairmairia gracilis collected at Wytham. The pedicelate nature of the egg furnishes during the early stages of parasitization, the sign if difference between a parasitised and an unparasitised host. During later stages the parasitised host appears hard compared to the soft-bodied unparasitised scale.

Two life tables were prepared for Greenisca placida for 1968 and 1969 generations respectively. The various mortalities suffered by Greenisca at various stages of its life history were studied by regular sampling, in an attempt to study the mortality factors affecting the various stages of its life cycle and to correlate the different mortalities with the adult density per unit area.

The reduction in natality was considerable but egg mortality was generally low. The nymphal mortality due to overcrowding was the highest. An insignificant mortality was caused by Coccinella septempunctata. Fungus attack was the cause of death of some nymphs and adults. Adult mortality was mainly due to host feeding, parasitism and predation and in a few cases the cause of death was unknown. Laboratory experiments showed, that the nymphal mortality is density dependent.