Insects infesting sorghum ( Sorghum bicolor L. Moench) panicles in northern Ghana. 1. Distribution, species composition, and damage potential

Surveys in the Upper East Region showed that sorghum panicles were attacked by an insect pest complex of which midge, mirid and pentatomid bugs and head caterpillars were most prominent. Midge was most important on late-planted sorghums while mirid bugs constituted the main pests of early sorghums. The mirid bug complex (Heteroptera: Miridae) was dominated by Eurystylus oldi (Poppius), but Creontiades pallidus (Rambur), Campylomma angustior (Poppius), Taylorilygus sp. and Megacoelum apicale (Reuter) also proliferated on most farms. Important predators associated with head bugs included earwigs, Forficula senegalensis Serville (Demaptera: Forficulidae) and assassin bugs, especially Cosmolestes pictus Klug (Heteroptera: Reduviidae). Yield loss estimates showed that controlling either head bugs or midge alone increased grain yields by 23-35 and 26-38 per cent, respectively, while controlling both pests increased yield by up to 63 per cent. Farmers recognized panicle feeders as pests on their crops, but usually did not think they caused economic damage; hence, farmers made no conscious efforts to control them. This is probably because most of them grow local guinense-type sorghums, which are known to be less susceptible to panicle pests compared to the improved caudatum types.

Because panicle-infesting insects feed directly on the reproductive parts of cereals, they often cause direct and irreversible damage. They are, therefore, often considered economic pests and their attack results in yield and quality losses. Changes in varietal susceptibility and farming systems have, over the years, induced a greater incidence of panicle-feeding insects, resulting in low adoption of improved varieties by farmers (Tanzubil, Zakaria & Alem, 2005). However, these improved varieties are believed to hold the key to increased and sustainable sorghum production because they combine early maturity with higher yields, and have better potentials for commercial exploitation and industrial use than the local varieties. Therefore, the need to develop effective management practices for panicle feeders cannot be overemphasized.
This study aimed at developing sustainable integrated pest management (IPM) systems for key pests of sorghum in the country.

Surveys
Field surveys and farmer interviews were used in all six districts of the Upper East Region (UER) between July and December 2003 to determine the distribution, species composition, and relative abundance of insect pests attacking sorghum panicles. The UER was chosen because it is the area of most intensive cultivation of sorghum in the country, with farmers growing early and late types of sorghum. Also, our preliminary observations had shown that the pest complex on sorghum was similar throughout northern Ghana, with the UER having the greatest diversity. In each district, 5 to 10 sorghum farms were randomly selected for the study with the assistance of the local office of Ministry of Food and Agriculture. The number of farms visited depended on the size of the district and intensity of sorghum cultivation. Site and farmer selection were guided by the variety of sorghum grown, history and intensity of cultivation of the crop, ecological variability, differences in cropping systems as well as ethnic and cultural differences, among others.
On each farm, 10 panicles were randomly selected at the milky and dough stages, covered with transparent polythene bags, severed from the plant and sent to the laboratory. All insects and other arthropods were then dislodged for analysis of species composition, population levels, natural enemy activity, and damage ratings. For each farm the variety grown, date of planting, cropping system and agronomic practices adopted by the farmer were recorded. Predation was also studied via visual observation.

Farmer interviews
The perceptions of the farmers on panicle pest infestation and damage to sorghum were also collected through the use of semi-structured interviews. Information collected included resource endowments, extension contact, scale of production, type of production system, varieties grown, main pests and diseases, relative importance of head bugs and other panicle pests, control measures used, yields recorded and losses incurred. Results from such an exercise could help identify feasible control options that could be adopted under the farmers' circumstances and resources. Data collected were subjected to quantitative and qualitative analyses.

Yield loss assessments
Controlled experiments were used in 2002 and 2003 to assess the relative importance of, and quantify damage caused by the various groups of panicle pests. Three varieties, namely local Kobori, Malisor 84-7 and Kapaala, were planted at the Manga Research Station to assess the nature and extent of yield losses caused by sorghum head insects, with emphasis on midge and mirid head bugs. The design was a split plot with planting dates (early June and early July) as main plots and varieties as sub-plots. Each treatment was replicated three times in plots consisting of five rows each 5 m long. At booting, 20 plants were tagged in each plot, from which five each were randomly selected at flowering and treated as follows: i. covered with head cage from head exertion up to harvest (full protection) ii. covered from head exertion to full anthesis (midge-controlled) iii. covered from full anthesis to hard dough stage (head bug-controlled) iv. not covered throughout reproductive development At maturity, each head was harvested separately, and yield and damage due to midge and head bugs were assessed using procedures described by Ratnadass, Doumbia & Ajayi (1993).

Field surveys
The surveys confirmed that sorghum panicles in all the study areas supported a pest complex of which midge, Stenodiplosis sorghicola ( Corq.), mirid head bugs, and pentatomid bugs were most dominant (Table 1). On early sorghum, midge incidence was low and mirid head bugs dominated the pest complex on all farms sampled. Latematuring sorghums, however, suffered more from midge damage than from mirid bugs.
Pentatomid bugs were few in the samples extracted from sorghum panicles, though field observations showed high populations. The main species included Miperus jaculus (Thunberg), Aspavia armigera (Fabricius), Riptortus dentipes considered as pests, observations in this study indicated that in sorghum panicles they fed on mirid bugs. Another predator occasionally encountered was the tree frog, Hyperilus sp.

Farmer interviews
Most farmers (53%) did not recognize head bugs as economic pests of sorghum, though 87% reported annual attack of their crops by these insects (Table 2). Bawku East recorded the highest (Fabricius), and Nezera viridula (Linnaeus).
Predatory arthropods recovered from sorghum panicles included assassin bugs (Cosmolestes pictus and Rhynocoris segmentarius (Germar), earwigs (Forficula senegalensis, Serville), and spiders (Fig. 2). Earwigs were recovered from each head examined, and sometimes up to 10 inhabited a single panicle. Though earwigs are sometimes responses (70%) for head bugs being important or very important followed by Bawku West (67%) while Bongo had the lowest, with only 20 per cent considering head bugs as important. Similarly, while 60 per cent of respondents in Bawku East thought the incidence of head bugs had increased over the past few years, most farmers in the other districts felt the trend in attack was either the same or fluctuating over the same period. None of the farmers interviewed took any measures to control panicle insects, except in the Bolga District where hand-picking was reportedly adopted against head caterpillars.

Yield loss assessments
Full control of all head insects resulted in 50

Discussion
Earlier studies in Ghana had recognized paniclefeeding insects as economic pests of sorghum. Bowden (1965) identified S. sorghicola and pentatomid bugs, M. jaculus and Riptortus spp., as the most important pests of sorghum panicles in northern Ghana.
Agyen-Sampong (1978) recorded 47 insect species associated with sorghum panicles at Nyankpala of which he singled out Contarinia (= Setenodiplosis) sorghicola, Riptortus tenuicornis (Dall.), and Anoplocnemis curvipes (Fabricius) as being most important. This study confirms the importance of midge and hemipteran head bugs as pests of sorghum. However, there has been a significant change in the pest profile over the years, with mirid head bugs now becoming more important than the pentatomids. None of the earlier studies mentioned mirid bugs as pests of sorghum. Mirid bugs feed and oviposit on developing sorghum grain, resulting in quantitative and qualitative losses. The action of digestive enzymes introduced into the grain during feeding further causes a breakdown of the endosperm, leading to loss of vitrosity. This change is probably associated with changes in varieties grown by farmers, from the local guinense types with loose droopy heads to the improved caudatum types with compact panicles.  (Descamps, 1954;Nwanze, 1985;McFarlane, 1989;Gahukar, Doumbia & Bonzi, 1989). In a review of panicle pests of sorghum in West Africa,  mentioned eight other mirid bugs as key pests. All the species of mirid bugs recorded in this study are included in this list, suggesting that the pest profile of sorghum is similar throughout the sorghum belt of the subregion.  established an economic injury level (EIL) of 0.97 to 2.52 E. immaculatus per sorghum panicle in Mali. Table  1 clearly shows that most farms sampled in the six districts supported higher populations than the EIL. Such populations would cause economic injury if left uncontrolled.
Pentatomid bugs, especially M. jaculus and A. armigera, proliferated on all farms visited, though few were recovered from sampled panicles. This is because most of these are very active and, thus, would escape capture under the whole panicle collection method used. As observed by Agyen-Sampong (1978), these bugs pierce and suck sap from milky grain, causing them to shrivel. This leads to losses in grain quality and quantity.
Though yet to be accurately assessed, there seems to be potential for biological control of panicle pests, especially mirid bugs. The controversy is over the status of earwigs in crop production. Though often abundant on sorghum panicles, some authors, including Nwanze (1985), consider them to be more of a nuisance to humans than the crop. Many farmers interviewed had similar perceptions about earwigs, with 66.7 and 100 per cent of farmers in Bolga and Bongo, respectively, describing them as nuisance organisms that bite them during harvesting. Krall, Youm & Kogo (1995) reported that nymphs and adult F. senegalensis feed avidly on millet spikelets, but damage inflicted on grain was negligible. In this study, they seemed to be more of beneficial organisms than pests, often observed feeding on head bugs.
Farmers recognized panicle feeders as pests on their crops, but did not usually think they caused economic damage; hence, they made no conscious efforts to control them. This is probably because most farmers grow local guinense-type sorghums, which are known to be less susceptible to panicle pests than the improved caudatum types Sharma et al., 1994). As the latter types of sorghum are introduced into the farming systems, the need is to sensitize farmers on the pest challenges likely to emerge and the possible actions that they can take to minimize crop damage. Similarly, farmers could not associate the chaffy heads observed on their fields with damage caused by midge. This phenomenon was usually blamed on drought, although the year under review experienced no significant drought throughout the growing season.
Yield loss studies consistently show that panicle insects are economic pests of sorghum that can cause up to 50 per cent yield losses in unprotected sorghum crops. The variety Malisor 84-7 seemed to be less damaged by head insects than Kapaala, but none of these was superior to the local variety in pest resistance. There would be the need to explore more germplasm and varieties to identify sources of resistance, which can be combined with other options in evolving sustainable IPM systems for sorghum head bugs and other panicle feeders.