The Other Insect Pests : Corn Planthoppers and Leaf Aphids
In this issue will be discussing not-so popular but equally destructive insects pests in corn. This will help corn farmers to know more about these pests and control them in their own fields.
Since we’ve been talking about major corn diseases in the past two issues, let’s hop now to a new set of pests: insects. When we talk of insect pests in corn, the first that come to mind are caterpillars (or worms in layman’s terms), which feed on the stalks, leaves, and ears. These borers, cutworms, armyworms, and earworms are a familiar sight in cornfields, and they can inflict serious damage on the crop if left uncontrolled and can result in significant yield losses. So farmers pay close attention to them, especially if the corn planted does not contain any protection from these insects. In the process, farmers become more knowledgeable on borers and its cousins than on other insects pests. As they say in Luzon, “It’s better to have your cornfield infested with any kind of pest than for it to be infested with corn borer.”
Just recently, some farmers in Ilocos brought to our attention an incident of hopper infestation in their off-season crop. It was their first time to see so many plant hoppers in a corn plant. Severely infested leaves, turn black because of the sooty molds or fungal growth on the honey dew that these hoppers produce. In most cases, plants become barren (without productive ears) and the ears are underdeveloped. For veteran farmers in that area, it was an unusual sight although most of them had already an idea of how to control them. Probably, it is just a similar approach to controlling leaf aphids or aplat in Ilocano.
Hence, our topic for the month is the sucking insectt pests of corn. This group may be of lesser importance than Lepidopterans (group of moths and butterflies), but it is also important to understand and recognize these insects and the damage they inflict on the corn plant. Briefly, we shall describe the common aphids and hoppers in corn and the control measures we can take to combat such problem.
In the Philippines, there are two documented species of planthoppers that attack corn. The most common and the only previously recorded planthopper species for corn is , Peregrines maidis (Ashmead). Just recently, another species was recorded to have infested thousands of corn fields in Mindanao, the Stenocranus pacificus (Kirkaldy). These planthoppers are locally called sip-sip or silam-silam in Ilocano, ngusong kabayo in Tagalog, and wayawaya in Cebuano and Ilonggo.
Adults are soft and pale yellowish after emergence, and its wings are whitish and devoid of spots. Its eggs can be shaped like sausage (Stenocranus), usually laid masses in longitudinal slits of the leaves near the stem, with incubation period of 5-10 days. Nymph or young Peregrinus sp. is yellowish, somewhat mottled black and wingless with eyes that are red and have antennae near short artista (bristle-like). On the other hand, Stenocranus sp. can be described as dirty white and with greenish eyes. These two species have the capacity to molt 5 times in 14-25 days. It is at this stage (nymph) when these planthoppers are the most destructive.
These insects are more common during dry season in South Cotabato, Sarangam, and Sultan Kudarat, and moderately observed in wet season. In South and North Bukidnon it was observed moderately present during the whole season of corn production. Its presence is quite low in other parts of the country although Pioneer agronomists have monitored them already in Isabela, Pangasinan, and Camarines Sur in both wet and dry seasons.
Damage. Planthopper damage in corn can be easily distinguished. They suck plant juices from young leaves and leaf sheaths which can lead to stunted plant growth in severe cases. The formation of droplets of honeydews on leaves and their toxins may cause galls along the veins and underneath leaf surface that resulted in loss of plant vigor and stunting.
Among Stenocranus sp.infested plants, white mold formation can be initially observed followed by the drying of leaves and formation of black sooty molds on the plant’s surfaces resulting in smaller ears and consequently low yields. Planthoppers can appear as early as during seedling emergence or at the two-leaf stage of the corn plant up to late whorl. Under heavy infestations, they can damage the corn plant until it reaches reproductive stage.
Alternate hosts. Just like other pests in corn, there are plenty of alternate weed hosts present in any cornfield like bugang or aguingay, goosegrass or palagtiki, and kulitis or amaranth.
CORN LEAF APHID
Sucking insects such as corn leaf aphids (Rhopalosiphum maidis Fitch) are quite common across all corn growing areas of the Philippines. Called aplat in Ilocano and dugos-dugos in Cebuano, the nymphs and adults are small, pearshaped, and pale to dark green in color. The head is marked with 2 longitudinal dark bands with two black spots on the abdomen on each side. Adults are winged measuring 1.3 mm long by 0.5 mm wide, and can reproduce in just 11 days even without males.
Aphids have been recorded in almost all corn growing areas of the country with yield losses ranging from 5%-20% although under heavy infestations, especially during the dry season, a reduction of 50% can be expected.
Damage. Aphids can attack corn from early whorl up to post-silking stage. Severe sucking by large colonies of aphids can cause seedlings to wither and die, while damage in the later stages includes stunted plant growth and yellowish mottling of leaves. Infested leaves and tassels and/or ear can dry prematurely as the plants move food supply from the ears to the affected portion.
Such response can also lead to poor ear development and even barren ears. Growth of sooty molds can also be observed on infested plant parts. Aphids ,are likewise known to transmit the sugarcane mosaic virus and maize dwarf mosaic virus.
Alternate host. Sugarcane and other grassy weeds are known as alternate hosts of corn leaf aphids.
COMMON GRASS DERBID
Among sucking insects, the grass derbid (Proutista moesta Westwood) is, perhaps, the least popular and is often dismissed as harmless insects. Also called sip-sip in Ilocano and dahon-dahon in Ilonggo, it is easy to find these pests in corn field because of the adults’ distinct bluish black forewings and deep brown body color. They are normally resting and sucking on the underside of the leaves. Nymphs, which are the destructive stage, are light brown in color with deep, red eyes. The head is blunt on the tip and the abdomen tapers posteriorly. Grass derbids have five moltings in 2030 days.
Corn plants are susceptible to grass derbid damage starting from seedling to late whorl stages although it has been reported to extend up to the reproductive stages under heavy infestations.
Damage. The common grass derbid can cause loss of plant vigor as they suck the plants’ sap. Heavy infestation especially just before flowering can lead to small, underdeveloped ears. Cases of gall formations along the veins and under leaf surface cari also be observed brought about by toxins from the insect. They are also capable of transmitting viral diseases such as maize stripe or maize mosaic virus.
Alternate hosts. It has been reported that grass derbids can also thrive in the ricefields. Other alternate hosts include sugarcane, sorghum, coconuts, and a number of grassy weeds such as aguingay (Rottboellia sp.) and sabung-sabungan (Eleusine indica).
Corn planthoppers may have not been a big issue to most farmers if due diligence in monitoring is made. Basic knowledge of the pest is also very important so farmers can make the proper control measures even before damage can become serious. Take note also that even insect-protected corn hybrids offer no resistance to sucking insects such as hoppers and aphids; Bt technology merely protects your corn from the Asiatic corn borer, the most prevalent among all insect pests in corn.
Here are some tips in controlling planthoppers, aphids, and grass derbids in your corn fields when threshold levels are exceeded.
1. Insecticidal seed treatment prior to planting should be similar to the control or management of corn seedling maggots.
2. Practice fallow period of 2-3 months prior to planting to deprive these pests of plant hosts; plow under corn debris after harvest to kill remaining eggs, nymphs, and adults.
3. If you can afford to do so, practice wider spacing during planting (70 cm between rows and 28 cm between hills) to achieve a lower plant population (< 50,000 plants/hectare). This is to allow better sunlight penetration within the cornfield and minimize shade that favors hoppers to multiply.
4. Practice crop rotation with cotton, rootcrops, beans and other non-graminae (grass) crop to break the life cycle of the pests and allow more natural enemies to increase their population.
5. Balance fertilization and split the application of nitrogen, for greener plants reportedly attract aphids and hoppers.
6. Spray thiamethoxam, carbaryl, cypermethrin, deltamethrin, chlorpyrifos, and methomyl.
7. Employ biological method to control hoppers by using spiders, earwigs, mirid bugs, dryinid wasps, fairy fly, and microbials to suppress the hoppers’ population. Ensure that farming practices are consistent in maintaining a healthy population of these natural enemies.