Insect fungi for the control of brown planthopper Nilaparvata lugens, and Malayan rice bug, Scotinophara coarctata

Introduction : Many potential pest organisms are normally kept at densities below damage thresholds by naturally occurring natural enemies in virtually all agricultural crops. This natural control can be enhanced by introduction of new biological agents ("classical biological control") or manipulation of indigenous organisms (Chapter 1). Integrated pest management (IPM) combines biological control with other forms of pest control, such as cultural, mechanical, and chemical methods. However, natural biological control should be the basis for all integrated pest management systems, and certainly in rice - which is, at present, a low input crop.Key pests. There are very few key pests in tropical rice. Key pests occur on a regular basis, and can cause substantial damage without being provoked by man's activities, e.g. excessive pesticide usage. For example, in tropical rice Malayan rice bug is a key pest, brown planthopper a secondary pest. Control programs should focus on specific control of these key pests without damaging natural enemy populations and provoking secondary pests.Pesticide use. In several rice growing areas, e.g. on Guadalcanal (Solomon Islands), and on Java and Sumatra (Indonesia) disastrous pest problems are being created through excessive pesticide usage. Programs aimed at minimizing pesticide use and restauration of natural enemy complexes have been initiated in these areas.Brown planthopper and Malayan rice bugNilaparvata lugens , brown planthopper (BPH). Introduction of rice varieties resistant to BPH, the use of broad spectrum insecticides on a large scale, and other agricultural measures provoked BPH to become a major insect pest of rice in tropical Asia. In this area, BPH is an example of an induced, or secondary, pest.In rice growing areas in Korea and Japan BPH outbreaks are caused by immigration waves from China. After the fallow period of the winter the relatively low populations of natural enemies are overwhelmed by the immigrating BPH populations. However, also in these temperate areas the massive population growth is stimulated by insecticide usage. Broad spectrum insecticides kill natural enemies and thereby render biological control less efficient.Scotinophara coarctata , Malayan rice bug (MRB). This shield bug was probably inadvertently introduced in Palawan (Philippines) about one decade ago. Now, damage can be found in large areas of rice all over Palawan, although its relative occurrence can differ between different areas on the island. Effective insecticides were identified, but chemicals are expensive for the Palawan subsistence rice farmer, and some of the effective insecticides can resurge BPH populations. Introduction of parasitic wasps from Australia, Malaysia, and the USA were not successful; the wasps established in the MRB populations, but parasitization levels are low. MRB is a key pest on Palawan - damaging populations build up regularly without provocation by man's activities, such as insecticide applications.Specific control. Conventional insecticides are, in general, expensive, can cause secondary pests, and insects develop resistance to the compounds. Therefore, development of specific and inexpensive control methods for BPH as well as for MRB are needed.Insect pathogens. Insect pathogens have been used for pest control. Several of the organisms (insect bacteria, -fungi, -nematodes) can be produced on artificial media, and applied in the field.A literature review on insect diseases, i.e. insect fungi, -viruses, -bacteria, -microsporidia, and - nematodes, of rice pests is presented (Chapter 2). The bibliography contains 278 references, and the major groups of insect pathogens as they relate to rice pests are briefly discussed. Available records on diseases, insect hosts and locations of collections are tabulated. Also, the references are grouped by subject, e.g. by insect pathogen, host, laboratory vs. field studies, etc.It can be concluded that there is much information on various diseases of rice insects in the literature. However, only few publications report biological control experiments with insect pathogens in the field.Insect fungi. Insect fungi infect the insect host by penetration through the cuticle rather than by oral ingestion; the fungi can infect sucking insects such as plant- and leafhoppers and pentatomid bugs. These insects do normally not ingest microbes from the internal parts of the plant; natural epizootics of insect fungi are often observed in BPH populations, and also the MRB can regularly be collected infected by fungal pathogens.The insect fungi are safe for important groups of natural enemies such as predators and parasites as well as to wildlife and man, they can be isolated and grown on artificial media, and various production methods are available. Fungus products can be stored, and applied in the field using conventional spray equipment.These fungi might be of use in the biological control of rice pests, in particular of sucking pests such as BPH and MRB.Field tests. Field experimentation with various insect fungi for BPH and MRB control is discussed in Chapter 4. The fungi are applied as suspensions of conidia and as suspensions of a dry mycelium product . The mycelium is grown in fermentors, dried, and milled. Conidia are produced on the mycelium clumps sticking to the plants in the field - and these conidia can infect the insects.BPH. Five entomopathogenic Hyphomycetes were tested under field conditions for biological control of BPH. Suspensions of conidia of Metarhizium anisopliae, M. flavoviride var. minus, Beauveria bassiana, and Hirsutela citriformis were applied equivalent to a rate of 4.5x10 12conidia/ha. In addition, M. anisopliae and Paecilomyces lilacinus were applied as preparations of marcescent mycelium at a rate of 1.5-2 kg/ha. Mortality due to fungus infection ranged from 63 to 98% 3 weeks after application. There were no consistent differences between the fungi. The mycelium sporulated on the plant and was as effective as the conidia suspensions in causing high levels of fungus infection.In a second experiment different rates of dry mycelium and suspensions of M anisopliae conidia were evaluated for BPH biological control. The mycelium was applied at an equivalent rate of 700, 3500, and 7000 g/ha.; the conidia were applied at a rate equivalent to 2.5 x 10 12conidia/ha. Significant control of the BPH populations was achieved at 2 weeks after application and up to harvest of the rice; differences between treatments were present but not consistent.These field experiments show that a) the insect fungi can infect BPH in the field, b) M. anisopliae infective materials can significantly suppress BPH populations, and c) dry mycelium can be equally effective for infection as well as suppression compared to conidia of the same fungus. These results justify research on maximizing the marcescent process (Chapter 5).MRB: In this wet season experiment the effects of the entomopathogenic fungi B. bassiana, M. anisopliae, and P. lilacinus on MRB populations were studied. AD fungi occur naturally in MRB field populations, although natural infection levels are low (5-10%). B. bassiana and M. anisopliae were collected from Philippine MRB populations, and P. lilacinus from a Malaysian population. The latter species was introduced in the Philippines in the course of these studies; the species established and could be isolated from MRB collected near the original experimentation site 1 year after the field studies.In the experiment adult MRB were kept on rice plants in cages in the field. Cages were placed in 4 different plots in 2 different fields. Mass produced marcescent mycelium, as well as suspensions of conidia of the fungi were tested. MRB numbers were significantly reduced in all fungal treatments compared to the control over a period up to 9 weeks, except in one of the plots where severe drought occurred. In addition, numbers of nymphs were suppressed in the irrigated plots. The overall performance of the different species of fungi and different materials was similar.Mass-production experiments. Possibilities for large scale application of entomogenous fungi for rice pest control would be greatly enhanced if dry mycelium can be used rather than conventional products containing insect fungus conidia. In the marcescent process dry myceIium materials are produced by liquid fermentation. The slurry is filtrated, stabilized with additives, dried, and milled. The product is thus a dry mycelium powder. This material can be formulated, stored, and applied in the field with conventional spray equipment.In preliminary experiments it was found that B. bassiana mycelium. remains viable after washing and drying without protectants. This is in contrast to mycelium of M. anisopliae and M. flavoviride var. minus, which die after washing and drying without protectants. Also, B. bassiana mycelium produces about 5-10 times as much conidia per mg dry mycelium compared to M. anisopliae. Therefore, B. bassiana was selected for these growth experiments.The constituents of the liquid fermentation medium is of key importance for mycelium growth in the fermentor and subsequent sporuIation on the plant in the field. In this chapter the effects of the composition of liquid media on B. bassiana growth and conidiation are reported. Two carbohydrate sources (sucrose and maltose), and one nitrogen/vitamin source (yeast extract) were tested for growth and conidiation. Maximum mycelium growth (12.31 mg/ml) was in the sucrose(3.5%)/ yeast extract(3.5%) medium, but washed mycelium from a maltose(2%)/yeast extract(0.75%) medium produced the maximum of 4.6x10 6conidia/mg.In commercial production yields per fermentor volume, rather than yields per ing dry mycelium are important, especially when relatively cheap media such as sucrose and yeast extract are used. With the data the yield per volume was calculated. The sucrose(3.5%)/ yeast extract(3.5%) and the maltose (2%)/ yeast extract(0.75%) media produce most conidia (equivalent to 3.52-3.72x10 7conidia/ml) per fermenter volume.Production estimates. Industry can grow B. bassiana in large scale fermenters at an efficiency rate up to 25 mg dry mycelium /ml. MRB and BPH can be controlled, in the wet season, with a dosage of about 2.5-5x10 12conidia/ha. It can thus be estimated that about 20-40 l fermenter space can produce material for treatment of 1 ha of rice for these pests.Conclusions. In this chapter the economic feasibility of large scale production of B. bassiana for rice pest control is discussed. It is estimated that a small production unit, producing material for treatment of 17.500 ha/yr, and functioning over a 5 year period can produce the mycelium at a cost of about US$ 15.-/ha. This basic price dictates that the final market price will certainly be, more than US$ 30.-per ha, which is more than for which most carbamate insecticides sold in tropical Asia.However, the fungal products are specific, do not cause resurgence, are safe for humans and wildlife - and their use should be stimulated with government subsidies or international funds.Finally, it is concluded that, in rice, microbial and other selective pesticides should be sparingly used. They should be merely regarded as purely corrective measures for the occasional key pest. The applications are relatively expensive, and with the present trends in world rice prices the profit margins become very narrow - which dictates farmers to minimize pest control inputs and rely on natural biological control as much as possible.Taxonomy . During the course of field experimentation numerous rice insects infected by insect fungi were collected. These collections contained the new taxon Metarhizium flavoviride v ar. minus var.nov., and the rare insect fungus M. album Petch (Chapter 3).M. flavoviride Gams and Rozsypal can be divided in the variety flavoviride from curculionid beetles and soil from temperate areas, and in the new variety minus which was isolated from homopteran insects from the tropics. The new variety was found on BPH in the Philippines and Solomon Islands, Recilia dorsalis (Cicadellidae) in the Philippines, and a grasshopper in the Galapagos Islands; its conidia are smaller (mostly 4.5-7 x 2-3 μm) and more consistently ellipsoidal to ovoidal than those of var. flavoviride . The new variety may form synnemata in culture. The varieties differ in the morphology and dimensions of the conidia and phialides, and in characteristics of the colonies on agar media.In a second taxonomy study the species M. album Petch is restored for a species from plantand leafhoppers of rice. In the Philippines and Indonesia M. album caused epizootics in populations of Nephotettix virescens and Cofana spectra respectively. M. brunneum Petch is a synonym of M. album; the species is characterized by the pale brown color of the conidial masses, clavate phialides, 10-12.5 x 2-3.5 μm, ovoid to ellipsoidal conidia, (3)4-6 x 1.5-2.5 μm, and growth of bulging masses of hyphal bodies rather than mycelium prior to sporulation. It is suggested that the primary criteria for delimiting species of Metarhizium are the -shapes of conidia and conidiogenous cells, -presence or absence of a subhymenial zone of swollen hyphal bodies, and -whether the conidia adhere laterally to form prismatic columns. The occurrence of many natural and artificial color variants of Metarhizium species suggests that colors of conidial masses and mycelium have only secondary taxonomic value. Conidial size is useful in delimiting species. A synoptic key to the taxa of Metarhizium is provided.