Elsevier

Crop Protection

Volume 27, Issue 1, January 2008, Pages 17-24
Crop Protection

Evaluation of Spodoptera exempta nucleopolyhedrovirus (SpexNPV) for the field control of African armyworm (Spodoptera exempta) in Tanzania

https://doi.org/10.1016/j.cropro.2007.04.005Get rights and content

Abstract

The African armyworm Spodoptera exempta is a major episodic migratory crop pest over much of Eastern and Southern Africa. Control of this pest has been reliant on the use of synthetic chemical insecticides. However, this approach fails to protect poor farmers and is becoming unacceptable for environmental and cost reasons. A programme of field trials was conducted in Tanzania to evaluate the endemic baculovirus, the S. exempta nucleopolyhedrovirus (SpexNPV), as an alternative control. Field trials demonstrated that both ground and aerial application of SpexNPV to armyworm outbreaks on pasture can initiate outbreaks of NPV disease and population collapses. The SpexNPV was effective when applied at 1×1012 occlusion bodies (OB) per hectare if applied to outbreaks early, when larvae are in I–III instar—mass mortalities appear 3–10 days post treatment. The data from these trials indicate that SpexNPV can have a potential role as a substitute for chemical insecticides in strategic armyworm management programmes.

Introduction

The African armyworm (Spodoptera exempta) is a serious pest of rangeland and cereals that erupts in episodic plagues across sub-Saharan Africa. These outbreaks vary from year to year but have a major impact on cereal production and livestock in the outbreak countries (Scott, 1991). The annual outbreaks of armyworm most often start in the identified primary outbreak areas of Tanzania and Kenya in January following the main rains (Haggis, 1987). Adults from these outbreaks then migrate to new areas following the seasonal rainfall patterns, to start new outbreaks in other parts of Eastern and Central Africa, and this sequence may continue through until June spreading North, East and South from the primary areas (Rose et al., 2000). Outbreaks may extend over many square kilometres with larval densities in excess of 1000 m2. Outbreaks are annual but intensity varies greatly from year to year. In Tanzania alone, during bad outbreak years many hundreds of thousands of hectares of crops may be attacked, but in some years no serious outbreaks may occur (Scott, 1991; Njuki et al., 2004). Control of this pest is routinely done through application of synthetic chemical insecticides. While these are technically effective, there is increasing concern over the environmental impact of these chemicals applied over wide areas. In addition, the cost of chemical insecticide is beyond the resources of farmers or national control agencies so that in many years only 30% of outbreaks are treated, with considerable loss of crops and damage to rangeland (Njuki et al., 2004). These shortcomings have stimulated the search for other more specific biological control options that would be safer and more sustainable.

It has long been known that the African armyworm has a number of natural enemies and pathogens, including viruses, fungi and protozoa, but the most important was reported to be a specific baculovirus, the S. exempta nucleopolyhedrovirus (SpexNPV) (Rose et al., 2000). This virus had been known since 1965 (Brown and Swaine, 1965) and its potential as a control agent has been highlighted on a number of occasions (Tinsley, 1979), but little progress to develop the NPV was made, probably because cheap and effective broad spectrum chemicals were available and considered acceptable. Research studies have confirmed that this disease is endemic in many parts of east Africa (Odindo, 1983) and that this NPV is highly pathogenic to armyworm (Odindo, 1981). Case studies on armyworm outbreaks confirmed that natural NPV can be a major cause of mortality in armyworm outbreaks (Persson, 1981). However, the NPV is rarely apparent in primary outbreaks of the pest, only appearing later in the season, and even then it can be highly localized, affecting only small parts of the outbreak area (McKinley, 1975). Trials to use crude macerated suspensions of infected larvae containing the NPV as an insecticide against armyworm had showed that this approach had promise (Brown, 1966). Subsequently, the virus was characterised (Harrap et al., 1977) and cross infectivity studies with a range of NPVs have indicated that the armyworm was susceptible only to the SpexNPV (McKinley et al., 1977, A.C. Cherry personal communication). The SpexNPV has been safety tested following FAO/WHO recommended protocols and no evidence of toxicity to mammals or non-target hosts was found (Harris, 1973) in agreement with a major recent safety review of Baculoviruses, which showed no evidence of adverse environmental impact from any use of baculoviruses as crop protection agents (OECD, 2002).

A new study to evaluate SpexNPV as an alternative to existing chemical and other control options began with a laboratory programme to optimise the mass production techniques for SpexNPV (Cherry et al., 1997) and the build up of a stock of SpexNPV for laboratory evaluation prior to field trials in Tanzania. Initial fieldwork began in 1999 at the end of the first phase, and then resumed in 2001 when a follow on study had begun. The field work in Tanzania was aimed at assessing SpexNPV in comparison to the existing chemical insecticide based control and also the use of local neem formulations, which had been reported to show promise against armyworm (Broza et al., 1999).

This paper reports on the field trials undertaken to evaluate SpexNPV as a control for armyworm outbreaks in Tanzania. The field trials reported here were carried out on pasture land, as it is commonly on pasture that eggs are laid and in which armyworm pass the early instars before migrating onto nearby land to attack cereal crops. In evaluating SpexNPV it was important to determine its viability when applied by lever operated knapsack systems used by smallholders, motorised mist blower, which is the method of choice for many farmers, and aerial application, which is the mainstay of the national control programme and also used by the largest commercial farmers.

Section snippets

The virus

The SpexNPV was a multiply enveloped NPV isolate (#0045), one of a number collected originally from wild S. exempta in Tanzania and Kenya in 1974. The virus was mass produced in third instar larvae using methods previously reported (Cherry et al., 1997). The NPV produced was processed using standard protocols developed at NRI (Hunter-Fuijita et al., 1998). The insects were, after storage at −20 °C, thawed out and macerated in distilled water to release the NPV. The suspension was then filtered

Results

The results of the preliminary ground trials at the PCS site (Fig. 1) confirmed that knapsack sprayer applied SpexNPV gave effective control of armyworm, even at the lowest SpexNPV rate used, 5×1011 OB ha−1; 96% of larvae were dead after seven days.

The results of the ground spray trial in 2002 (Fig. 2) illustrate the wide variation in armyworm densities seen in the field with small plot trials. In the control plots, the armyworm counts increased after treatment as insects hatched out during the

Discussion

In this series of trials the higher field application rate of 1×1012 OB ha−1 showed consistent control of armyworm, while the lower rate of SpexNPV applied, 5×1011 OB ha−1 did not reduce outbreak numbers in the 2002 field trial, although it had previously shown promise in glasshouse and preliminary field trials. The selection of the higher rate of 1×1012 OB ha−1 was a balance between rates showing efficacy with the need to keep rates low enough to be economic. The rate of 1×1012 OB ha−1 is similar to

Acknowledgements

This publication is an output from a research projects R6746 and R7954 funded by the Crop Protection programme of the United Kingdom Department for International Development (DFID) for the benefit of developing countries. The views expressed are not necessarily those of DFID.

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